Pterosoma planum Lesson 1827

Introduction

Pterosoma is a monotypic genus represented by the Indo-Pacific species P. planum. The transparent trunk, with whitish minute dish-shaped inclusions and surface tubercles, is expanded as an oblong disk that is flattened dorso-ventrally. The trunk terminates in a tapering tail. The visceral nucleus arises from a short, mobile stalk and is covered by a flattened shell.  The shell is divided medially into two lateral halves by a low carina, homologous with the keel in Carinaria. The carina runs the length of the shell, beginning at the posterior end abutting the protoconch. While swimming, the visceral nucleus is raised dorsally, exposing the gills to inflowing water, but if disturbed it is retracted into a longitudinal groove on the surface of the trunk. The proboscis is moderately short, extending ventrally beneath the anterior margin of the trunk.

Brief Diagnosis

Carinariid heteropods with:

• Trunk expanded, forming an oblong disk that is flattened dorso-ventrally
• Visceral nucleus arises from short stalk and is covered by a flattened shell
• Tapering tail extends posteriorly from trunk
  
• Proboscis extends ventrally from beneath anterior margin of trunk

Characteristics

1. Body morphology
   1. Trunk greatly expanded laterally, forming an oblong disk that is  dorso-ventrally flattened
      

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      Figure. Dorsal view of Pterosoma planum. ©

   2. Trunk highly transparent, covered on the surface with whitish tubercles and embedded with minute whitish, disk-shaped inclusions
      

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      Figure. Dorso-lateral view of elevated visceral nucleus and gills in Pterosoma planum. In the photograph, low whitish tubercles are seen on surface of the trunk, and imbedded minute, disk-shaped inclusions are visible below and to the left of the gills. ©

   3. Visceral nucleus dorso-ventrally compressed, arising from a flexible short stalk (see above photo)
      
   4. Fin sucker present in both males and females
      
   5. Tail extends posteriorly from beneath the trunk, tapering to a narrow terminal section (see above photos)
      
   6. In young, post-metamorphic individuals, the expanded trunk disk (about 5 mm in length in the specimen below) is well developed and is fringed by about two dozen, prominent white tubercles. Numerous small white inclusions are also evident in the disk. Also, at this stage the viscereal nucleus stalk is prominent and the adult shell (teleoconch) is just beginning to form
      
      

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      Figure. Juvenile specimen of Pterosoma planum. Left: Ventral view of animal; note the large white tubercles on the fringe of the trunk disk, the multiple small white inclusions in the disk, the single left tentacle ahead of the left eye, and the yellowish-tan bolus of food in the esophagus (above the anterior part of the swimming fin). Right: Dorso-lateral view of the visceral nucleus and shell. At this stage of development, the adult teleoconch is just beginning to form © Allan Connell

2. Shell
   1. Shell dorso-ventrally compressed, covering the visceral nucleus
   2. Shell divided medially (in the anterior-posterior axis) by a low carina whose height increases somewhat anteriorly
   3. Coiled protoconch (larval shell) at posterior end of teleoconch (adult shell)
      

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      Figure. Shell of Pterosoma planum viewed from the left side. Protoconch at posterior end of shell, and height of carina increases somewhat toward anterior end of teleoconch. ©

   4. Larval shell globular, right side of shell with smooth surface except for low radiating ridges on second whorl, and umbilicus open with radiating striae on wall of umbilicus
      

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      Figure. Larval shell of Pterosoma planum; views of right side, left side, and aperture (left to right images, respectively). Shell diameter = 0.70 mm (right and left images) and 0.65 (center image). ©

3. Swimming fin well-developed; fin sucker large, located dorsally on the posterior margin of the fin; sucker present in both sexes (see title illustration)
   
   
4. Eyes and tentacles
1. Eyes broadly triangular in dorsal view
   
2. Left tentacle large and right one very reduced (note prominent left tentacle in title illustration)
   

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   Figure. Dorsal view of head region in Pterosoma planum. Right and left tentacles are out of focus, but visible in photograph. ©

Behavior

Observations of free-swimming Pterosoma planum in aquaria aboard ship (Seapy, unpublished) revealed that the stalked visceral nucleus is normally elevated dorsally, exposing the gills and the visceral nucleus to inflowing water (see photographs under 1.a above). However, when individuals were prodded with a blunt probe, the visceral nucleus (covered dorsally by the flattened shell) was rapidly retracted into a longitudinal groove on the dorsal surface of the trunk. Hypothetically, the act of prodding an individual simulates disturbance by a predator, and the vulnerable soft tissues of the gills and visceral nucleus are protected beneath the shell.

In the book on holoplanktonic gastropod snails by Lalli and Gilmer (1989:32), the in-situ field observations of swimming Pterosoma planum by Gilmer were described: "Pterosoma has large opalescent spots covering the central body in both sexes; these spots continually change in brilliance and pattern as the animals swim."  While the "central body" undoubtedly refers to the trunk, the nature of the "opalescent spots" is unclear since the only structures clearly visible on the trunk surface in net-captured specimens (Seapy, pers. obs.) are the surface tubercles described above under 1.b.  Conceivably, the "opalescent spots" and the behavior observed by Gilmer may be limited to animals in-situ. Despite observing active and apparently healthy specimens swimming freely in aquaria aboard ship for extended periods of time (Seapy, pers. obs.), the observations of Gilmer were never seen.

Other Names for Pterosoma planum Lesson 1827

• Pterosoma Lesson 1827

References

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

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

Seapy, R. R. and C. Thiriot-Quievreux. 1994. Veliger larvae of Carinariidae (Mollusca: Heteropoda) from Hawaiian waters. Veliger 37:336-343.