Thorny Seahorse

Meet a fish that looks like a chess piece and is armed like a medieval mace. The thorny seahorse (Hippocampus histrix) carries spines that Museums Victoria describes as very elongate and extremely sharp, often tipped in black. Five more spines crown the top of its head.

It is not big. Its height tops out at 150 mm, about 6 inches, and seahorse height is measured from the tail tip to the top of the crown. Wild lifespans are an open question: Project Seahorse says natural seahorse lifespans are virtually unknown, with most estimates coming from captivity. Known lifespans across the group run from about one year in the smallest species to an average of three to five years in the larger ones. That is the group’s figure, not this species’.

Dad does the pregnancy The female uses an ovipositor, an egg-laying tube, to place her eggs in a pouch under the male’s belly. The male fertilizes them, aerates them and regulates their salt balance. He may also give them some nourishment until they are born.

Project Seahorse writes that seahorse pregnancy lasts about two weeks to one month, and most males give birth to around 100 to 200 babies. Newborns of most species measure between 7 and 12 mm. These are group figures, not measurements of this species.

Suction dining without a stomach

Seahorses have no stomach and no teeth. They suck prey through a tubular snout and pass it through an inefficient digestive system. For the thorny seahorse specifically, Fishes of Australia says it presumably preys on small crustaceans and other planktonic organisms.

Spines, camouflage and a muddled name

Its base colour is highly variable to match its surroundings, from cream to grey, greenish, yellow or brown to burgundy-red. Seahorses in general are more adapted to maneuvering than to speed, and therefore rely on camouflage to avoid predators more than on speed to escape them.

It lives on both hard and soft bottoms, often attached to soft corals or sponges at 10 to 95 m, usually below 15 m. The range covers the tropical and subtropical Western Pacific, including Bali, Papua New Guinea, New Caledonia and Japan. In Australia it has been recorded south of Ashmore Reef in the Timor Sea, on the Great Barrier Reef and at Port Stephens in New South Wales.

The name has caused confusion. Museums Victoria notes that H. histrix has often been used for many spiny seahorses, making them look widespread when they have relatively restricted ranges. The IUCN rates the species Vulnerable. It is rarely collected for the aquarium trade due to its preferred habitat in moderately deeper waters, but specimens may be dried and sold in the traditional medicine trade or as curios.