Pterotracheidae Rafinesque, 1814

Introduction

All pterotracheids lack a shell as adults, although they possess one as larvae (the shell is shed following metamorphosis). Their bodies are elongate and basically cylindrical, consisting of a proboscis, trunk and tail. The maximal recorded body length is 38.5 cm (Pterotrachea coronata). The head lacks tentacles anterior to the eyes, except in male Firoloida demarestia. The viscera are compacted into a long, cylindrical 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. 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 zone. All pterotracheids have cosmopolitan distributions in tropical to subtropical waters.

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 following 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 after metamorphosis
• All species with cosmopolitan distributions in tropical to subtropical waters
  

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 a long and narrow visceral nucleus
4. Esophagus extends from the buccal mass, at the end of the proboscis, to the visceral nucleus
   


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Figure. Location of buccal mass, esophagus, and 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 in juvenile P. hippocampus. Right: broadly triangular eye in adult P. hippocampus. © R. R. Seapy

   2. Tentacles absent, except in Firoloida demarestia males
      
      
5. Radula
   1. 24-30 tooth rows
   2. Central rachidian tooth (lower center in photo below) polycuspid with a  prominent middle cusp that is flanked on either side by numerous short cusps
      
      

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      Figure. High magnification view of central rachidian tooth (lower, middle) and elongate, pointed lateral teeth (upper left and right in photo) 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	Tentacles anterior to the eyes	Posterior egg string
   Pterotrachea	well developed	absent in both sexes	absent
   Firoloida	very short, ventral	present in both sexes	present

Other Names for Pterotracheidae Rafinesque, 1814

• pterotracheids
• pterotracheid heteropods

References

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

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

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), University of Amsterdam. UNESCO Publishing, Paris.

Tesch, J. J. 1949. Heteropoda. Dana Report 34, 53 pp., 5 plates.