Denise's pygmy seahorse (Hippocampus denise) is a highly variable species, which like its close relative Bargibant's pygmy seahorse, is a habitat specialist living in an obligate relationship with gorgonian corals. They were scientifically described by Lourie & Randall in 2003. Unlike Bargibant's, Denise's pygmy seahorse is a relative generalist is terms of the number of host species it inhabits. During my PhD study on these species biology I recorded H. denise from a total of eight gorgonian genera (Acanthogorgia sp, Annella sp, Echinogorgia sp, Ellisella sp, Melithaea sp, Muricella sp, Verrucella sp, Villogorgia sp), compared to only one for H. bargibanti. Once settled on a gorgonian following some time floating in the plankton as a juvenile, the small seahorse settles, at around 1.3 cm, to a host where it remains for the rest of its life. Once fully-grown they reach a maximum length of 2.4 cm.
During my PhD I was lucky enough to observe Denise's pygmy seahorse mating, fighting and even giving birth. I found the gestation period to be 11-12 days when the male would give birth to between 6 and 16 young. The young settle to a host and over a few days take on its exact colouration. This results in a wide range of colours and skin textures in this species with some individuals being smooth and others bumpy, depending on their host's polyp prominence.
The habitat specificity for the gorgonian-associated pygmy seahorses, H. denise and H. bargibanti, puts them at a higher risk of extinction than non-specialist species. Gorgonians can live for 100 years but can easily be damaged by natural phenomena such as storms, but also human induced threats such as diver damage, anchor damage, destructive fishing practices and even climate change. Denise's pygmy seahorse is found living on the lush coral reefs of southeast Asia, from Borneo to New Guinea, Solomon Islands and Palau. This region is also sadly under the greatest threat from human damage. The best method for protecting these tiny fish is within Marine Protected Areas (MPAs) and by careful regulation of diver interactions so the seahorses are not accidentally damaged by interested divers.
Follow this link to more facts about the diet, habitat and diversity of pygmy seahorses.