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Pneumoderma heronensis Newman and Van der Spoel, 1989

Overview

This is a small naked pelagic snail, more than 1 cm long. It is oval, with a slightly transparent skin and usually brown to black pigmentation. The wings are well developed as is the large posterior and lateral footlobes. The buccal mass has two lateral arms, each with up to 12 suckers. It lives in the Pacific Ocean in the upper water layers although never abundant (Pneumoderma heronensis 1).

Taxonomic Description

The extended body is barrel-shaped, with brown and black chromatophores over all surfaces excluding the wings. The median footlobe is long and pointed; the lateral footlobes joint at the anterior margin. The wings are short, and broad distally. The lateral gill is well developed and fringed. The posterior gill is fringed, with four radiating crests that join medially. The visceral mass does not extend to the posterior end of the body (Pneumoderma heronensis). The everted buccal apparatus displays two lateral arms with 12 stalked suckers arranged alternately in two rows. The suckers are flat, disc-shaped, on short stalks and unequal in size. The largest suckers (up to 1 mm in diameter) are situated medially on the buccal arm. The terminal four suckers are extremely small. The proboscis can extend to the same length as the body and, uniquely, bears two short 'tentacles' medially (Pneumoderma heronensis buc). The hook sacs are long, approximately one-half to two-thirds the length of the proboscis. Each hook sac is armed with numerous (approximately 100) fine hooks. In Pneumoderma heronensis hooksacs are arranged in four longitudinal rows along the inner margin, decreasing in size towards the proximal end; even smaller hooks are distributed over the entire surface of the hook sac. The Pneumoderma heronensis radula formula is 6-0-6, with 23- 25 rows (Pneumoderma heronensis radula, Pneumoderma heronensis buccal).
Lengths of the contracted specimens (stored in 70% ethanol) ranged from 5.85 to 10.2 mm.

Structure and Morphology

Pneumoderma heronensis closely resembles Pneumoderma mediterraneum which has been previously reported from the Great Barrier Reef (Russel and Coleman, 1935). The major difference between these species is found in the number of lateral buccal arm suckers, there being 7 suckers per arm in Pneumoderma mediterraneum and 12 in Pneumoderma heronensis. The arrangement of these suckers also differs between species as they alternate in two rows in P. heronensis, but are arranged in a single row in Pneumoderma mediterraneum. Russel and Coleman (1935) noted that the lateral arm, in their one specimen, was only partially extended. Without dissection, these authors may not have observed all the suckers present and hence mistakenly ascribed it to Pneumoderma mediterraneum. Additionally, Tesch (1950) noted that there is an increase in the number of suckers with age in Pneumodermatidae. However, Pneumoderma heronensis cannot be confused with Pneumoderma mediterraneum, as adults of the latter species have fewer suckers despite their larger body size (length 20 mm; Van der Spoel, 1976). The two formae of Pneumoderma atlanticum have between 8 to 16 suckers on each lateral buccal arm. Forma eurycotylum (Meisenheimer, 1905) differs from Pneumoderma heronensis in having a reduced number of lateral radula teeth, a median radular tooth, a rounded median footlobe, and only the posterior gill is fringed. Forma pygmaeum (Tesch, 1903) differs in possessing four lateral radular teeth, a short lateral gill, eight suckers, and being small in size (length 3 mm; Van der Spoel, 1 976). Pneumoderma heronensis differs from Pneumoderma degraaffi in possessing twice the number of buccal suckers (stalked), whereas in the latter, the suckers are attached to the buccal wall. The radula formula also differs, whereby in Pneumoderma heronensis, there is double the number of lateral teeth (cf. Van der Spoel and Pafort-van Iersel, 1982).

Juveniles

A special description is not available.

Reproduction

This species is a protandric hermaphrodite.

Ecology

Heron Island specimens were observed in situ, within a plankton swarm predominantly of ctenophores, larvaceans, siphonophores and the euthecosomatous pteropod Diacavolinia longirostris (De Blainville, 1821). The Northwest Shelf specimens were also found in samples full of Diacavolinia longirostris. Gymnosomes are carnivorous and their selective feeding on euthecosomatous pteropods has been investigated by Lalli (1970 a and b). Boas (1886) found specimens of Pneumoderma with Cavolinia tridentata in their guts. It is assumed that Pneumoderma heronensis was feeding on the abundant euthecosome Diacavolinia longirostris both at Heron Island and off the Northwest Shelf. These gymnosomes were observed to be rapid swimmers, frequently avoiding capture by a SCUBA diver or dipnet.

Distribution

So far the species is only known from its type locality. See the Pneumoderma heronensis map.

Types

Pneumoderma heronensis Newman and Van der Spoel, 1989: 81, fig. 1-2.
Holotype (Pneumoderma heronensis 1): Collected while SCUBA diving, from surface waters off the north side of Heron Reef, April 19, 1987. Sexually mature, 12.5 mm long when relaxed (Australian Museum 156728.
Paratypes: 6 specimens collected by dipnet from surface waters off the northeast side of Wistari Reef (23O 27'S 151° 55'E) April 20, 1987. Relaxed body lengths ranged from; 10.9 to 15.9 mm, preserved in buffered 5% formalin and lodged at the Australian Museum, Sydney Australia C#156729 -156732, and ZMAN, ZMA. Moll. 389010.
Type Locality: North side of Heron Reef, southern Great Barrier Reef, off Gladstone, Queensland, Australia (23° 27'S 151° 55'E); bottom dept approximately 20 m. 23 specimens were identified from CSIRO plankton samples, collected on October 27, 1983 from the Northwest Shelf, off Western Australia (19° 25'S 119° 30'E).
Etymology: the name is derived from the name Heron Island Research Station, University of Queensland.

Pneumoderma heronensis