Nematocyst composition of the cubomedusan Chiropsalmus quadrigatus changes with growth

Abstract

The nematocysts of Chiropsalmus quadrigatus (Cubozoa; Cubomedusa; Chirodropidae) were examined to determine if their composition changes with an increase in body size. Fixed tentacles of specimens collected in Okinawa, Japan, were homogenized and their nematocysts were observed under a differential interference contrast microscope. Six nematocyst types were observed in medusae of all sizes: microbasic mastigophores (MM), large and small trirhopaloids (lTR and sTR), holotrichous isorhizas (HI), ellipsoidal isorhizas (eI), and ovoid isorhizas (oI). Two other nematocysts, large ovoid isorhizas (loI) and microbasic euryteles (ME), were observed only in small individuals. There was also marked difference in proportion of tentacular nematocysts between small and large individuals. HI was the dominant type in small specimens, while MM and eI were predominant in large specimens. Nematocyst composition in the bell and pedalia also differed between small and large individuals. Bells of small medusae contained oI and sTR, while only oI were observed in most large individuals. The pedalia of small medusae had clusters of MM, ME, sTR, and oI. Such single clusters on pedalium bases were characteristic of small individuals. The pedalia of large individuals contained scattered oI. Tentacles of medusae are used for prey capture, so the changes in the major type of nematocysts in tentacles may reflect changes in prey type.