Wednesday, 14 October 2015

Getting hip to the Hippocampus

I feel like talking about seahorses today. I suppose we all have days like that.

We know what seahorses are, right? A genus of odd little bumpy fish that swim about in an upright rather than horizontal posture, have cute snouty faces and hold onto things with their prehensile tails. Their genus name is Hippocampus, which means 'sea monster horse'. Hippocampus is, incidentally, also the name of a seahorse-shaped structure in the vertebrate brain, which deals with memory.

Through work I've learned a lot of stuff about seahorses lately. It's fairly common knowledge that in seahorses the males 'give birth' to the young, but I always thought that meant that their pouch is just a safe storage area for the fertilised eggs to sit in until they hatch, rather like a waterproof bumbag for stashing your valuables while you scuba-dive.

But there's way more to it than that. The pouch is a complex structure, much more comparable to a mammalian uterus than to a bumbag. The eggs implant in it, and the wall of the pouch becomes thick, spongy and highly vascularised. Salinity within the pouch is regulated and adjusted through the pregnancy to suit the developing eggs. And when they hatch, after two to four weeks of gestation, the male goes through contractions that are stimulated by oxytocin, the same hormone that does the job in mammals, to expel the tiny little seahorses into the big bad underwater world (where most of them immediately get eaten by something, perhaps even by other seahorses, but that's fish for you.) Functionally, it's a proper pregnancy and it ends with a proper birth.

The male can't put his fins up for a nice long rest after squeezing out up to 1,500 perfect mini-hims, though. The whole process has to start again pretty much immediately. So closely and intimately linked are the breeding cycles of paired seahorses that the female has her next batch of eggs ready to go within hours of the male giving birth.

Because of this close synchrony, seahorses are usually monogamous, hardly the norm for fish. The pair bond may not last a lifetime but persists at least through the breeding season, and is reinforced every day by a short but sweet greeting display. Each morning the female goes to her mate and he lets go of his holdfast to swim with her. They become brighter in colour and twirl around together, sometimes taking hold of each other's tails. On the day after the male has given birth, the greeting ceremony is longer and ends with mating. When she turns up to see him, they do their usual thing and he shows her his empty pouch. The two then let go of whatever they were holding onto and drift upwards together, and the female puts her ovipositor into his pouch and gives him her next batch of eggs.

After the daily greeting, the female goes off to feed, leaving her partner behind (as he is pretty much permanently pregnant he doesn't wander as far as she does), but she'll be back the next day and they'll repeat the whole touching process.

There are about 45 species of seahorses in the world. And (here comes the inevitable bad news) virtually all of them are either threatened to some degree or are 'data deficient' by the IUCN's reckoning. Lots of them are caught deliberately to be dried out and sold as curios, but lots, lots more die as bycatch, especially from shrimp trawling (more than two-thirds of what's brought up by shrimp trawlers is non-target bycatch, and shrimps and seahorses live in the same sorts of habitats). In all, we take out some 40 million seahorses every year. And all that close pair-bonding described above means that when a seahorse loses its mate, it also loses months of potential breeding time because finding and bonding with a new mate takes time.

Have a look at iSeahorse and Project Seahorse for all manner of stuffs about seahorses and what people are doing and can do to help safeguard their future. And here's a sweet video of a seahorse pair doing their dance.