CT scans reveal structure of the Paradoxical Seahorse

Paradoxical Seahorse, _Hippocampus paradoxus_.  Image: Ralph Foster (image used with permission of Magnolia Press).

Paradoxical Seahorse: the holotype of Hippocampus paradoxus (SAMA F10490).

The Paradoxical Seahorse, Hippocampus paradoxus, is a recently described species from the Great Australian Bight (Foster and Gomon, 2010). Known only from a single specimen collected in 1995, this small species breaks the planning rules of seahorse anatomy. It is the only known seahorse without a dorsal fin, instead having a series of fleshy lobes down the back and tail. It also lacks the rigid external bony plates that characterise most seahorses. The numbers of plates ringing the body and tail of a seahorse are key taxonomic (identification) features but were very difficult to determine in the specimen because of the fleshy dermis. To visualise the remnants of the rings and other features hidden beneath the skin, South Australian Museum researchers turned to micro-CT scanning. The technique proved to be remarkably useful allowing detailed, non-destructive examination of not only the skeleton but some soft tissue features, as well, such as unlaid eggs in the abdomen. Some of the CT scan images used to scientifically describe Hippocampus paradoxus are presented here, along with CT scans of two other seahorse species, H. breviceps and H. denise, for comparison.

 

Reference

Foster, R. and Gomon, M. F. (2010). A new seahorse (Teleotei: Syngnathidae: Hippocampus from south-western Australia. Zootaxa 2613: 61–68.

 

CT scan 2 Paradoxical Seahorse

CT scan of holotype of Hippocampus paradoxus (SAMA F10490): left ventrolateral view of last trunk vertebra (V10) with associated scaffold-like elements of 8th trunk ring and first caudal vertebra (V11) showing haemal spine.

 

Hippocampus paradoxus from South Australian Museum on Vimeo.

Paradoxical seahorseCT scan of holotype of Hippocampus paradoxus (SAMA F10490): lateral view of head and trunk.

 

Paradoxical seahorseCT scan of holotype of Hippocampus paradoxus (SAMA F10490): reduced transparency view revealing eggs in abdomen.

 

CT scan Paradoxical seahorseA second reduced transparency CT scan of holotype of Hippocampus paradoxus (SAMA F10490) revealing eggs in abdomen.

 

Shorthead SeahorseCT scan of a Shorthead Seahorse, Hippocampus breviceps, specimen. This small species is commonly found in shallow bays in SE Australia. In contrast to H. paradoxus, the characteristic seahorse bony armour is robust and encloses the entire body.

 

Denise's Pygmy SeahorseCT scan of Denise’s Pygmy Seahorse, Hippocampus denise.This tiny species, restricted to tropical reefs, lives on Gorgonian octocorals. Although common in regions to the north of Australia, this recently collected specimen (SAMA F13806) is the first one ever found in Australian waters. This specimen is a subadult male with a Standard Length of only 14 mm. Like H. paradoxus, this species has reduced ossification of the bony plates, particularly of the abdomen.