Pterotracheidae Rafinesque, 1814

Introduction

All pterotracheids lack a shell as adults, although they possess one as larvae (the shell is shed at metamorphosis). Their bodies are elongate and basically cylindrical, consisting of a proboscis, trunk and tail. The maximal recorded body length is 33 cm. The head lacks tentacles anterior to the eyes, except in male Firoloida. The viscera are compacted into a fusiform visceral nucleus. The swimming fin is large, located about midway between the anterior and posterior ends of the trunk, and bears a sucker only in males (presumably used in mating). Pterotracheids are mostly epipelagic (dwelling in the upper several hundred meters of the water column), although the vertical ranges of two species of Pterotrachea extend into the mesopelagic. Nocturnal vertical migration is seen in Pterotrachea.

The Pterotracheidae are widely regarded as the most highly derived of the heteropod families. Features supporting this contention include: (1) enlargement, elongation and narrowing (to a  basically cylindrical shape) of the body in the anterior-posterior axis, resulting in a streamlined body with enhanced swimming abilities, (2) shedding of the larval shell at metamorphosis, with the result that buoyancy problems are reduced since a calcareous shell (present in the adults of the other two families) is lacking, (3) compaction of the viscera into a pyriform visceral nucleus, which is largely enveloped by the gelatinous body at the posterior end of the trunk.

Brief Diagnosis

Heteropod molluscs with:

• Body elongated and basically cylindrical, divided into proboscis, trunk and tail
  
• Viscera compacted into a pyriform visceral nucleus
• Shell lacking in adults; larval shell shed at metamorphosis
  

Characteristics

1. Shell present in larvae only; cast off following metamorphosis
   
2. Body morphology
1. Elongate, basically cylindrical; streamlined for rapid swimming
2. Proboscis, trunk and tail regions well developed
   

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   Figure. Body regions in Pterotrachea coronata. © R. R. Seapy

3. Viscera compacted into pyriform visceral nucleus
4. Esophagus elongated, connecting buccal mass with visceral nucleus
   


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Figure. Location of esophagus, extending from the buccal mass to the visceral nucleus, in Pterotrachea coronata. © R. R. Seapy

3. Swimming fin
   1. Located about midway between head region and visceral nucleus
      
   2. Fin sucker small; present only in males
      

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      Figure. Pterotrachea coronata.  Left: swimming fin in female, viewed from right side of body. Right: swimming fin and sucker in male, viewed from right side. © R. R. Seapy

4. Head
   1. In dorsal view, shape of eyes either rectangular (with a narrow retinal base) or triangular (with a broad retinal base that curves upward toward the lens)
      
   

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   Figure. Dorsal views of eyes in Pterotrachea. Left: rectangular eye of P. coronata. Center: narrowly triangular eye of juvenile P. hippocampus. Right: broadly triangular eye of adult P. hippocampus. © R. R. Seapy

   2. Tentacles absent, except in Firoloida males
      
      
5. Radula
   1. 24-30 tooth rows
   2. Central rachidian tooth polycuspid with prominent central cusp, flanked on either side by numerous short cusps that decrease in size laterally
      
      

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      Figure. High magnification view of central rachidian tooth (lower, middle) and lateral teeth (upper, directed inward) from radula of Pterotrachea hippocampus. Scale bar = 20 µm. © G. Richter

Comments

Two genera are included in the Pterotracheidae, one of which, Firoloida, is monotypic. The genera can be distinguished by the following characters:

Genus	Tail	Posterior egg string	Tentacles
Pterotrachea	well developed	absent	absent in both sexes
Firoloida	very short, ventral	present	present in males

References

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

Spoel, S. van der, L. Newman and K. W. Estep. 1997. Pelagic molluscs of the world. World Biodiversity Database, CD-ROM Series. Expert Center for Taxonomic Identification (ETI), Amsterdam, The Netherlands, UNESCO, Paris.

Tesch, J. J. 1949. Heteropoda. Dana Rep., 34: 1-54.