Advancing marine conservation
We base our decisions on species names and identities accepted by the IUCN SSC Seahorse, Pipefish and Seadragon Specialist Group, which we host and which includes experts on taxonomy, phylogeny and nomenclature; we contribute to that expertise.
The taxonomy of seahorses has gone through a great deal of upheaval with as many as 140 names in use at various times. However, Project Seahorse formally revised the taxonomy in 2016 and settled on 41 genuine species. Since then, colleagues have added five more species that we consider genuine, based on genetic and morphological differentiation, bringing the total to 46 species. Other species have been proposed as well but do not, thus far, hold up under full analytical scrutiny. Without doubt, though, more species will emerge, especially among the pygmy seahorses.
Of the new species in the past five years, three are pygmy seahorses, the tiny species with adults less than 24 mm in length, which now number eight species in total. Given that the most recent pygmy seahorse species was found on the other side of the Indian Ocean (South Africa), 8000 km west of previously known pygmy species (Indonesia),1 more will almost certainly be discovered.
Number of seahorse species recognised to date
Range in adult size (mm) across seahorse species
Seahorses range in adult size from about 14 mm to 350 mm (0.5 to 14 inches) when fully extended. Species differ from one another in relative proportions, shape of the head and body, location and number of head spines, and fin rays. Colour and skin filaments are not reliable identification characteristics for species; they change over time for any individual and vary greatly even within a population.
In developing identification guides, Project Seahorse emphasises taxonomic distinctions that can be detected by an interested non-specialist. Some species look very similar to each other to the casual eye yet must be distinguished for fisheries and trade management. We favour approaches and methods that Customs officers and other professionals can use reliably when quickly assessing the very large numbers of individuals they may need to check, especially in trade of dried seahorses.
[Updated 17 May 2021]