Pterotrachea hippocampus Phillipi 1836

Introduction

The body morphology of Pterotrachea hippocampus is elongate; similar to that in P. coronata but not as streamlined (see the P. coronata page). Instead, the proboscis is shorter and thicker, the visceral nucleus is shorter and tear-drop shaped, and the tail is shorter and less well developed. The eye shape is unique in the genus, with a narrowly- to broadly-triangular shape.  The trunk cutis anterior to the swimming fin is thickened ventrolaterally (forming a bib) and the tail terminates in two small, leaf-like lobes, as in P. coronata. Like the other species of Pterotrachea, a swimming fin sucker is present only in males. The geographical distribution is cosmopolitan in tropical to subtropical waters.

Brief Diagnosis

A species in the genus Pterotrachea with the following characteristics:

• Body elongate, comparable in form with P. coronata
  
• Proboscis similar to that in P. coronata, but shorter and thicker
• Trunk cutis anterior to swimming fin thickened as ventro-lateral folds, forming a bib
• Visceral nucleus tear-drop shaped, like that in P. scutata
• Eye shape, in dorsal view, narrowly to broadly trianguler

Characteristics

1. Body morphology
   1. Proboscis tubular and moderately long (see title illustration); relatively shorter and thicker than in P. coronata
      
   2. Trunk cutis anterior to swimming fin thickened ventro-laterally, forming a bib (see title illustration)
      
   3. Viewed dorsally, eyes narrowly (juveniles) to broadly (adults) triangular in shape. Change in shape due to lengthening of the retinal base (compare the three photographs below), and quantified by the ratio of the eye length to retinal base width that, in Hawaiian specimens, decreases from 1.6 in juveniles to 1.0 in adults (Seapy, 1985)
      
      

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      Figure. Eye shape in Pterotrachea hippocampus as a function of age. Left: eyes in a juvenile specimen, ca. 15-20 mm body length. Center: eyes in a young adult, ca. 30-35 mm body length. Right: eyes in a old adult, ca. 50-55 mm body length. ©

   4. Visceral nucleus tear-drop shaped, with a length to width ratio decreasing gradually with age and averaging, in Hawaiian specimens, 2.9 (Seapy, 1985), nearly identical with that for P. scutata (3.0), but differing dramatically with that for P. coronata (4.8)
      

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      Figure. Tear-drop (pyriform) visceral nucleus in Pterotrachea hippocampus.©

   5. Swimming fin sucker medial on the ventral margin of the fin
      

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      Figure. Ventral portion of the swimming fin and sucker in Pterotrachea hippocampus, viewed from the right side of the body with the ventral margin directed upward. ©

   6. Swimming fin closer to the visceral nucleus than the head (see title illustration), like in P. coronata
2. Shell present only in larvae and is cast off after metamorphosis
3. Larva unknown, but is probably represented by either larva 2 or 3 of Richter (1968); see Pterotrachea larvae
4. Radular morphology similar to that in the other species in the genus, with a median cusp that is conspicuously broader and longer than the lateral cusps
   
   

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   Figure. Scanning electron micrograph of central rachidian tooth (lower center) and inwardly-directed lateral teeth (upper) in Pterotrachea hippocampus. Photograph modified from Ricther and Seapy (1999, Fig. 4D). © G. Richter.

Taxonomic Comment

Until recently Pterotrachea minuta was recognized as a valid species that was closely related to P. hippocampus. However, this species is now regarded as a junior synonym of P. hippocampus (Seapy, 2000). In an earlier study of the Pterotrachea fauna of Hawaiian waters (Seapy, 1985), examination of a broad size range of specimens that were tentatively identified as P. hippocampus and P. minuta revealed a continuum of change in the shape of the eyes and the visceral nucleus (the two primary criteria used to distinguish the two species). Young individuals were shown to possess narrowly triangular eyes and longer visceral nuclei which, with growth, transitioned to broadly triangular eyes and shorter visceral nuclei in older adults. The conclusion of the study was that there was no justification for recognizing P. minuta as a valid species; at least in the North Pacific Ocean. The subsequent (Seapy, 2000) study was a detailed examination of all the characters cited by Bonnevie (1920) to distinguish P. minuta from P. hippocampus, and was based on specimens collected from the North Atlantic Ocean. The results confirmed and amplified upon those of the earlier Hawaiian study.

Other Names for Pterotrachea hippocampus Phillipi 1836

• Pterotrachea minuta

References

Bonnevie, Kr. 1920. Heteropoda collected during the "Michael Sars" North Atlantic Deep-Sea Expedition 1910. Report on the scientific results of the "Michael Sars" North Atlantic deep-sea expedition 1910: 3(2)(Zoology): 3-16, 5 pl.

Lalli, C. M. and R. W. Gilmer. 1989. Pelagic snails. The biology of holoplanktonic gastropod mollusks. Stanford Unive. Press, Stanford, pp. 1-259.

Richter, G. 1968. Heteropoden und Heteropodenlarven im Oberflächenplankton des Golfs von Neapel. Pubblicazioni della Stazione Zoologica di Napoli 36: 346-400.

Richter, G. and R. R. Seapy. 1999. Heteropoda, pp. 621-647. In: D. Boltovskoy (ed.), South Atlantic Zooplankton. Leiden: Backhuys Publ.

Seapy, R. R. 1985. The pelagic genus Pterotrachea (Gastropoda: Heteropoda) from Hawaiian waters: a taxonomic review. Malacologia 26(1-2): 125-135.

Seapy, R. R. 2000. Species discrimination among pelagic heteropods: resolution of the Pterotrachea hippocampus - P. minuta problem. Journal of Molluscan Studies 66: 99-117.