BASTERIA, 65:57-60, 2001

A mysterious, living, ‘giant’ Gymnosomata species near the Maltese Islands (, Opisthobranchia)

Constantine Mifsud

4, Shepherds Street, Rabat, RBT 02, Malta

in Mysterious giantspecimens ofGymnosomata are recorded from near the Maltese Islands

the central Mediterranean Sea.

words: Key Gastropoda, Opisthobranchia, Gymnosomata, , Mediterranean Sea,

Malta.

INTRODUCTION

In the MediterraneanSea, species of of the gastropod order Gymnosomata De Most records from old Blainville, 1824, are rarely reported upon. are expeditions 1950; Pruvot-Fol, Van der (Tesch, 1954; Spoel, 1976). The order includes seventeen spe- from the cies Mediterranean, distributed over four families (Sabelli et al., 1990, 1992).

These molluscs do not shell and their possess a size rarely exceeds twenty mm. They use small for and have rather life wing-like appendages swimming a bentho-pelagic cycle.

They are usually caught at depths well above the sea bed in specially controlled trawl

nets.

DISCUSSION

trawlers few Fishing are in the Maltese Islands and the fishing season for these craft extends all the round. executed outside usually year Trawling is in areas a three mile limit from land, established by law, which in most cases is strictly enforced. An otter type oftrawl is employed and the catches, besides the fish, include various classes ofmolluscs,

such as cephalopods.

A recent examination ofthe discarded contents from a trawler's net revealed a small

number of juvenile specimens of cephalopod species, entangled in a large mass of the Udotea Also calcareous algae petiolata (Turra) Borgesen. present were some algae (maerl) and few a . The trawler was working at night, off the eastern coast of Malta at a

depth of 60-80 m.

The majority of the molluscs found in this material belonged to Loligo vulgaris (L.,

1758). All specimens were small, not exceeding 10 cm; some of them had been torn to

All the found isolated and in alcohol for parts. cephalopods were preserved study pur- this lot also female of poses. Among were one specimen Sepiola aurantiacaJatta, 1896, and two other strange specimens, which were isolated because they looked like cephalopods, but definitely did not belong to this order. 58 BASTERIA, Vol. 65, No. 1-3, 2001

1. 9 Fig. The mysterious, gymnosomatous gastropod from off Malta, measuring cm when still fresh.

sexual Abbreviations: a, appendage; b, body; f, anterior frilled fringe of the body; m, buccal mass;

lt, lateral ‘tentacles’; t, tail.

After careful examination, the specimens were found to resemble, morphologically, mollusc species from the order Gymnosomata. The specimens in particular resemble

of the 1886. species genera Thalassopterus Kwietniewsky, 1910, and Thliptodon Boas, One of the had the mantle and consisted of the buccal specimens missing essentially mass and the soft trailing parts, which measured 30 mm in length and 10 mm in width. The buc- cal of this dissected and of mass specimen was a pair laige jaws (fig. 2) and an arrowhe- radularibbon ad-shaped (fig. 3) were easily extracted. The radula containedsmall, green

mucus and a short of stalk, which have been of lumps piece may parts the alga Udotea. However, the mollusc may have chewed this material during its captivity in the trawl net and this therefore does not true diet. This because represent its is gymnosomes are known be to carnivores feeding mostly on pelagic pteropod molluscs, Chaetognatha and small medusae. The have brown serrated and the radula jaws a chitinous, cutting edge of consists a central large, thick, flat, pointed cusp with a similarly shaped large first late- thin ral on each side. The marginals are and shaped like bent crochet needles.There are

as many as 34 ofthese marginals on each side, at the greatest width ofthe radularibbon. The other specimen (fig. 1) seemed in perfect condition and when still fresh, measured

9 cm in length with a maximum width of 2.5 cm. This specimen and the parts of the

damaged small specimen were also preserved in alcohol for future studies and are kept

in the collection ofthe author.

The when had buccal head large specimen, fresh, a mass or measuring 15 mm across. The body or mantle was rather flattened, opaque and of a light grey colour, rounded with small flat tail the left end. There anteriorly a triangular at posterior are what look

like two small 'wings' and there are also a few minute, frilled tubercular appendages at the anterior of the mantle On the side of part edge. right the body a large (1 cm), club- sexual is followed and also shaped, appendage present, by an 'opening'. Posteriorly, on the much contracted stalk same side, is a gill with 35 tightly compressed leaves on each

side, in a seems with positioned staggered fashion. This large specimen complete, no severed parts or damaged. Mifsud: A Maltese Gymnosomata species 59

Gymnosomes are known to be pelagic and are usually found in trawl hauls from well above the bottom. It couldbe, depths that by chance the specimens were caught in the trawl its descent to the bottom its during or on way up. The of the general morphology trawled specimens suggest that they belong to the order and the of the inflatedbuccal Gymnosomata shape mass suggests that they belong the Clionidae The buccal to family Gray, 1840. apparatus is usually everted in species of this because are trawledfrom these family, they great depths. However, parts were retai- ned in perfect order in the Maltese specimens. The has external branch which be larger specimen an gill present, seems to lacking in of Boas, and species Thliptodon 1886, Gymnosomata in particular. The radula more par- ticularly resembles that of Thalassopterus Kwietniewsky, 1910. Specimens of Thalassopterus zancleus have been trawled Messina Kwietniewsky, 1910, near in Sicily, which is only a short distance from Malta. However, away the hundreds of specimens trawled never exceeded a few mm in and described length were as "probably larvae" and thereforepos- sess a different morphology.

Figs 2, 3. Jaws [2] and the complete radula ribbon [3].

CONCLUSION

At this stage it is difficult to classify the above-described Maltese trawled molluscs. More would have be obtained order specimens to in to make a complete study and assess- and make ment, to sure they are complete and hence determinethe correct placement of this species. Only one species ofthis order had been recorded for the Maltese Islands. This is krohni Clionopsis Troschel, 1854, and was recorded by Vayssiere (1913: 214). 60 BASTERIA, Vol. 65, No. 1-3, 2001

These molluscs have remained undiscovered for comparatively large a long time, which be due their fishermen discard such rubbish may to rarity. However, specimens as and shovel them back into the before land. This usually sea reaching finding demon-

in the need for strates particular more proper investigation and study in this small and enclosed ofthe Mediterranean nearly totally part sea.

ACKNOWLEDGEMENTS

I would like thank friend A. Sammut to my who provided me with the material from the trawlers inwhich the specimens were found. Thanks are also due to Mr. A.W. Janssen for providing important references and my colleagues R M. Sammut and C. Cachia for their help throughout.

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