Hippocampus paradoxus, Foster, Ralph & Gomon, Martin F., 2010

Foster, Ralph & Gomon, Martin F., 2010, A new seahorse (Teleostei: Syngnathidae: Hippocampus) from south-western Australia, Zootaxa 2613, pp. 61-68 : 62-66

Hippocampus paradoxus sp. nov.

Paradoxical Seahorse

Figs 1, 2 A, 3

Hippocampus sp D: Kuiter, 2009:141, 2 figs.

Holotype. SAMA F10490, female, SW of Esperance, Western Australia (approximately 34o 29’ S, 121o 32’ E), Station GAB 108, depth 102 m, epibenthic sled, RV Franklin, collected by S. Hageman, P. Bock & Y. Bone, 26 July 1995.

Diagnosis. A species of Hippocampus having: no dorsal fin, a series of fleshy, fin-like lobes along dorsal midline of trunk and tail, 8 trunk rings, 11 pectoral fin rays, extremely robust cleithrum, and prominent first nuchal plate.

Description of holotype. Counts: DF –; PF 11; AF 2; TrR 8; TaR 41.

Measurements: SL 64.6; HL 9.8 (15.2% SL); TrL 14.8 (22.9% SL); TaL 40.0 (61.9% SL); HD 8.4 (85.7% HL); SnL 2.5 (25.5% HL); OD 1.9 (19.4% HL); PO 5.3 (54.1% HL); CH 4.9 (50.0% HL); SnD 1.9 (76.0% SnL); PL 1.2. (1.9% SL).

Head, trunk and tail very compressed, extremely fleshy without obvious bony segments; abdomen, situated between TrR6 and TrR8, broad and protuberant; dorsal surface densely covered with microscopic papillae, papillae also ventrally in some areas, particularly tail, but much less dense; several minute papilliform cirri, visible on expanded supraoccipital.

Head prominent (HL 15.2% SL), axis approximately 90o to trunk axis; cleithrum particularly well developed and robust making head bulbous posteriorly; head deep, (HD 85.7% HL) rising steeply from short snout (SnL 25.5% HL) to a prominent supraoccipital; first nuchal plate also prominent dorsally, of similar height to supraoccipital, relative to horizontal axis of head and much higher than second nuchal plate; coronet only evident in radiograph as diffuse ossification, lying between supraoccipital and first nuchal plate and not in contact with supraoccipital; fleshy ridge-like crest extending posteriorly from supraoccipital, between gill openings, and over first nuchal plate; crest midway between supraoccipital and cleithrum—corresponding to position of coronet—the highest point on head ( CH 50.0% HL); gill openings narrowly separated at top of head just anterior to cleithrum; cleithrum prominent laterally and dorsally, reaching to top of first nuchal plate; no cleithral ring evident on dermis.

Orbit surrounded by 9 conical papillae; additional conical papillae on head visible with magnification, some corresponding in position to head spines of species that have them; largest, a broad based ‘nose spine’ just anterior to eyes with smaller papillae on snout and forehead, above eyes, on supraoccipital, on operculum, around gill openings and overlying second nuchal plate; very low circular mound-like tubercle, approximately 1.5mm in diameter, high on head above operculum; blunt fleshy ‘neck spine’, adorned with short cirri, at intersection of superior trunk ridge with TrR1.

Trunk and tail rings barely perceptible but discernable with transillumination; no trunk ridges clearly evident externally, though inferior trunk ridge discernible through dermis; position of superior trunk ridge inferable from cross sectional body shape; caudal segments quadrangular but tail ridges poorly defined.

Dorsal midline of trunk and tail with medially aligned dermal outgrowths (lobes) that superficially resemble fins: 2 on trunk, first on anteriormost 3 trunk rings low, second, covering TrR7 and anterior half of TrR8, resembling dorsal fin but very fleshy and without rays; remaining 10 lobes on tail, more delicate and membranous; base of first on tail covering TaR3, TaR4 and much of TaR5; anterior tail lobes fairly regularly spaced, each spanning about 2–2.5 rings and separated by about 1–1.5 rings; spacing less regular posteriorly, with lobes becoming smaller and less distinctively shaped, posteriormost reduced to small tubercle; second tail lobe considerably smaller than first and third; pairs of small, fleshy, flange-like, lateral outgrowths adjacent to inferior tail ridges ventral to the second and third tail lobes.

Urinogenital opening raised and surrounded radially by pleated skin folds.

Anal fin tiny, fleshy, both rays branching from base; number confirmed by CT scan that shows two pterygiophores at base.

Osteology. CT scan reveals mostly complete, but very light, ossification dorsally on body; superior and lateral trunk ridges clearly discernable but lateral trunk ridge lost with ossification decreasing just prior to tail; ossification ventral to lateral trunk ridge reduced with trunk rings not fused to neighbours. TrR1 completely encircling ‘neck’ and fused to cleithrum; remaining thoracic rings (TrR2 – TrR6) almost encircling trunk but unfused ventrally and ventral trunk ridge absent; TrR7 similar to anterior rings but because of expanded abdomen not encircling body; ossified ventral elements associated with last trunk ring (TrR8) scaffold-like, somewhat extended and strongly angled posteriorly, following contour of posterior surface of abdomen.

Segmented inferior trunk ridge remnants, originating from TrR6 ossified elements, follow contour of abdomen converging ventro-posteriorly, to run almost parallel adjacent to ventral midline, before diverging after TrR7 and recurving dorso-posteriorly and terminating just anterior to anus, approximately level with origin of tail (see fig 3); cross-shaped ridges on segments the only remnants of intersection with trunk rings posterior to TrR6.

Eleventh vertebra considered to be first caudal vertebra as it is first vertebra with haemal spine; associated 9th ring therefore regarded as first caudal ring; remaining caudal segments totally enclosed by bone, quadrangular.

Based on CT scan, ossified structures at base of second dermal lobe mid-dorsally on trunk (visible in radiograph and initially interpreted as remnants of dorsal fin pterygiophores) appear to be bony outgrowths of superior trunk ridges.

From late 2010, additional CT scan images of H. paradoxus can be viewed at www.samuseum.sa.gov.au/ ichthyology/research.

Colouration. Colour in life unknown. Colour in preservative yellow-cream, peppered with tiny brown dots; nuchal tubercles densely dotted; series of faint brown spots, some with pale centres, on dorsal midline between TrR3 and TrR6.

Reproduction. The holotype is a female with a greatly distended abdomen holding eleven hydrated oocytes that are each approximately 2 mm in diameter. It was collected in late July. Although male specimens are unavailable, the similarity of this species with H. minotaur would support a hypothesis that the pouch is caudal in position.

Habitat. The holotype was collected from the midcontinental shelf benthos (102 m) by researchers targeting bryozoans. High energy waves sweep the sea bed of the Great Australian Bight at this depth producing expanses of rippled sand interspersed with sponge and bryozoan stabilised ‘islands’ ( James et al., 2001). The collection site featured a complex, highly diverse assemblage of bryozoans, including Adeona , a number of bushy flexible species (Fam. Catenicellidae ), "lace corals" (Fam. Phidoloporidae ) and some sponges, on a coarse substrate of calcareous sand (Bock pers. comm.). Seafloor photographs in James et al. (2001:561, fig.9) illustrate this habitat type. Water temperatures in the western GAB are moderated by the warm Leeuwin Current during winter. At the time of collection, in late July 1995, the Leeuwin Current was distinct offshore and waters were isothermal ( James et al., 2001).

Distribution. Known only from the type locality south west of Esperance, Western Australia, on the extreme western margin of the Great Australian Bight (GAB). There are few records of Hippocampus from the more than 2500 km of high energy coastline, stretching from the extreme south west of Western Australia eastward across the GAB to the tip of Eyre Peninsular in South Australia, but the large mid-continental shelf regions have not been extensively collected.

Comparison with similar species. The only species with which H. paradoxus is likely to be confused is H. minotaur . The two are of similar size and body form and share TrR, TaR, and PF counts. All known specimens of H. minotaur have a dorsal fin with 7 or 9 rays ( Lourie & Kuiter, 2008) and a raised base on the ultimate trunk ring and first tail ring, whereas H. paradoxus entirely lacks a dorsal fin and has no indication of a raised base at this position. The cleithrum of H. paradoxus is much more robust and the first nuchal plate is more dorsally prominent with a height about the same as the supraoccipital, relative to the horizontal axis of the head. In H. minotaur the supraoccipital (‘coronet’ of Gomon, 1997) is tallest and the head lacks the fleshy crest and tapers into a much deeper anterior trunk. Hippocampus minotaur appears to have lost the thoracic rings ventrally whilst they are retained, in part at least, in H. paradoxus . The former also lacks the circumocular papillae of H. paradoxus .

Although H. minotaur does not feature the dramatic dorsal appendages of H. paradoxus , some specimens have mid-dorsal tubercular swellings that may be homologous. These swellings are particularly evident on the paratypes (see Gomon 1997, figs 1b & 3a, Kuiter 2009, p.141). They are situated along the body and tail in nearly the same positions and are very similar in appearance to the distal tubercular swellings on the tail of H. paradoxus . Radiography revealed other similarities. The female H. minotaur paratype (AMS IA.3509) has ossified structures on the dorsum similar to the bony prominences of the superior trunk ridges evident in the CT scan of H. paradoxus . More significantly, the extended TrR8 ossified elements are present and there are indications of inferior trunk ridge remnants girdling the anterior of the abdomen (see fig 2).

Etymology. Latin masc. adj. paradoxus , ‘strange, contrary to all expectation’, in reference to the unusual morphology relative to all other seahorses.

Figures

FIGURE 2. Radiographs of Hippocampus paradoxus holotype (A) and H. minotaur paratype AMS IA. 3509 (B).