Reference:Biol.Bull.169:291-296.(August,1985) GROWTH OF OCTOLASMIS COR (AURIVILLIUS, 1892) ON THE GILLS OF SCYLLA SERR.4TA (FORSKAL, 1755) WILLIAM B. JEFFRIES', HAROLD K. VORIS 2 , AND CHANG MAN YANG 3 1Departmen!ofBiology,DickinsonCollege.Carlisle,Pennsylvania17013;2DepartmentofZoology, FieldMuseumofNaturalHistory,Chicago,Illinois60605;*ZoologyDepartment, KentRidge,NationalUniversityofSingapore,Singapore0511 ABSTRACT A unique experimental procedure to determine the growth ofOctolasmis cor used to advantage the suspended crab cage operation in the Straits of Johore, Singapore. This allowed growth measurements of a previously inaccessible symbiotic organism in a natural situation. The resulting data suggest that growth to sexual maturity is rapid, occurring within a two-week period. It is speculated that this is an adaptation to an ephemeral substrate which allows O. cor to maximize its reproductive capacity by increasing egg production and the number of potential hosts. INTRODUCTION Although there are well-documented reports on the growth of certain species of Balanomorpha (Barnes and Powell, 1953; Costlow and Bookout, 1953; Mawatari el al., 1954; Barnes, 1956; and Daniel, 1958), there are relatively few such reports on the Lepadomorpha (Evans, 1958; Skerman, 1958; Page, 1983), and none on epizoic barnacles such as Octolasmis which live attached to certain Crustacea. Scylla serrata (Forskal, 1755) was the host selected for this study because in earlier work (Jeffries et al, 1982), all of the 45 individuals examined had Octolasmis in their gill chambers. S. serrata is indigenous to the Johore Straits (Chuang, 1961), an estuary fed by several rivers and rich in plankton flora and fauna (Khoo, 1967; Tham et al., 1970). Large numbers are also imported to Singapore by air from nearby Southeast Asian countries and held in cages suspended in the Johore Straits until ready for market. The purposes of this research was to measure the growth rate of Octolasmis cor from cyprid to large adult on the gills of S. serrata under nearly natural conditions and to consider the implications for coevolution of the epizoite and its host. MATERIALS AND METHODS Methods of managing the hosts are essential prerequisites for the study of growth in epizoic organisms. Clean substrates must be provided for attachment of epizoites, the time of attachment must be known or at least inferable, and the hosts must be maintained under natural or nearly natural conditions. The crab cage operation in the Straits of Johore provided a unique opportunity to meet these conditions. In turn, this allowed the development of techniques to determine the rate of growth of a species of Octolasmis which, because they occur within the gill chambers of decapods, are inaccessible to direct observation and sequential measurement. From late May until the middle of August 1983, S. serrata were obtained within 48 hours of arrival in Singapore by air freight from Indonesia. Crabs 83 mm to 1 15 Received9January1985;accepted23May1985. 291