D. G. Nagle á F. T. Camacho á V. J. Paul Dietary preferences of the opisthobranch mollusc Stylocheilus longicauda for secondary metabolites produced by the tropical cyanobacterium Lyngbya majuscula Received: 28 January 1997 / Accepted: 24 April 1998 Abstract Pure compounds isolated from the cyanobac- terium Lyngbya majuscula Gomont were evaluated in an arti®cial diet for their in¯uence on the feeding prefer- ences of the sea hare Stylocheilus longicauda (Quoy and Gaimard, 1824), which lives in and feeds on this ®la- mentous cyanobacterium (blue-green alga). Microcolin B, ypaoamide, malyngolide and other natural products acted as feeding deterrents at natural concentrations. At lower concentrations, sea hares were indierent to ypaoamide and malyngolide in their diets. In contrast, barbamide stimulated sea hare feeding at the concen- trations normally found in L. majuscula. Malyngamides and majusculamides, the most common natural prod- ucts found in samples of L. majuscula from Guam, increased sea hare feeding at low concentrations and inhibited feeding at the higher concentrations that occurred in some collections of L. majuscula. Dietary selection of cyanobacteria by S. longicauda may be regulated by the concentration of speci®c chemical cues produced by L. majuscula. Introduction The feeding speci®city of herbivores may be regulated by several factors, including the availability of ``enemy-free space'', resource-partitioning, mating behavior, and host-plant chemistry (Fox and Morrow 1981; Bernays and Graham 1988; Schultz 1988; Thompson 1988; Bernays and Cornelius 1989; Hay 1992). The relative importance of these factors is a topic of considerable debate. Because specialist herbivores are relatively rare in marine systems, at least compared with insects in terrestrial systems, the importance of dierent factors that select for host-plant specialization may be easier to determine in marine communities (Hay 1991, 1992). Like many specialist terrestrial herbivores (Rank 1992; Rowell-Rahier and Pasteels 1992), marine specialist herbivores are often unaected by, or even stimulated to feed by, host natural products (Hay et al. 1989, 1990). The sea hare Stylocheilus longicauda (Quoy and Gai- mard, 1824) (Opisthobranchia: Anaspidea) specializes on the benthic, ®lamentous cyanobacterium Lyngbya majuscula Gomont (Oscillatoriaceae) [  Microcoleus lyngbyaceus KuÈtzing] (Switzer-Dunlap and Had®eld 1979; Paul and Pennings 1991), and sequesters natural products, which can function in defense against preda- tors, from this cyanobacterium (Paul and Pennings 1991; Pennings and Paul 1993a; Pennings et al. 1996). Switzer- Dunlap and Had®eld (1977) reported that larval S. longicauda metamorphosed on L. majuscula and sev- eral species of red algae, but that only L. majuscula supported rapid growth of juveniles. Large numbers of S. longicauda can be found on L. majuscula in the ®eld (Switzer-Dunlap and Had®eld 1979; Paul and Pennings 1991; Nagle and Paul personal observation), S. long- icauda prefers L. majuscula over other algae in labora- tory choice-tests, and of six algae tested, only L. majuscula supported rapid growth in laboratory assays (Paul and Pennings 1991). Thus, considerable evidence suggests that this sea hare is a specialist on L. majuscula. Like many cyanobacteria, Lyngbya majuscula is a rich source of biologically active natural products including both deterrent and toxic compounds (Moore 1981, 1982; Orjala and Gerwick 1996). The L. majuscula metabolites Malyngamides A and B (Paul and Pennings 1991; Pennings et al. 1996; Thacker et al. 1997), Majusculamides Marine Biology (1998) 132: 267±273 Ó Springer-Verlag 1998 Communicated by M.F. Strathmann, Friday Harbor D.G. Nagle 1 á F.T. Camacho á V.J. Paul (&) Marine Laboratory, University of Guam, U.O.G. Station, Mangilao, Guam 96923, USA Fax: +671 734-6767 e-mail: vpaul@uog9.uog.edu Present address: 1 University of Mississippi, Department of Pharmacognosy and National Center for the Development of Natural Products, Research Institute of Pharmaceutical Sciences, School of Pharmacy, University, Mississippi 38677, USA