Ecology of Sipuncula

Abstract

ABSTRACT: The marine Sipuncula are regarded as consisting of 1 class, 4 families, 13 genera and 367 species and subspecies. Sipuncula are widespread in the benthos of the World Ocean: from 82" N and 77" S to the equator, and from the littoral to a depth of 7000 m; 63 % of the species are confined to the shelf. Temperatures tolerated range from - 1.9 to 29 "C; however, most species are found within the 20 'C isotherm. The geographical distribution is confined to seas with normal oceanic salinity. The range for survival and reproduction is 27 to 44 9~ S. Sipuncula are mainly deposit feeders. According to the mode of feeding and motility, the Sipuncula comprise 4 ecological groups: (1) burrowers, indiscriminately swallowing the substratum; (2) worms hiding in vacant shelters and collecting detritus; (3) waiting sestonophages; (4) sessile worms, scraping off food from the substratum. INTRODUCTION Sipunculans' may be classified as unsegmented, vermiform, bilaterally symmetrical coelomate Protostomia, which in the course of evolution have rather prominently differentiated from the main line of Protostomia, leading to the Annelida and Mollusca. At present, 367 species and subspecies of Sipuncula are known, forming 13 genera, 4 families, and 1 class. High densities in certain biotopes, averaging 8000 individuals m-' (Chapman, 1955), the muscular body devoid of calciferous formations, and the lack of protective shell or tube render the sipunculans an easily available prey for other animals. Sipunculans have been reported from the alimentary tracts of actinians, molluscs, crabs and seastars. They are the sole food source for the gastropod Mitra litterata, a common inhabitant of the Hawaiian waters, and constitute a rather important part of the ration of benthophage fishes. They were found in the stomachs of over 50 fish species, belonging to 2 orders of elasmobranch and 17 families in 6 orders of teleosts, including such important food-fish as cod, plaice and haddock (Kohn, 1975). Sipunculans were considered edible some 200 yr ago: Pallas (1774) named one of the first scientifically described sipunculan worms Lumbricus edulis (= Siphonosoma edule). Native populations of If the phylum is to be called Sipuncula it seems reasonable and logical to use the term 'sipunculan' instead of 'sipunculoid' or 'sipunculid' (Stephan and Edmonds, 1972) O Inter-Research/Printed in F. R. Germany Malaya, Caroline Islands and South China consume large specimens of the genera Sipunculus, Siphonosoma and Phascolosoma, reaching 60 cm in length and 2 to 2.5 cm in diameter (Sato, 1935; Chin, 1947). According to recent American studies (Rice and McIntyre, 1972; Rice, 1976) these soft-bodied creatures are able to bore into hard rock, thus playing an important role in the destruction of limestone and coral reefs. The present study has 2 aims: (l) to analyse the depth distribution of sipunculans in the benthos of the World Ocean; (2) to generalise data on the ecology of sipunculans, to reveal their reaction to the principal abiotic factors and to arrange them into major ecological groups. MATERIAL