ULOPHYSEMA OERESUNDENSE BRATTSTROM, 1936 (CRUSTACEA, ASCOTHORACICA)

Ulophysema oeresundense Brattstrim, 1936, is a highly specialized crustacean that spends its adult life enclosed in different organs of its sea urchin host. The morphological adaptations of the integument of U. oeresundense for making close contact with the host cells were studied. The body is enclosed in a double-walled mantle covered on the outside by a single layer of choanocyte-like cells of host origin. The outer mantle epithelial cells underlie a thin electrondense epicuticle, which has a dense network of highly differentiated villi-like structures. The wall of the mantle contains branches of the intestine, but the gut epithelium in adult specimens shows pronounced degeneration and its role in nutrient uptake is doubtful. The mouth parts are atrophied. The mantle cavity functions as a brood chamber, and its integument is provided with "anchoring" hooks that, together with the long and hairy projections of the body surface, probably prevent the eggs and larvae from being expelled. A great number of sensory organs with a putative rheoreceptive function are observed among the outer epithelial cells of the mantle cavity. The specialized epicuticle and the microvilli of the epithelial cells of the mantle are considered a route for molecular interchange. The Ascothoracica, an order of the subclass Cirripedia, are parasitic crustaceans living as endo- or ectoparasites in different species of coelenterates and echinoderms. A peculiar feature of their morphology is the presence of a soft bivalve carapace or mantle enclosing the entire body. In females of some species the mantle is highly modified, developing a branching system of roots extending into all parts of the body cavity of the host, and containing the gut. Ulophysema oeresundense Brattstrom, 1936, is an endoparasite of irregular sea urchins. It spends its adult life most often attached to the genital glands of the sea urchin, but is also found unattached in the perivisceral cavity. The larval development, part of the life cycle, and gross morphology of U. oeresundense have been described by Brattstrom (1947, 1948a, b). The present paper deals with the ultrastructure of the integument of the parasite, with the purpose of understanding the process of nutrition. As far as we know, no work of the ultrastructural level has been published, except for a paper by Grygier (1982) concerning sperm ultrastructure, and a short note by Bresciani and Jespersen (1983). MATERIAL AND METHODS Specimens of U. oeresundense were collected from the host Echinocardium cordatum (Pennant) dredged at Disken, 0resund, south of Helsingor, at a depth of 7-9 m. The sea urchins were taken in May and June 1982, brought to the laboratory alive, and dissected. The infestation was rather high (20-25%). Small specimens (2-3 mm) were found free in the coelomic cavity of the host, while older specimens (up to 20 mm in length and with folded mantles) were attached to the degenerated gonads. Small pieces of the mantle and body of the parasite were fixed for electron microscopy in trialdehyde (Lake, 1973) or in 3% glutaraldehyde in filtered sea water, blockstained for 2 h at room temperature in 0.5% aqueous uranyl acetate, and postfixed in 1% osmium for 1 h at 4?C. Araldite or an Epon-Araldite mixture was used as embedding medium for all the material. The sections were cut with a diamond knife on a Reichert Om3U ultramicrotome, stained with uranyl acetate and lead citrate, and examined with a Siemens Elmiskop I. For scanning electron