Morphology of the nematocysts of the medusae of two scyphozoans, Catostylus mosaicus and Phyllorhiza punctata (Rhizostomeae): implications for capture of prey
We examined the cnidomes (total complement of nematocysts) of medusae of the zooxanthellate and azooxanthellate jellyfishes Phyllorhiza punctata and Catostylus mosaicus (Rhizostomeae, Scyphozoa), and compared the assemblages of zooplankton captured on the oral arms of each species to determine whether differences in the types or amount of zooplankton captured were consistent with possible differences in the cnidomes. Cnidomes were described using light and scanning electron microscopy. Each species had a distinct cnidome and, in general, P. punctata appeared to have far fewer nematocysts than C. mosaicus. Four types of nematocysts were identified in C. mosaicus; two types of holotrichous isorhizae, rhopaloids, and birhopaloids. Oral arms and bell margins of C. mosaicus possessed all of these types, but the cnidomes of the two regions differed in relative abundances and sizes of isorhizae and rhopaloids. Five types of nematocysts were identified in P. punctata, although not all types were found in all specimens. Round holotrichous isorhizae were found in bells only, while oval holotrichous isorhizae, rhopaloids of two distinct size ranges, and birhopaloids, were found in bells and oral arms. Cnidomes of bells and oral arms in P. punctata also differed in the relative abundances and sizes of oval isorhizae and medium-sized rhopaloids. Although there were clear differences in the overall cnidomes and absolute abundances of nematocysts in each species, the oral arms (feeding appendages) of both C. mosaicus and P. punctata had similar types and relative abundances of nematocysts. Zooplankton sampled from the oral arms of each species showed that both C. mosaicus and P. punctata preyed predominantly on copepod nauplii, and larval gastropods and bivalves. C. mosaicus captured approximately 10 times more gastropod larvae and five times more bivalve larvae than P. punctata, and P. punctata captured approximately twice as many copepod nauplii as C. mosaicus. Differences in the relative abundances of zooplankton captured by each species could not be adequately explained by differences in the cnidomes of the oral arms.