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Coelacanths + my favourite gar! I'm so happy with all my little fishes (stay tuned for sturgeons soon) <3

P.S: If you look in the background there, you may spot a Special Little Guy made by @moldspace doing a great job guarding my ficus elastica!

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Wet Beast Wednesday: seahorses

This Wet Beast Wednesday post is about the weirdest damn fish out there. Seahorses are what you get when a fish tries to be as un-fish-like as possible. This makes them weird as hell but also very interesting. There are currently 46 recognized species of seahorse, with more possibly out there, waiting to be discovered. Seahorses are members of the genus Hippocampus and are named after a famous mythical creature. I am of course referring to the horse.

(Image: a common seahorse (Hippocampus kuda). Ir is a yellow fish with a profile similar to a chess knight. It has an upright body with multiple bony ridges and a forward facing head with a long, tubular snout. The tail is long, serpentine, and coiled in a spiral. The only fin is on the back near the base of the tail. End ID)

Seahorses are one of three groups of fish that make up the family Syngnathidae, the others being the pipefishes and sea dragons. The sea dragons are more closely related and look similar, but can be distinguished from seahorses because they lack prehensile tails. Genetic and molecular analysis indicates that seahorses and sea dragons are descendants of pipefish. Seahorses have upright, rigid and angular bodies with scaleless skin and long, prehensile tails. The head is positioned at an angle to the body and allegedly looks like the mythical horse. I think it looks more like a chess knight after a weird manufacturing error. The head is tipped with a long, tubular mouth that only opens at the very end. The only fin on the body is the dorsal fin on the back. The eyes on opposite sides of the head and each can move independently of each other, allowing the seahorse to look in two directions at once. The largest species of seahorse is the big-belly seahorse (Hippocampus abdominalis) which can reach a length of 35 cm (14 in) with its tail uncurled. The smallest species is Satomi's pygmy seahorse (Hippocampus satomiae), which reaches 14 mm (0.5 in).

(Image: a group of three Satomi's pygmy seahorses clinging to some polyps. They are tiny seahorses with tan, spiny bodies. End ID)

Seahorses and other Syngnathids have a unique(ly bizarre) skeletal system consisting of a series of squarish bony rings emerging from and surrounding the spine. These rings are widest at the abdomen, providing space for the internal organs, and narrowest around the tail. This structure provides extra support to the body ad well as acting kind of like armor, making it somewhat similar to an exoskeleton. The tip of the rings can be seen through the seahorse's skin, giving them a distinct ridged texture. The jaw bones are fused together to make up the tubular mouth. Only the lips at the end of the mouth can open and close. The name Syngnathidae means 'together jaw' and refers to the fused jaws that all members have.

(Image: a seahorse skeleton showing off the ring-like structure of the skeletal system. It looks like a wireframe model of a seahorse. A dried seahorse is next to it. End ID).

Seahorses live in temperate and tropical marine waters and are found worldwide, which is impressive for some of the worst swimmers in the world. They typically live in shallow water with plenty of structures such as seaweed beds, seagrass beds, coral reefs, and mangrove roots. As mentioned above, they are some of the worst swimmers in the world with only one fin. The slowest swimming fish is the dwarf seahorse (Hippocampus zosterae), which has a top speed of 1.5 meters (5 ft) per hour. Swimming is also very energy intensive for them, further reducing their ability to swim. Seahorses are one of only two types of fish who swim in a vertical position, the other being the (unrelated) razorfish Aeoliscus strigatus. Still a more realistic method of movement than those horse myths. A giant deer thing that lives to run but has fragile toothpick legs and will die if it breaks one? That's just silly. Instead of swimming, seahorses prefer to use their prehensile tails to grab onto bits of plants or corals. Seahorses have been found in the open ocean clinging to floating bits of debris. It is speculated that riding the currents like this is how they became so widely dispersed. Animals riding floating vegetation to new habitats like this is called a rafting event and they are very cool.

(Image: a hedgehog seahorse (Hippocampus spinosissimus) clinging to a coral branch. It is mostly pale tan, but with dark blotches. End ID)

You wouldn't expect an animal that can barely swim to be a voracious predator, and yet seahorses have one of if not the highest hunt to kill success rations of all animals. Naturally, seahorses aren't exactly running down their prey and fighting it into submission. They have a very different strategy. Seahorses prey primarily on planktonic crustaceans like copepods and mysids, but will also eat other invertebrates and larval fish. They hunt by using their mouths to create suction, pulling the prey in. This is a common strategy amongst fish, but the seahorses put their own spin on things. Being extremely slow actually helps the seahorses in hunting, as they can gradually move into clouds of prey without being noticed. In addition, the hydrodynamic shape of their heads lets them get in without disturbing much water, which helps keep the prey from being scared off. The long, tubular snout also helps the seahorse get in close. Once in position, the head snaps upwards and the mouth opens, creating suction that draws prey into the mouth. The flicking of the head has been shown to let seahorses catch more prey than pipefish, which otherwise use the same feeding method. The suction isn't very strong, which requires the seahorse to get its mouth very close to the target. This hunting method has a 90% success rate. In comparison, wolves, one of the animals most effective for being expert hunters, have a roughly 20% success rate (though this heavily depends on what type of prey they hunt). While seahorses are extremely successful at catching prey, they aren't as good at digesting what they catch. In fact, they don't even have stomachs, just esophagus straight to intestine. Their digestion is so inefficient that seahorses need to eat near constantly to avoid starving to death. It may be hard to understand how such an animal could evolve, but remember that these are animals with a very sedentary lifestyle that doesn't consume much energy. Evolution doesn't make organisms perfect, it makes them good enough to survive or they die. Seahorses have found a niche that they can effective survive in and did it by evolving into low-energy pursuit predators. As weird as a slow-swimming, extremely effective predator that barely digests its food and needs to constantly eat sounds, it is good enough to keep them from going extinct and thus they exist. Unlike horses.

(Image: a group of 11 big-belly seahorses. They are mostly yellow, but with black spots. The abdomens of several of them are notably distended. End ID)

Because seahorses don't sound enough like the invention of a drunk speculative biology writer yet, we need to talk about their reproduction. Seahorses are famous because the males are the ones who get pregnant. Sort of. Seahorses reproduce seasonally and, in many species, a male and female will form a pair bond that lasts for the duration of the season, mating only with each other. It is often said that seahorses mate for life, but this is not the case. Some species are less monogamous and will mate with multiple partners each season while two, the big belly seahorse (Hippocampus abdominalis) and the knobby seahorse (Hippocampus breviceps) mate in groups with no pair bonds. In most species, pairs will go through an extensive courtship prior to mating. While there is some variance for species, courtship usually follows 4 steps.

Step 1: about half an hour after dawn, potential mates approach each other. The male will brighten his colors and begin quivering rapidly while turning toward her. The female will then brighten herself and quiver while turning away from him. They hold on to the same object while doing this and will gradually circle it. This step is usually repeated for at least 2 days before moving on to the next step.

Step 2: similar to step 1, but the roles reverse, with the female leaning toward the male and him leaning away. This can be repeated for up to an hour, after which they will take a break that can last several hours before the next step.

Step 3: the female brightens and assumes a pointing posture. The male follows suit, then leaves. This step is repeated several times before moving on.

Step 4: after completing step 3, instead of the male leaving, the pair will let go of the object they're holding and float upwards. They will mate and then sink back down. This is repeated up to 8 times in succession.

(image: a pair of seahorses mating. They are floating in the water with their bellies touching. Both have notably distended abdomens. End ID)

During mating, the two face each other and press their bellies together. Male seahorses have a brood pouch on the abdomen that they open and flood with sea water. The female has an ovipositor (a tube that eggs are laid through) that she inserts into the brood pouch to release her eggs. The male fertilizes them with his sperm. Flooding the brood pouch with seawater is believed to help the sperm fertilize the eggs. The number of eggs can range from 5 to 2,500 based on the size of the species. The eggs become embedded in the wall of the brood pouch and are held in place by spongy tissue. The brood pouch acts like an incubator where the father can regulate the water within, controlling salt and oxygen levels, providing immunological support, and releasing lipids and calcium that the eggs take up to help the embryos develop. The brood pouch is initially kept at a low-salinity standard, but as hatching approaches, the salt level will increase until it matches seawater. The eggs hatch internally and the father will use muscular contractions to force the newborns out of the brood pouch. Newborn seahorses receive no parental care and only about 0.5% of them will survive to adulthood. The male usually releases the larvae at night and will be ready to mate again by morning. Seahorses mate multiple times per season.

(GIF: a seahorse giving birth. It is repeatedly flexing its body, forcing tiny seahorses out of a hole in its belly. End ID).

So why and how did all of this evolve? The origins of male pregnancy are uncertain, though they almost certainly predate seahorses as pipefish and sea dragons do the same thing. The most prominent hypothesis is that the male taking over parental duties improves reproductive success by taking the burden off the female and giving the offspring a head start. The male incubating the eggs provides them more shelter than if they were exposed to the environment and helps ensure that the maximum number of eggs develop properly thanks to the environmental controls and provided nutrients the brood pouch offers. All this further begs the question of why the mother isn't the one with the brood pouch incubating the eggs like in mammals and (ovo)viviparous fish. The answer may come down to energy costs. Both the male and female expend considerable energy in mating and rearing, but tests have shown that the energy cost of the female producing the eggs is higher than the male incubating them. If the female had to do both roles, she likely wouldn't have enough energy to get the job done. Therefore, the male takes over and shares the burden to maximize reproductive success. This is likely the origin of the extensive courtship as well. When reproduction is so costly to both parties, they both have an incentive to vet the other and make sure their investment won't go to waste. The courtship process likely allows each partner to examine the other's level of fitness while demonstrating their own to make sure their partner will likely be successful.

(Image: a group of juvenile seahorses swimming in a glass jar. They are extremely small and dark brown. nd ID)

Seahorses are highly specialized animals, and specialization comes with a cost. Specialized species are very good at surviving stable environments and very bad at adapting to cope with change. Unfortunately, the ocean is changing rapidly due to human activity and seahorse numbers are declining. A lot of the decline is due to habitat loss as seagrass beds and coral reefs are being hit hard by climate change. Seahorses are also susceptible to bycatch and are fished for the aquarium trade, food, and traditional Chinese medicine. TCM often claims that dried seahorses can be used to treat pain, wheezing, bedwetting, and impotence as well as induce labor. These claims have either not been scientifically tested or lack supporting evidence. It is estimates that up to 20 million seahorses a year are caught for use in TCM. Unfortunately, seahorses are understudied due to their elusive nature so it is hard to tell what kind of conservation needs they have. Unless we act to protect them seahorses could one day be just as nonexistent as the horse.

(Image: a short-snouted seahorse (Hippocampus hippocampus) clinging to some kelp. It is a small, brown seahorse. End ID)

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