Fish in focus - The West Australian Seahorse

Glenn Moore's blog | Created 2 years ago

Continuing our series of endemic fish species, here is a very special fish that is endemic to the west coast of Western Australia.

The West Australian Seahorse: Hippocampus subelongatus  Castelnau, 1873

A live West Australian Seahorse

West Australian Seahorse
Image copyright WA Museum

Seahorses have long been part of children’s fantasy alongside mermaids and other mythical sea creatures. However, seahorses are not mythical at all and what’s more they are fish - true fish, just like a herring, or a goldfish. They swim with fins, they ‘breathe’ using gills, they control buoyancy with a swimbladder. There’s nothing different in that regard but they are very special fish of course. Although seahorses have scales, they have been modified into bony plates. There are two aspects of seahorses that makes them ‘unfish-like’.  Firstly, they don’t have a tail fin - instead they’ve evolved a prehensile tail, just like a monkey or a possum, and they use it to hold on. Secondly, seahorses have a bent neck and swim vertically, using their tiny dorsal fin for propulsion. Seahorses hold a very special place in my life and I have been studying West Australian Seahorses for more than 17 years.

The West Australian Seahorse is a very special fish for us because it is an endemic - found only on the west coast, from Cape Leeuwin to Shark Bay. Especially during the summer breeding season, this species loves muddy, silty habitats – places like the Swan River Estuary and Cockburn Sound. It usually lives in water less than 20m deep and can be found holding onto sponges, sea-squirts and seaweed.

The West Australian Seahorse is quite a large species on a world scale. They get to about 25cm and weigh more than 10 grams, perhaps even a little more than that when they’re pregnant. They come in brown, white, yellow, orange, red and even purple, but they can change colour too. They can’t swim fast to catch prey or escape predators, so they rely on a mastery of camouflage.

A female specimen of the West Australian Seahorse

Female West Australian Seahorse
Image copyright WA Museum

A preserved male specimen of the West Australian Seahorse

Male West Australian Seahorse
Image copyright WA Museum

One of the most fascinating things about seahorses is that the males get pregnant! So, then, if the male gets pregnant, why is it a male? It’s a very simple answer. What makes a male a male is that he has sperm and what makes a female a female is that she has the eggs.

Like most seahorses, West Australian Seahorses are monogamous, repeatedly mating with the same partner (we use genetics to determine the parents of the babies). We also know that this species mates size-assortatively (that is, large males and females pair up and small males and females pair up). The pair goes through a series of courtship rituals for several days before mating. This involves synchronised swimming, colour changes and other ritualised behaviour. After this elaborate courtship, the female inserts her ovipositor into the male’s brood pouch (on his tail) and deposits a clutch of eggs (usually 200 - 600). The male fertilises the eggs inside his pouch and incorporates them into the lining, forming a pseudo-placenta. The developing embryos are nutured and oxygenated for 2-3 weeks, after which the male goes through labour. After birth, the young seahorses fend for themselves.

A carapice measuring the head of a West Australian Seahorse

Measuring a West Australian Seahorse
Image copyright WA Museum

West Australian Seahorses are protected. They are listed under CITES Appendix II (International trade agreement) and listed under IUCN as being data deficient. We just don’t know enough about these wonderful fish, so this protection is in place to be cautionary. No doubt their habitat is threatened by development and pollution and many are taken for aquaria, but as always, much more research is needed.