A STUDY OF THE POPULATIONS OF PARASITICA (COUCH, 1842) IN SYMBIOSIS WITH ANOMURAN DECAPODS IN THERMAIKOS GULF (N. AEGEAN SEA)

BY

J. CHRISTIDIS1), CH. CHINTIROGLOU1,3)and M. B. CULLEY2) 1) Aristotle University of Thessaloniki, School of Biology, Dept. of Zoology, Box 134, GR-54006, Thessaloniki (Macedonia), Greece 2) University of Portsmouth, School of Biological Sciences, The Marine Laboratory, Ferry Road, Hayling Island, PO 11ODG, U.K.

ABSTRACT

A wide range of meristic characteristics of the participants of the symbiotic relationship of Calliactis parasitica with decapod anomurans living in gastropod shells, from the North Aegean Sea (Thermaikos Gulf), was investigated. Using these data, attempts were made to determine the relative age (to size classes) of the anemones from this area, at their first association with an anomuran. It was found that anemones possessing a mean column diameter (mdC) of less than 0.5 cm, or a retracted column height (hC) of less than 0.4 cm, or anomurans possessing a cephalothorax length of less than 0.7 cm were not found living symbiotically in the study area. The anemone population also showed a shift in its distribution towards smaller and lighter individuals, indicating a possible preference for such anemones by the anomurans or the existence of an upper limit with regard to size for this particular symbiosis in the area and depths studied. We believe that both anemones and anomurans seek actively to initiate their symbiotic relationships from a relatively young age (in the first year of their life). The driving force behind this tendency for symbiosis should be attributed rather to biotic parameters which affect these organisms ( and increased feeding possibilities) than to abiotic.

RÉSUMÉ

Recherches sur bon nombre des caractéristiques méristiques des participants à l'association sym- biotique de Calliactis parasitica avec des anomoures vivant dans des coquilles de Gastéropodes, dans le nord de la mer Égée (golfe de Thermaikos). D'après ces données, un essai été tenté en vue de déterminer l'âge relatif (classes de taille) des anémones de cette région, à leur première association avec un Anomoure. Il a été constaté que, dans la zone étudiée, il n'y avait pas de vie symbiotique chez les anémones possédant un diamètre columnaire moyen (mdC) de moins de 0,5 cm, ou une colonne rétractée haute de moins de 0,4 cm, ou chez les anomoures à longueur de thorax inférieure à 0,7 cm. La population d'anémones a aussi montré un déplacement dans sa

3) To whom correspondence should be addressed. 228 distribution vers les plus petits et les plus légers individus, ce qui indiquait une préférence possible pour de telles anémones de la part des anomoures ou l'existence d'une limite supérieure en ce qui concerne la taille pour cette symbiose particulière dans la zone et aux profondeurs étudiées. Nous pensons qu'anémones et anomoures cherchent activement à commencer leur associations à une âge relativement jeune (dans la première année de leur vie). La force d'impulsion tendant à la symbiose serait principalement à attribuer aux paramètres biotiques qui affectent ces organismes (prédations et possibilités accrues d'alimentation) plutôt qu'aux paramètres abiotiques.

INTRODUCTION

Among the most investigated symbiotic relationships between and other invertebrates are those of sea anemones and decapod Crustacea. From 50 such relationships investigated by Ross (1967, 1974a, b), it appears that in the Mediterranean Sea among the most common symbioses are those of the Calliactis parasitica (Couch, 1842) and the anomurans Dardanus arrosor (Herbst, 1796), (Linnaeus, 1767), Pagurus excavatus (Herbst, 1791 ), and Pagurus alatus (Fabricius, 1775). The study of these relationships has yielded valuable information on their role and importance in their ecosystems (Ross, 1960, 1967, 1971, 1974a, b). In these relationships, it frequently became obvious that the gastropod shell inhabited by the anomurans was also of great importance, something that led Ross & Sutton (1963) to name these symbiotic relationships "ecological triangles". Calliactis parasitica appears to be an active member of the symbiosis, as it can move from shell to shell on its own. Additionally, Mainardi & Rossi (1969) and Ross (1974a, b, 1980) noted that the anomurans also frequently "steal" anemones from each other, in order to establish partnerships. Behaviour of this sort is encountered in anomurans both inter- and intraspecifically (Ross, 1974a, b). Through these actions the anomuran gains protection, as the cephalo- pod molluscs, which are considered their main predators, retreat in the presence of an anemone on the gastropod shell (Ross, 1971; Brooks, 1991; Chintiroglou, unpubl. data). For the anemone, the nutritional gains obtained from movement with the anomurans are thought to be significant (Stachowitsch, 1980; Chin- tiroglou & Koukouras, 1991). It would appear therefore, that for both participants this symbiosis comprises an important evolutionary step towards their survival (Ross, 1974a, b; Brooks, 1991). One of the less investigated parameters concerning these symbiotic relation- ships, is the size/age structure of the populations of anemones and anomurans, when the symbiosis is first established (Ross, 1974b, 1980). The present paper identifies the true range of size classes (size being relative to age) of C. para- sitica specimens involved in symbioses with anomuran decapods in Thermaikos