Thorny Seahorse, Hippocampus histrix Kaup 1856

Recorded in Australia from south of Ashmore Reef, Timor Sea, the Great Barrier Reef, Queensland, and in Port Stephens, New South Wales. Elsewhere the species occurs in the tropical and subtropical Western Pacific, from Bali, Indonesia, Papua New Guinea, New Caledonia and Japan. 

Inhabits areas with both hard and soft bottoms, often attached to soft corals or sponges at 10-95 m, usually below 15 m. The Thorny Seahorse may also be found on shallower algae-rubble or rocky reef areas in about 10 m.

Meristic features: Dorsal fin 17-19; Anal fin 4; Pectoral fin 17-18; trunk rings 11; tail rings 33-34; subdorsal rings 2 + 1. 

Head long, about equal to trunk length, angled down to near trunk; snout long, equal to or longer than head length; body narrow; dorsal fin short-based.

Spines very elongate, extremely sharp; nape spine immediately before coronet, elongate and directed upwards, slightly longer than spine above eye; lower shoulder-ring spine elongate, single, directed laterally and slightly forward; spine above eye about equal to eye diameter; nose spine slightly longer than spine above eye; cheek spine single; 2 sharp neck spines, similar to coronet spines in length; trunk ridge spines on nearly all trunk and tail junctions, only absent on 1st ring of lateral ridge, starting on 4^th; subdorsal spines 3/0,1,0.

Coronet of moderate height, large, with 5 elongate divergent spines on apex.

Base colour highly variable to match surroundings, from cream to grey, greenish, yellow or brown to burgundy-red; body plain or with saddle-like markings, often with small darker spots and blotches on sides and dorsal surface; small areas with tiny reddish spots on nape and dorsal surface; snout usually with one to several fine white bars; tips of spines often black.

Like other seahorses, this species presumably preys on small crustaceans and other planktonic organisms.

The female uses an ovipositor to transfer her eggs into an elaborate enclosed pouch under the abdomen of the male. The male not only fertilizes the eggs inside the pouch and provides physical protection for the developing embryos, he also osmoregulates and aerates the embryos and may provide some nourishment until the offfspring are born.

Rarely collected for the aquarium trade due to its preferred habitat in moderately deeper waters. Specimens may be dried and sold in the traditional medicine trade or as curios.

Listed under Appendix II of the Convention on the International Trade in Endangered Species of Wild Flora and Fauna (CITES). As a result, the species is subject to the Convention at http://www.cites.org/. H. histrix is listed as Data Deficient by IUCN.

The name H. histrix has often been used for many spiny seahorses inhabiting tropical regions, giving the impression that these spiny species are widespread rather than having relatively restricted distributions.

H. histrix is most often confused with H. multispinus, from which it differs in having a single long shoulder-ring spine rather than the long double lower shoulder-ring spines of H. multispinus that diverge from one another in a V-shape. In addition, the nape spine is close to the coronet and is directed upwards in H. histrix, whereas it is removed from the coronet and directed forwards in H. multispinus.

Hippocampus histrix Kaup 1856, Cat. Lophobranchiate Fish: 17, Pl. 2 (fig. 5), Japan.

Bray, D.J. 2020