Strange new seahorse has no dorsal fin

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A unique new species of seahorse, Hippocampus paradoxus, has been described from Australia. It differs from all other known seahorses by having no dorsal fin. Instead it has a series of fleshy, fin-like lobes along the dorsal midline of the body and tail.

Only one specimen, a female, is known. The fish was collected as long ago as 1995, by scientists researching bryozoans, and had lain in a museum collection ever since, until discovered recently by seahorse specialists examining the collection.  

The specimen was collected to the south west of Esperance in Western Australia, on the extreme western edge of the Great Australian Bight. 

Because it was collected during scientific research, some details are available of the site:  a coarse substrate of calcareous sand populated by a varied mix of bryozoans, including Adeona, a number of bushy flexible species (Family Catenicellidae), "lace corals" (Phidoloporidae), and a number of sponges.  The site lies on the midcontinental shelf at a depth of 102 metres, and is one of numerous "islands" of sponges and bryozoans found on the expanses of rippled sand created by wave action.

Interestingly, the putative closest relative of the new species, Hippocampus minotaur, is likewise a small, mid-shelf species from comparable depths but in south-eastern Australia, more than 2000 km away at the opposite end of the Great Australian Bight.

The two share features previously considered unique to H. minotaur and are thought to be sister species, but there are clear differences, most obviously the lack of a dorsal fin and the strange fleshy lobes in H. paradoxus. The area between their known distributions has not been well sampled so it is unknown how far the range of each species extends or whether there are other, similar species in between.

Although the two species are tiny, they appear not to be closely related to the “pygmy seahorses” (eg. H. bargibanti) of the Indo-West Pacific. The true pygmy seahorses are "abdominal brooders" that lack a typical seahorse caudal pouch and brood their eggs and young within the abdomen, and the group is thought to have split off from the typical seahorses at an early stage. 

However, in some respects H. minotaur and H. paradox have affinities with both groups.  For example, H. minotaur has a brood pouch that is technically caudal but very close to the abdomen, and may be an intermediate state. Unfortunately the single specimen of H. paradoxus is a female and hence throws no light on the brooding habits of the species.

The scientific name paradoxus and the suggested common name Paradoxical Seahorse refer to these and other puzzling questions the new species raises.

For further information see the paper: Foster, R. & M. F. Gomon (2010) A new seahorse (Teleostei: Syngnathidae: Hippocampus) from south-western Australia.  Zootaxa 2613: 61–68.