ALONE OF THE FISHES, THE MALES BEAR THE YOUNG. WHEN COURTING SEASON COMES ROUND, MATING PAIRS GET TOGETHER AND PERFORM A DAILY COURTSHIP DANCE, ENTWINING WITH ONE ANOTHER AND SWIMMING SIDE BY SIDE.
A seahorse is a long way from the classic idea of a fish, those streamlined racing shapes seen flashing past in schools of tuna or wobbling lopsidedly under the waves of children's drawings. Seahorses seem to have developed with a very different game plan in mind.
Their heads look like horses' heads with crowns on top. Their eyes can swivel independently just like those of chameleons. Their monkey-like tails are prehensile and they don't even have scales: their bodies are encased in skin which covers a series of rings of bony armour. As for fins, they have tiny frivolous-looking efforts which peek out from their backs and behind their eyes. They're not great at swimming and have the distinction of one of their number, the dwarf seahorse (Hippocampus zosterae), being the slowest-moving known fish with a top speed of 1.5m per hour. Seahorse racing probably wouldn't make a great spectator sport. However, the Ancient Greeks thought that seahorses pulled Poseidon's chariot -- maybe seagods don't need speed. Or maybe something magical happened once they started to pull.
As indeed seahorses, at even a slightly closer look, are magical. They are the only fish to have a neck, which means they can remain stationary to look around, probably a good thing given their lack of swimming skills. They and the razorfish are the only fish to swim vertically, though razorfish swim with their heads down.
Seahorses use their prehensile tails to cling to a support -- depending on their environment that could be a seafan, sea grass or a convenient coral, and will change their colour to blend in to their environment. Some of them have spines or fronds of skin which help them match their surroundings and the smallest members of the group, the pygmy seahorses, did such a great job of this that the first species (Hippocampus bargibanti) was only discovered by accident in a collection of gorgonian seafans in 1970. They are close to the smallest vertebrates known (current title being held by a 7mm long frog from Papua New Guinea), the smallest one being a minuscule 14mm in total length.
They are hungry creatures despite their slow-motion lifestyle and feed continuously, using their long snouts like vacuum nozzles to suck up small shrimps.
Their adult life has enduring fascination. Alone of the fishes, the males bear the young. When courting season comes round, mating pairs get together and perform a daily courtship dance, entwining with one another and swimming side by side. This, rather unromantically, is done to determine mating readiness of each partner. Once ready, the real deal begins, and lasts around eight hours. The male opens his egg pouch to show readiness and the female deposits her eggs into the pouch. The male then fertilises them and the embryos develop in his pouch for 9-45 days depending on species. While there, the male supplies the eggs with a hormone, prolactin, identical to that produced by female mammals (yes, humans too) when lactating. He also provides oxygen; and as the brood grows, the fluid in the pouch changes to become more like sea water, until eventually, when they hatch, the baby seahorses are ready for life in the open sea.
Hatching sounds easy, but the male must perform a series of muscular contractions to get the babies out. There is typically no further care of the offspring which must seek their own fortune on the reefs. Survival rates are low: out of what may be a litter of thousands, less than one percent will survive to adulthood. Birth generally takes place at night so that, when the female next visits, the male is ready to receive the next batch of eggs.
Once a mating pair is established, and during the whole of the male's pregnancy, the female will visit daily and the pair will dance together. Again, and rather unromantically, this daily dance has a reproductive purpose. The female uses the dance to establish how close to readiness the male is for her to deposit the next batch of eggs. With such low survival rates, the male will remain pregnant for almost all of the breeding season. Before we all start thinking what a wonderful example the seahorse male is as a parent, let's just consider the fact that energy calculations suggest that the female loses 30% of her body mass with each batch of eggs she deposits in the male's pouch. The male's energy expenditure while incubating was shown to be less than this, which suggests that energetically, the female's burden is still heavier than the male's.
But how did the switch from female pregnancy to male pregnancy occur? That is a mystery known only to the seahorses and the pipefishes, their horizontal-swimming ancestors.
Unluckily for seahorses, though, they are thought to be good luck and to be useful in curing a variety of ailments. It is for this reason that seahorses hauled up as by-catch in shrimp trawls are kept aside and dried for sale to traditional Asian medicine markets. Much to the misfortune of these unique fish.
text: Georgina Jones
photo: Jenny Strömvoll / Georgina Jones
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