Firoloida desmaresti Lesueur, 1817
Overview
This is a medium sized naked pelagic snail, up to 4 cm long, with an almost completely transparent body. Only the broad oval visceral nucleus forms a dark part, the large eyes with black retina and the mouth organs are clearly seen. The intestine is frequently full and visible. The body is very long cylindrical with one swimming fin has no tail but in females the egg ribbon may be attached as a 'tail' (Firoloida desmaresti female 2, Firoloida desmaresti female caudal). It hunts large prey animals like fish larvae, shelled pteropods and other heteropods. It lives in the upper layers of warm waters of all.
Taxonomic Description
The species Firoloida desmaresti is a very common, small species characterised by the absence of a real tail. The transparent, long cylindrical body has a fin in the centre, a short not curved proboscis anteriorly and a globular visceral nucleus posteriorly. Sexual dimorphism is present: in males a fin sucker, tentacles before the eyes and a voluminous penis are present and a rudimentary tail the visceral nucleus terminates in a filamentous thread. In females this tail is absent and only two obtuse, hook-shaped lobes are present between which the egg string (frequently present in preserved females) protrudes. Owre (1964) described this egg string as a permanent anatomical structure in adult females. The three gills are extremely small. Muscles of body and fin are fused into a homogeneous layer. The Firoloida desmaresti eyes are slightly triangular by the curved retina which is slightly broader than the eye lens. The Firoloida desmaresti radula formula is 2-1-1-1-2. The median teeth are provided with a typical "star-shaped" median cusp (Firoloida desmaresti SEM)
Body length up to 40 mm.
Morphology and Structure
The chromosome number is 31 (2N) in males and in females 32 (2N) (Thiriot-Quievreux, 1990).
Juveniles
The Firoloida desmaresti juv. have a small dextrally coiled shell with about 3 whorls, regularly increasing in width. The suture is clear but not very deep. The aperture is oval to rounded (Firoloida desmaresti juv 1). The shell is slightly elevated thus not completely planorboid. The shell is yellow-whitish and the surface is smooth. Only the first embryonic whorl has a rough granulated surface. The operculum is composed of two separate layers, a small inner and a larger outer layer with has about 2 to 3 whorls. They were described also under the name Tholapex solutum (cf. Waren, 1992) and Cyclostrema solutum (cf. Nofroni, 1984). The foot is covered with cilia (Firoloida desmaresti juv 2).
Reproduction
As in all Pterotracheidae the veliger in this species first has a shell with two whorls. Protoconch I has fine punctuation, protoconch II has transverse growth lines. The organogenesis is fully described by Thiriot-Quievreux (1971 a). The anatomy is described in detail by Gabe (1966). The sexes are separate.
Ecology
This species is carnivorous and epipelagic. It is sometimes parasited by trematodes of which the rediae are described (Lester and Newman, 1986).
Distribution
This species is found between 40°N and 40°S which is slightly further southwards than most Heteropoda, see the Firoloida desmaresti map. It forms scattering layers. A 14 kc/s echosounder detected sonic scattering layers in the upper 20 meters of the sea over the continental shelf is reported. Generally the layer is in the upper 10 meters both during day and night (Blackburn, 1956).
Types
Firoloida desmaresti Lesueur, 1817c: 39, pl. 2, figs. 1-1b.
Types were not located.
Type locality: Off Martinique, Atlantic Ocean. Coll.: Péron and Lesueur.