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  • Male whitespotted bamboo shark (Chiloscyllium plagiosum) biting the pectoral fin of a female shark during courtship. Unusually for sharks, these bamboo sharks seem to mate on a one-to-one basis. In other shark species, females are usually mobbed by multiple males. The male's two claspers are clearly visibly in this image, as are the male's siphon sacs, which are inflated with sea water for the purpose of washing the shark's semen into the female's cloaca during the actual act of mating. The siphon sacs depicted here seem to be inflated to a greater degree than seen in most other shark species. The entire courtship and mating process spanned at least eight hours. From the time that the male grasped the female's pectoral fin to the end of actual mating was approximately one hour.
    mating-whitespotted-bamboo-sharks-ch...tif
  • Red Light District: A pair of mandarinfish (Synchiropus splendidus) mating at late dusk, photographed to appear as if illuminated against a red lantern suggestive of red light districts. Photographed in the Lembeh Strait, North Sulawesi, Indonesia.
    mating-mandarinfish-at-night-synchir...tif
  • Male long-spine porcupinefish (Diodon holocanthus) pursuing a female fish. This was the culmination of several hours of activity involving a number of males pursuing this female, until finally only one male was left. During the pursuit, the female swam at rapid speed in seemingly random directions, as if she were trying to lose the males. She hid in crevices at times, forcing the males to search for her. The males nipped and bit her constantly. The actual mating event took place well after sundown, with the fish swimming out to a deep part of the channel at about 23 meters, then rushing toward the surface, releasing eggs and sperm at about 12 meters depth.
    diodon-holocanthus-mating-long-spine...tif
  • Pair of Risbecia tryoni nudibranchs mating. Lembeh Strait, North Sulawesi, Indonesia
    Risbecia-tryoni-nudibranchs-mating-l...tif
  • Mating mandarinfish, just under the jetty of Tufi Dive Resort. This photograph is a film scan.
    Mandarinfish-mating-Synchiropus-sple...tif
  • Pictured here is a pair of olive ridley turtles (Lepidocheyls olivacea) engaged in mating.
    lepidocheyls-olivacea-olive-ridley-t...tif
  • Male long-spine porcupinefish (Diodon holocanthus) pursuing a female fish. This was the culmination of several hours of activity involving a number of males pursuing this female, until finally only one male was left. During the pursuit, the female swam at rapid speed in seemingly random directions, as if she were trying to lose the males. She hid in crevices at times, forcing the males to search for her. The males nipped and bit her constantly. The actual mating event took place well after sundown, with the fish swimming out to a deep part of the channel at about 23 meters, then rushing toward the surface, releasing eggs and sperm at about 12 meters depth.
    long-spine-porcupinefish-mating-diod...tif
  • Portunid swimming crabs mating at night in Lembeh Strait, North Sulawesi, Indonesia. The male crab is on top. The female crab is underneath, flipped upside down.
    portunid-swimming-crabs-mating-at-ni...tif
  • Mating cream-tinted Hypselodoris bullocki nudibranchs, Ambon, Indonesia
    mating-Hypselodoris-bullocki-nudibra...tif
  • Nembrotha kubaryana nudibranchs mating on a coral reef in Ambon, Indonesia
    Nembrotha-kubaryana-nudibranchs-mati...tif
  • Two beautifully colored Hypselodoris bullocki nudibranchs mating. Lembeh Strait, North Sulawesi, Indonesia.
    colorful-Hypselodoris-bullocki-nudib...tif
  • Mating whitespotted bamboo sharks (Chiloscyllium plagiosum), with the male above, insertion of right clasper clearly visible. This photograph depicts the final stages of copulation, which spanned approximately five minutes. Consequently, the male's siphon sacs are almost entirely deflated, indicating that the bulk of sperm insertion has already taken place. Just over one hour elapsed between the time that the male was able to grasp the female's pectoral to the final act of copulation. The male pursued this female for at least seven hours prior to being able to bite the female's pectoral fin, with the entire sequence of events taking place in water that was between two and six meters deep.
    mating-whitespotted-bamboo-sharks-ch...tif
  • Portunid swimming crabs mating at night in Lembeh Strait, North Sulawesi, Indonesia. The male crab is on top. The female crab is underneath, flipped upside down.
    portunid-swimming-crabs-mating-at-ni...tif
  • This is a pair of broadclub cuttlefish (Sepia latimanus) mating. Having succeeded in gaining the female's acceptance, the male on the right has wrapped his arms around the female's head. He is in the process of deliverying a package of sperm with his hectocotylus (modified left tentacle). The male typically stays with the female while she deposits fertilized eggs, to guard against copulation by other males.
    sepia-latimanus-cuttlefish-mating-ja...tif
  • This is a pair of mating Pteromys volans orii flying squirrels engaged in copulation on a cold winter day.
    pteromys-volans-orii-flying-squirrel...tif
  • This is a pair of Pteromys volans orii flying squirrels mating. Copulation occurs multiple times over an extended period of time. This can happen inside the nest or outside, as shown here. Mating can take place in the day time or at night.
    pteromys-volans-orii-flying-squirrel...tif
  • This is a pair of mud crabs (Scylla sp.) preparing to mate in the late summer. The male is on top, with the female upside-down beneath.
    scylla-mud-crabs-mating-japan-202208...tif
  • This is a pair of mud crabs (Scylla sp.) preparing to mate in the late summer. The male is on top, with the female upside-down beneath.
    scylla-mud-crabs-mating-japan-202208...tif
  • To initiate the courtship process, the male whitespotted bamboo shark (Chiloscyllium plagiosum) approaches the female and bites her to secure a grip. In order to mate, the male must keep up with the female as she struggles to break free of his grip. This involves thrashing and swimming over an extended period and area. To position himself properly for copulation, the male needs to bite the female's pectoral fin. If his initial bite is on another part of the female's body, the male must adjust his position to secure himself to the pectoral fin, as pictured here. In this image, the female and male have come to a brief rest after a prolonged swim of nearly an hour, with the female just about to take off again with suitor in tow. Copulation occurred within a couple of minutes of this photo being taken.
    whitespotted-bamboo-shark-courtship-...tif
  • This is a female Sepia latimanus broadclub cuttlefish preparing to deposit eggs into the Acropora coral. Females adopt this characteristic scrunched up face prior to depositing eggs. The male that has just mated with her is standing guard in the background to keep other males away.
    sepia-latimanus-cuttlefish-courtship...tif
  • A pair of Nembrotha cristata nudibranchs enjoying a morning tryst
    green-and-black-Nembrotha-cristata-n..tiff
  • The final few minutes during copulation of whitespotted bamboo sharks (Chiloscyllium plagiosum). Biting the female's right pectoral fin, the male has aligned his body to use the clasper on the right side of his body to inseminate the female. Clearly visible is the nearly deflated siphon sac structure on the ventral surface of the male, which provides a visual indicator of progress. The male uses his siphon sacs, which he has filled with seawater while swimming with the female during the courtship leading up to copulation, to flush semen into the female. Shortly after this photo was taken, copulation ended. The female immediately swam off, while the male collapsed onto the reef and remained motionless for an extended time.
    mating-whitespotted-bamboo-sharks-ch...tif
  • This is a view of olive ridley turtle (Lepidocheyls olivacea) copulation from beneath the turtles. The male has inserted his penis (which is situated in his tail) into the female's cloaca to inseminate her.
    lepidocheyls-olivacea-olive-ridley-t...tif
  • This is a male whitespotted bamboo shark (Chiloscyllium plagiosum) working his way into a crevice in order to approach a female shark for mating. The male had to work from a relatively inconvenient angle in order to position himself to bite the female's pectoral fin to initiate the courtship process. The female was lodged in a narrow space, with limited access for potential suitors. From the time the male succeeded in biting the female's pectoral fin until the act of copulation took about one hour. The male's two claspers (the organs used for inseminating the female shark) are clearly visible here.
    male-whitespotted-bamboo-shark-clasp...tif
  • In the days leading up to mating among whitespotted bamboo sharks (Chiloscyllium plagiosum), empty egg sacs like the one pictured here appear. These eggs sacs do not contained fertlized embryos. They are empty, and they are not old egg sacs from which juvenile sharks have already emerged. The appearance of these empty eggs sacs seems to coincide with heightened male shark activity, with ensuing courtship and copulation.
    whitespotted-bamboo-shark-empty-egg-...tif
  • Male whitespotted bamboo sharks (Chiloscyllium plagiosum) initiate the courtship process by biting and holding on to the female shark, striving to get a firm grasp on one of the pectoral fins as depicted here. The female appears to struggle and attempt to escape, leading the male on an extended swim. At some point, the female seems to give in, and the male has an opportunity to mate. During this sometimes intense swim, the male gradually inflates his siphon sacs with seawater, visible here as the bulge at the ventral area of the male. The male uses seawater to flush his sperm into the female's cloaca when the time comes to mate. As shown here, these bamboo sharks seem to mate one-to-one, which is in contrast to observed behavior in other shark species, when a single female attracts the attention of many males at once.
    courtship-whitespotted-bamboo-shark-...tif
  • This is a male broadclub cuttlefish (Sepia latimanus) engaging in courtship. He is stroking the female with his tentacles. If she accepts, they will mate and she will deposit eggs while the male maintains vigil to warn off other males.
    sepia-latimanus-broadclub-cuttlefish...tif
  • Shown here is a male Sepia latimanus cuttlefish (right) approaching a female (left) in order to attempt to mate. The male has split his body pattern and coloration into two distinct halves for courtship. The half facing the female is a subdued whitish tone, which presumably communicates calm, soothing intent. The other half is normal pattern, possibly to ward off other males.
    sepia-latimanus-cuttlefish-courtship...tif
  • Male and female humpback whale diving. The male has his penis extended. I observed this pair hanging out for extended periods of time in between dives. The female had a habit of hanging in a head-down position, with the ventral surface of her fluke held parallel to the surface of the water, at or just above the water line, while the male stayed nearby. It is possible that the two whales were engaged in courtship behavior.
    male-humpback-whale-penis-visible-ma...tif
  • A pair of bigeye jacks (Caranx sexfasciatus), with the male temporarily turned black in preparation for spawning. Another pair of fish also getting ready to spawn is visible in the background. These fish were among a large school of several hundred jacks swimming above the Blue Corner dive site in Palau.
    bigeye-trevally-mating-spawning-blue...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) engaged in copulation. This is an extended process spanning many hours, during which the male often needs to fend off rivals while engaging in repeated bouts of copulation.
    japanese-dwarf-flying-squirrels-mati...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) engaged in copulation. This is an extended process spanning many hours, during which the male often needs to fend off rivals while engaging in repeated bouts of copulation.
    japanese-dwarf-flying-squirrels-mati...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) engaged in copulation. This is an extended process spanning many hours, during which the male often needs to fend off rivals while engaging in repeated bouts of copulation. As shown here, the female can, and often does, put up some measure of resistance, occasionally dislodging the male completely. The male’s penis is visible.
    japanese-dwarf-flying-squirrels-mati...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) engaged in copulation. This is an extended process spanning many hours, during which the male often needs to fend off rivals while engaging in repeated bouts of copulation.
    japanese-dwarf-flying-squirrels-mati...tif
  • Three porcupine pufferfish (Diodon holocanthus) pursuing a female. There were originally five males in pursuit when I first came across this group. The courtship process lasts for several hours, culminating when only one or two males remain. This happened in the evening, around 20:00 hours. The female swam to deep water, around 23m, then rushed for the surface at high speed with the males in pursuit. She released a cloud of eggs at 12m, and the males followed with release of sperm. The courtship process is energetic, with the males chasing and biting the female. The female hid from the males on several occasions, forcing them to find her.
    Diodon-holocanthus-porcupinefish-cou...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) engaged in copulation. This is an extended process spanning many hours, during which the male often needs to fend off rivals while engaging in repeated bouts of copulation.
    japanese-dwarf-flying-squirrels-mati...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) engaged in copulation. This is an extended process spanning many hours, during which the male often needs to fend off rivals while engaging in repeated bouts of copulation.
    japanese-dwarf-flying-squirrels-mati...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) engaged in copulation. This is an extended process spanning many hours, during which the male often needs to fend off rivals while engaging in repeated bouts of copulation. As shown here, the female can, and often does, put up some measure of resistance, occasionally dislodging the male completely. The male’s penis is visible.
    japanese-dwarf-flying-squirrels-mati...tif
  • A group of bigeye trevally (Caranx sexfasciatus) swimming around a grey reef shark (Carcharhinus amblyrynchos), with one of the fish rubbing its body against the shark's skin. Bigeye trevallies and other species of fish engage in this activity from time to time, perhaps to remove parasites or other irritations from their skin. For the most part, the sharks do not seem to mind, though they sometimes take off at high speed. Note also that one of the bigeye trevallies is dark. The change in coloration is associated with reproduction. Photographed at Blue Corner in Palau.
    bigeye-trevally-rubbing-against-grey...tif
  • Close-up view of the pectoral fin of a female humpback whale (Megaptera novaeangliae) that was one of the two whales in a courting pair. She was very friendly and approached closely on a number of occasions. Note what appear to be parasites attached to some of the barnacles on the pectoral fin.
    close-up-view-pectoral-fin-humpback-...tif
  • Female humpback whale (Megaptera novaeangliae) engaged in courtship (male visible in the rear), slapping the water and creating an impressive bubble stream. These whales were extremely friendly. They engaged for several hours with people in the water.
    humpback-whales-engaged-in-courtship...tif
  • During courtship, humpback whales (Megaptera novaeangliae) sometimes swim together in a graceful manner reminiscent of a choreographed performance, and take an interest in boats and people. The two whales pictured here are a good example. The closer whale is the male, while the darker whale in the background performing a spyhop is the female. I came across this pair outside the Hunga island area in Vava'u, Tonga.
    humpback-whales-engaged-in-courtship...tif
  • A pair of humpback whales (Megaptera novaeangliae) engaged in courtship, with the female in the foreground exhaling bubbles.
    humpback-whale-courtship-blowing-bub...tif
  • Banggai cardinalfish (Pterapogon kauderni) with hatchlings in its mouth. This is one of only two known fish species whose babies return to the parents at night. The other species is the convict fish (Pholidichthys leucotaenia). Lembeh Strait, North Sulawesi, Indonesia.
    endangered-Banggai-cardinalfish-Pter...tif
  • Pair of humpback whales (Megaptera novaeangliae) engaged in courtship. The female is in the foreground. She is at the surface, re-entering the water after performing a spyhop. Note how graceful and delicate the movement of her pectoral fin is.
    humpback-whale-courtship-megaptera-n...tif
  • Female humpback whale (Megaptera novaeangliae) doing a spyhop, with motor boat and person in the background. This whale was the female of a pair engaged in courtship. The whales were extremely inquisitive and friendly, proactively approaching and engaging people in the water. The person in the background is Captain Ongo, who was one of the people who started the whale watching industry in Vava'u, Tonga.
    humpback-whale-spyhop-with-boat-and-...tif
  • Banggai cardinalfish (Pterapogon kauderni) with clutch of eggs in its mouth, eyes of the developing embryos visible. These fish are listed as endangered on the IUCN Red List. Lembeh Strait, North Sulawesi, Indonesia.
    Pterapogon-kauderni-endangered-Bangg...tif
  • Pictured here are two male broadclub cuttlefish (Sepia latimanus) engaged in a conflict. The bold striped pattern signals aggression. The males were fighting to establish dominance in the area around this large Acropora coral colony, where multiple females had gathered for spawning.
    sepia-latimanus-cuttlefish-males-fig...tif
  • Female grey reef sharks (Carcharhinus amblyrhynchos) with visible scarring from mating activity. During the mating season, many female sharks appear with scars like this, inflicted by amorous male sharks. Mating is a rough affair among sharks, ften with multiple males biting and holding on to female sharks. Photographed at Blue Corner dive site in Palau.
    grey-reef-shark-mating-scars-carchar...tif
  • Pictured here is a pair of Japanese dwarf flying squirrels (Pteromys volans orii) shortly before mating. The female is in front. She has lifted her tail, letting the male check her status by smell. They mated one hour after this photo was taken. This perch is stained with urine, and there are feces visible. This is an indication that it is frequented by these and possibly other flying squirrels.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a male and female pair of Japanese dwarf flying squirrels (Pteromys volans orii), with one inside the nest and the other above. This female (inside the nest) is in oestrus. The male is standing guard. Another male approached repeatedly, resulting in heated confrontation, with the males chasing one another, occasionally leaping and gliding among trees.
    japanese-dwarf-flying-squirrel-matin...tif
  • This is a male Pteromys volans orii flying squirrel standing guard at the entrance of a nest. Inside is a female that has entered estrus. This has attracted the attention of another male. At this point, the interloper male had made numerous attempts to mate with the female, hence the protective position of this male. The two males faced off a number of times. Interlopers are sometimes successful in mating with females while the primary male is unaware or otherwise occupied.
    japanese-flying-squirrel-pteromys-vo...tif
  • The two Pteromys volans orii flying squirrels pictured here had just emerged from their nest, taking a moment here prior to foraging. This pair mated 35 days after this photo was taken. One of the pair was taken by a northern goshawk (Accipiter gentilis) about a week after mating took place. The survivor left the nest, most likely joining other individuals in another nest.
    pteromys-volans-orii-flying-squirrel...tif
  • This is a female broadclub cuttlefish (Sepia latimanus) depositing eggs among branches of table coral (Acropora sp.) shortly after mating.
    sepia-latimanus-cuttlefish-depositin...tif
  • This is a pair of Korean seahorses (Hippocampus haema) engaged in unusual post-mating courtship activity. Earlier in the morning, the pair had completed hours of courtship, culminating with the female (background) depositing eggs into the brooding pouch of the male (foreground). Once this takes place, the pair usually return to independent activity, most often foraging for food. In this instance, the pair stayed together. They maintained body contact, often with their prehensile tails intertwined. The male broods the eggs for three to ten weeks. After hatching the juveniles, the male engages in courtship with the female and mates again throughout the spring and summer breeding season. Pair bonds are reasonably consistent, though there seems to be some mixing that takes place. Described in 2017, this species is found in the waters of Korea, as well as along the southern and western coasts of Japan.
    hippocampus-haema-korean-seahorse-co...tif
  • This is a palr of Korean seahorses (Hippocampus haema) engaged in unusual post-mating courtship activity. Whenever the two seahorses separated and were not in direct physical contact, they maintained frequent eye contact, as depicted here with the male in the background (belly full of eggs) looking toward the female. Earlier in the morning, the pair had completed hours of courtship, culminating with the female depositing eggs into the brooding pouch of the male. Once this takes place, the pair usually return to independent activity, most often foraging for food. In this instance, the pair stayed together. The male broods the eggs for three to ten weeks, depending upon the time of year. After hatching the juveniles, the male engages in courtship with the female and mates again throughout the spring and summer breeding season. Pair bonds are reasonably consistent, though there seems to be some mixing that takes place. Described in 2017, this species is found in the waters of Korea, as well as along the southern and western coasts of Japan.
    hippocampus-haema-korean-seahorse-co...tif
  • Female humpback whale calf (Megaptera novaeangliae) and its mother with multiple escort male humpbacks. There was a frenzy of males around this mother and calf pair. I have seen this type of behavior on multiple occasions. I believe it may have something to do with the adult female being in estrus, receptive to potential mating opportunities for a limited duration.
    humpback-whale-female-with-calf-mult...tif
  • This is a pair of Pteromys volans orii flying squirrels that have emerged from the nest at dusk. The female seated above is entering estrus. The male is checking her readiness for mating.
    pteromys-volans-orii-flying-squirrel...tif
  • This is a male Pteromys volans orii flying squirrel standing guard at the entrance of a nest. Inside is a female that has entered estrus. This has attracted the attention of another male, which means that this male must maintain continual vigil. The two males faced off a number of times. Interlopers are sometimes successful in mating with females while the primary male is unaware or otherwise occupied.
    pteromys-volans-orii-flying-squirrel...tif
  • This is a palr of Korean seahorses (Hippocampus haema) engaged in unusual post-mating courtship activity. Whenever the two seahorses separated and were not in direct physical contact, they maintained frequent eye contact, as depicted here. Earlier in the morning, the pair had completed hours of courtship, culminating with the female (right) depositing eggs into the brooding pouch of the male (left). Once this takes place, the pair usually return to independent activity, most often foraging for food. In this instance, the pair stayed together. They maintained body contact, often with their prehensile tails intertwined as pictured here. The male broods the eggs for three to ten weeks, depending upon the time of year. After hatching the juveniles, the male engages in courtship with the female and mates again throughout the spring and summer breeding season. Pair bonds are reasonably consistent, though there seems to be some mixing that takes place. Described in 2017, this species is found in the waters of Korea, as well as along the southern and western coasts of Japan.
    hippocampus-haema-korean-seahorse-co...tif
  • This is a pair of Korean seahorses (Hippocampus haema) engaged in unusual post-mating courtship activity. Earlier in the morning, the pair had completed hours of courtship, culminating with the female (right) depositing eggs into the brooding pouch of the male (left). Once this takes place, the pair usually return to independent activity, most often foraging for food. In this instance, the pair stayed together. They maintained body contact, often with their prehensile tails intertwined as pictured here. The male broods the eggs for three to ten weeks, depending upon the time of year. After hatching the juveniles, the male engages in courtship with the female and mates again throughout the spring and summer breeding season. Pair bonds are reasonably consistent, though there seems to be some mixing that takes place. Described in 2017, this species is found in the waters of Korea, as well as along the southern and western coasts of Japan.
    hippocampus-haema-korean-seahorse-co...tif
  • This is a small crab infested with a Sacculina barnacle parasite, a genus of barnacles that is a parasitic castrator of crabs. These barnacles are pelagic during their larval stage. Female larvae seek out host crabs. Once a suitable host is located, the female barnacle attaches to a joint area, molts into a form known as a kentrogon, and then injects its soft body into the crab, discarding its hard shell in the process. Once inside, the Sacculina develops into two parts: the interna, which comprises root-like threads that wrap around the crab’s internal organs; and the externa, which is a bulbous reproductive organ that protrudes from the crab’s abdomen, as pictured here. Male Sacculina barnacles inject themselves into a pocket in the female’s body in order to undertake the sole function of producing spermatozoa to fertilize the female. In other words, the male becomes parasitic to the female, which is parasitic to the crab. Once infected, a crab is unable to molt, though it otherwise functions normally. Nutrition that the crab ingests is siphoned off by the barnacle. Even more amazing, when a female Sacculina sets upon a male crab, as pictured here, it sterilizes the crab and causes the male crab to release hormones that cause the shape of the body to change such that it more resembles a female crab, with wider, flatter abdomen. The formerly male crab can even act like a female, performing female mating dances. When the female Sacculina is ready to release her brood into the water, she causes her host crab to do exactly what a female crab would normally do to release her own eggs. Namely, the crab finds high ground, grooms the brooding pouch on its abdomen and shoots out clouds of larvae via the hole visible at the center of the abdominal area, using its claws to stir the water to assist the newborn larvae, in this case barnacle larvae, not crabs. Once infected, a crab devotes its life to the reproduction of Sacculina. There are more than 100 species of Saccu
    sacculina-barnacle-infested-male-cra...tif
  • Aerial panorama of Kannoura fishing port in Kochi Prefecture, Japan. This is the area where I have documented white-spotted bamboo sharks (Chiloscyllium plagiosum) mating in the wild.
    kannoura-fishing-port-kochi-prefectu...tif
  • A group of twenty three sperm whales (Physeter macrocephalus) traveling together. These whales (all females and immature individuals) were part of a much larger aggregation or superpod that I estimated to comprise around 100 individuals. The reason for such a large gathering is unclear, though it may have something to do with reproduction. Some time after this encounter, I came across another large group in the same area, which included a mature bull. Mature male sperm whales spend most of their time in colder waters, visiting tropical and temperate waters for the purpose of mating. The cloud of dark substance in the foreground is defecation from one of the whales, something which sperm whales seem to do often when they are at the surface.
    superpod-sperm-whales-sri-lanka-miri...tif
  • This is a small crab infested with a Sacculina barnacle parasite, a genus of barnacles that is a parasitic castrator of crabs. These barnacles are pelagic during their larval stage. Female larvae seek out host crabs. Once a suitable host is located, the female barnacle attaches to a joint area, molts into a form known as a kentrogon, and then injects its soft body into the crab, discarding its hard shell in the process. Once inside, the Sacculina develops into two parts: the interna, which comprises root-like threads that wrap around the crab’s internal organs; and the externa, which is a bulbous reproductive organ that protrudes from the crab’s abdomen, as pictured here. Male Sacculina barnacles inject themselves into a pocket in the female’s body in order to undertake the sole function of producing spermatozoa to fertilize the female. In other words, the male becomes parasitic to the female, which is parasitic to the crab. Once infected, a crab is unable to molt, though it otherwise functions normally. Nutrition that the crab ingests is siphoned off by the barnacle. Even more amazing, when a female Sacculina sets upon a male crab, as pictured here, it sterilizes the crab and causes the male crab to release hormones that cause the shape of the body to change such that it more resembles a female crab, with wider, flatter abdomen. The formerly male crab can even act like a female, performing female mating dances. When the female Sacculina is ready to release her brood into the water, she causes her host crab to do exactly what a female crab would normally do to release her own eggs. Namely, the crab finds high ground, grooms the brooding pouch on its abdomen and shoots out clouds of larvae via the hole visible at the center of the abdominal area, using its claws to stir the water to assist the newborn larvae, in this case barnacle larvae, not crabs. Once infected, a crab devotes its life to the reproduction of Sacculina. There are more than 100 species of Saccu
    sacculina-barnacle-infested-male-cra...tif
  • Southern right whale (Eubalaena australis) with pectoral fin raised above the ocean surface. This was a group of several whales, perhaps engaged in courtship/ mating activity. The whale with its pectoral fin in the air was the focal animal, which suggests there was a high probability it was female. Visible in the background are fields of canola, a major crop in South Africa. Photographed with the permission of the Department of Environmental Affairs, South Africa.
    southern-right-whale-pectoral-fin-ra...tif
  • When female Pteromys volans orii squirrels go into estrus, there is usually a primary male with her. He fends off challengers and does his best to monopolize copulation opportunities. Females will mate with other males when the opportunity arises however. Here, the female has snuck out to copulate with another male. The primary male has just realized what is happening and has rushed out of the den to confront the interloper.
    pteromys-volans-orii-flying-squirrel...tif
  • This is a male hairchin goby (Sagamia geneionema) protecting a brood of eggs. Males of this species mate several times with females, which lay their eggs on the upper surfaces of confined spaces and then leave the area to the care of the male. In this case, this burrow was at 20m depth, with water ranging between 14 and 16 degrees Celsius. The males protect the eggs until they hatch, somewhere between one and two weeks depending on water temperature, and then mate again. Throughout this process, the male does not eat much. As a result, after several matings, the males become too weak to carry on, and they die. The name for this species in Japanese is sabihaze (サビハゼ).
    hairchin-goby-sagamia-guarding-eggs-...tif
  • Depicted here is a pair of Pteromys volans orii flying squirrels sharing a nest high up in the canopy. The larger individual on the left is female, the other male. During the peak of reproductive season, males and females often pair up like this. The bonds are not permanent though, and fidelity is not certain. It is not unusual for females to mate with more than one male.
    pteromys-volans-orii-pair-japan-2021...tif
  • This is a large male broadclub cuttlefish hovering above a coral reef during the reproductive season. Multiple males like this were engaged in competition for females that were ready to mate and deposit eggs.
    sepia-latimanus-broadclub-cuttlefish...tif
  • Competition between rival males for the right to mate with a female flying squirrel (Pteromys volans orii) in estrus can become heated. The two males pictured here chased one another up and down the tree, culminating in one executing a full body-slam punch, sending the other into the air.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is the moment when a male Pteromys volans orii flying squirrel inside the nest has heat-butted a potential rival male that was looking into the nest with the objective of gaining access to the female inside, which was approaching estrus. Even after being chased off, rival males like this often return to try again. Females sometimes choose to mate with multiple partners.
    pteromys-volans-orii-flying-squirrel...tif
  • This is a Japanese dwarf flying squirrel (Pteromys volans orii), gliding from the top of one tree to another. Known locally as ezo-momonga, this sub-species of Siberian flying squirrel is found only in Hokkaido, Japan. Mature females measure up to 15cm, males up to 18cm (not including tail). These animals weigh up to 120g and are capable of gliding considerable distances. During flight, they use their patagia (membranes of skin between their forelimbs and hind limbs) and tails (10-12cm) to achieve lift, directional control and maneuvering capability. One study in Japan recorded a maximum glide distance exceeding 49m, though most flights fell into the 10m to 20m range. Although these animals are primarily nocturnal, they sometimes emerge during the day. Usually this is for brief durations to urinate or defecate. In this instance, this particular individual was one of two males competing for the right to mate with a female. Such reproduction-related competition sometimes takes place during daylight hours.
    japanese-dwarf-flying-squirrel-airbo...tif
  • In the midst of competition for the right to mate with a female Japanese dwarf flying squirrel (Pteromys volans orii), this male launched itself from the top of the tree stump just fractions of a second before I pressed the shutter. The squirrel landed centimeters away from my arm on a tree that I was braced against, then rushed upward and launched itself again in pursuit of the other male.
    japanese-dwarf-flying-squirrel-male-...tif
  • Pictured here is a 3cm female skeleton shrimp (Caprella bispinosa) with her small white babies clinging to her body. The magenta-red spikes on her body take on this coloration during this time of year. The spines contain venom. Females carrying young are thought to use these spines to ward off potential predators targeting the young. This may include males of her own species, which may attempt to kill the young in order to mate with the female, similar to the behavior of cats. Males of this species and other skeleton shrimps, which are actually amphipods, also have venom-laden spikes, which they use for competition between males.
    caprella-bispinosa-female-carrying-y...tif
  • This is a Pteromys volans orii Japanese dwarf flying squirrel male perched at the top entrance to a nest. On this day, he was guarding a female inside the nest. She was in estrus, and the male was warding off other males. This pair mated several times throughout the day, but the female also mated with another male while this one was unaware. This photo is one in a series of three which illustrate the height of the nest.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a Pteromys volans orii Japanese dwarf flying squirrel male perched at the top entrance to a nest. On this day, he was guarding a female inside the nest. She was in estrus, and the male was warding off other males. This pair mated several times throughout the day, but the female also mated with another male while this one was unaware. This photo is third in a series of three which illustrate the height of the nest.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a Pteromys volans orii Japanese dwarf flying squirrel male perched at the top entrance to a nest. On this day, he was guarding a female inside the nest. She was in estrus, and the male was warding off other males. This pair mated several times throughout the day, but the female also mated with another male while this one was unaware. This photo is first in a series of three which illustrate the height of the nest.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a pair of Japanese dwarf flying squirrels (Pteromys volans orii). The one in front is male. The one peeking out from behind is female. They are situated at the top entrance to their nest, caressed by warm light from the setting winter sun. This pair mated several times throughout the day. There was another male in the area, a challenger. The female mated with the other male as well, when this one was not looking.
    japanese-flying-squirrel-pteromys-vo...tif
  • This male and female Pteromys volans orii flying squirrels have paired up for the reproductive season. They had just finished foraging for food high in the canopy and were sitting together like this during light snow. The pair mated a few days after this photograph was taken. The male is on the right, female left.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a pair of Korean seahorses (Hippocampus haema) engaged in spawning. The lighter-colored female on the left is depositing eggs into the brooding pouch of the male (right, looking toward the camera). The male’s pouch is swollen and distended to make room for the eggs, which he fertilizes once they are deposited. The male broods the eggs for three to ten weeks. After hatching the juveniles, the male engages in courtship with the female and mates again throughout the spring and summer breeding season. Pair bonds are reasonably consistent, though there seems to be some mixing that takes place. Described in 2017, this species is found in the waters of Korea, as well as along the southern and western coasts of Japan.
    hippocampus-haema-korean-seahorse-sp...tif
  • This is a pair of Pteromys volans orii flying squirrels that have just emerged from their nest early in the morning during the winter reproductive season. The individual on the left is female, right male. After preening together, the pair ascended into the canopy to forage. They mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a Pteromys volans orii flying squirrel male grooming itself after emerging from his nest on an early winter morning. This male shared the nest with a female. The pair mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a Pteromys volans orii flying squirrel male grooming itself after emerging from his nest on an early winter morning. This male shared the nest with a female. The pair mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a male striped poison-fang blenny mimic (Petroscirtes breviceps) watching over multiple batches of eggs that are at different stages of development. The eggs are adhered to the inside of the bivalve shell, with the eyes of many of the developing fish larvae visible. This male will have mated with several different females at various times, with each female depositing eggs to the inner surface of the shell. The male guards the eggs until they hatch. Pictured here, the male has just opened his mouth for a split-second.
    petroscirtes-breviceps-guarding-eggs...tif
  • This is a Pteromys volans orii flying squirrel male pausing after grooming himself during an early winter morning. This male shared the nest with a female, which joined him shortly after this photo was taken. The pair mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a pair of Pteromys volans orii flying squirrels that have just emerged from their nest early in the morning during the winter reproductive season. The individual on the left is female, right male. After preening together, the pair ascended into the canopy to forage. They mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a Pteromys volans orii flying squirrel male grooming itself after emerging from his nest on an early winter morning. This male shared the nest with a female. The pair mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a Pteromys volans orii flying squirrel male grooming itself after emerging from his nest on an early winter morning. This male shared the nest with a female. The pair mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a portrait of a Pteromys volans orii flying squirrel male illuminated by the light of an early winter morning. He had just emerged from his nest, followed shortly thereafter by a female. The pair mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a mature male Asian sheepshead wrasse (Semicossyphus reticulatus). The fish has taken on whitish pastel coloration for the breeding season. During this season, the male mates with multiple females, and defends his terrority against other males. Wrasses are protogynous hermaphrodites, so this male, like all other males of this species, started life as a female.
    asian-sheepshead-wrasse-semicossyphu...tif
  • This is a mature male Asian sheepshead wrasse (Semicossyphus reticulatus). The fish has taken on whitish coloration for the breeding season. During this season, the male mates with multiple females, and defends his terrority against other males. Wrasses are protogynous hermaphrodites, so this male, like all other males of this species, started life as a female. At 100% magnification parasites attached to the fish’s face are visible.
    asian-sheepshead-wrasse-semicossyphu...tif
  • This is a mature male Asian sheepshead wrasse (Semicossyphus reticulatus). The fish has taken on whitish coloration for the breeding season. During this season, the male mates with multiple females, and defends his terrority against other males. Wrasses are protogynous hermaphrodites, so this male, like all other males of this species, started life as a female.
    asian-sheepshead-wrasse-semicossyphu...tif
  • This is a pair of Pteromys volans orii flying squirrels that have just emerged from their nest early in the morning during the winter reproductive season. The individual facing the camera is male. After preening together, the pair ascended into the canopy to forage. They mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a pair of Pteromys volans orii flying squirrels that have just emerged from their nest early in the morning during the winter reproductive season. The individual on the left is female, right male. After preening together, the pair ascended into the canopy to forage. They mated 10 days after this photo was taken.
    japanese-flying-squirrel-pteromys-vo...tif
  • This is a palr of Korean seahorses (Hippocampus haema) preparing to spawn. About 30 seconds after this photo was taken, the seahorses swam up from the reef, where the lighter-colored female (left) deposited eggs into the abdominal pouch of the male (right). Once spawning takes place, seahorses usually  usually return to independent activity, most often foraging for food. In this instance, the pair stayed together post-spawning and continued courtship-like behavior. They maintained body contact, often with their prehensile tails intertwined. The male broods the eggs for three to ten weeks, depending upon the time of year. After hatching the juveniles, the male engages in courtship with the female and mates again throughout the spring and summer breeding season. Pair bonds are reasonably consistent, though there seems to be some mixing that takes place. Described in 2017, this species is found in the waters of Korea, as well as along the southern and western coasts of Japan.
    hippocampus-haema-korean-seahorse-co...tif
  • Pteromys volans orii flying squirrels engaged in copulation during daylight hours
    pteromys-volans-orii-flying-squirrel...tif
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