Overview

Brief Summary

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

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Comprehensive Description

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.

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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)

      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. © Roger R. Seapy

    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)

      Figure. Tear-drop (pyriform) visceral nucleus in Pterotrachea hippocampusRoger R. Seapy

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

      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. © Roger R. Seapy

    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

    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.

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Ecology

Habitat

Mesopelagic
translation missing: en.license_cc_by_4_0

© WoRMS for SMEBD

Source: World Register of Marine Species

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Epipelagic
translation missing: en.license_cc_by_4_0

© WoRMS for SMEBD

Source: World Register of Marine Species

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Depth range based on 8 specimens in 1 taxon.
Water temperature and chemistry ranges based on 8 samples.

Environmental ranges
  Depth range (m): 203 - 1549
  Temperature range (°C): 5.640 - 21.576
  Nitrate (umol/L): 1.322 - 27.408
  Salinity (PPS): 34.728 - 35.970
  Oxygen (ml/l): 0.597 - 5.164
  Phosphate (umol/l): 0.342 - 2.410
  Silicate (umol/l): 0.908 - 32.458

Graphical representation

Depth range (m): 203 - 1549

Temperature range (°C): 5.640 - 21.576

Nitrate (umol/L): 1.322 - 27.408

Salinity (PPS): 34.728 - 35.970

Oxygen (ml/l): 0.597 - 5.164

Phosphate (umol/l): 0.342 - 2.410

Silicate (umol/l): 0.908 - 32.458
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

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