Morphology of the nematocysts of the medusae of two scyphozoans, Catostylus mosaicus and Phyllorhiza punctata (Rhizostomeae): implications for capture of prey

Abstract

Abstract. 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 assemblage 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, specimens of P. punctata appeared to have far fewer nematocysts than those of C. mosaicus. Four types of nematocysts were identified in medusae of C. mosaicus; 2 types of holotrichous isorhizae, rhopaloids, and birhopaloids. In C. mosaicus, the oral arms and bell margins possessed all of these types, but the cnidomes of the 2 regions differed in relative abundances and sizes of isorhizae and rhopaloids. Five types of nematocysts were identified in medusae of P. punctata, although not all types were found in all specimens. Round holotrichous isorhizae were found only in the bell, while oval holotrichous isorhizae, rhopaloids of 2 distinct size ranges, and birhopaloids were found in the bell and oral arms. Cnidomes of the bell and oral arms in specimens of P. punctata also differed in the relative abundance and sizes of oval isorhizae and rhopaloids. Although there were clear differences in the overall cnidomes and absolute abundances of nematocysts in each species, the oral arms (feeding appendages) of specimens 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 species preyed predominantly on copepod nauplii and larvae of gastropods and bivalves. Medusae of C. mosaicus captured ∼10 × more gastropod larvae and 5 × more bivalve larvae than those of P. punctata. Specimens of P. punctata captured approximately twice as many copepod nauplii as those of C. mosaicus. Differences in the relative abundance of types of zooplankton captured by each species could not be adequately explained by differences in the cnidomes of the oral arms.

Functional characteristics of nematocysts found on the scyphomedusa Cyanea capillata

“…2) and nomenclatural classifications, had the most similar velocities and lengths and, therefore, are most likely to function similarly. The lack of correspondence between a nematocyst's nomenclatural classification and its function suggests that caution is appropriate when using medusan nematocyst assemblages as indicators of prey selection and trophic role (Purcell and Mills, 1988;Carrette et al, 2002;Peach and Pitt, 2005). Further, the lack of correspondence between capsule size, discharge velocity and kinetic energy may confound the use of nematocyst sizes to infer prey size (Purcell, 1984;Carrette et al, 2002).…”

“…Previous studies using scanning electron micrographs of preserved samples of nematocysts on captured prey have repeatedly found euryteles and O-isorhizas of scyphomedusa to penetrate both hard and soft bodied prey for the full length of their tubule (Heeger and Möller, 1987;Heeger et al, 1992;Östman and Hydman, 1997;Peach and Pitt, 2005). In contrast, a-isorhizas are less effective penetrants (Heeger and Möller, 1987;Peach and Pitt, 2005) and have been found to only partially penetrate their prey, leaving the rest of their tubule to stick to the surface of the prey (Heeger and Möller, 1987). This observation is consistent with their lower discharge velocities and kinetic energies.…”

“…The majority of described nematocyst types are assumed to be penetrants because the tip of their tubules are open and are inferred to be capable of delivering the toxins characterizing cnidarian stings. Beyond this, most of our current knowledge on prey capture by nematocysts is based upon studies which either relate the nematocyst assemblage found on a medusan species to its diet (Purcell and Mills, 1988;Carrette et al, 2002;Peach and Pitt, 2005), or studies using microscopy to examine preserved samples of nematocysts and their captured prey (Purcell, 1984;Heeger and Möller, 1987;Heeger et al, 1992;Östman and Hydman, 1997;Peach and Pitt, 2005). Due to the inherent difficulty in observing the fastest cellular process identified, few studies have directly observed and described the discharge properties of different types of nematocysts (Holstein and Tardent, 1984;Nuchter et al, 2006).…”

“…Therefore, we do not know whether the tubules of similar types of nematocysts discharge similarly or whether their discharge properties are related to the nematocyst categories identified by taxonomists. Since tubules are the part of nematocysts typically observed clinging to prey (Purcell, 1984;Heeger and Möller, 1987;Heeger et al, 1992;Östman and Hydman, 1997;Peach and Pitt, 2005), their properties likely determine how nematocysts capture prey, and hence, their functions.…”

Jellyfish modify the response of planktonic assemblages to nutrient pulses

“…Jellyfish had a considerable impact on the mesozooplankton community, with concentrations of all of the most abundant taxa decreasing in mesocosms exposed to jellyfish. Catostylus mosaicus captures a range of mesozooplankton, but most notably copepods, mollusc veligers and polychaetes (Browne & Kingsford 2005, Peach & Pitt 2005, and most of these groups decreased substantially in treatments containing jellyfish. Jellyfish also caused changes in the assemblages of microzooplankton during both the May and September experiments.…”