Growth Dynamics of the Radial Shields of the Euryalid Snake Star Astrobrachion constrictum (Echinodermata: Ophiuroidea)

Growth bands are evident in sections of the radial shields of Astrobrachion constrictum and on direct examination of dissected material. These bands result from the sequential addition of scales to the radial shields, a phenomenon not typical of ophiuroid radial shields. The rate of addition of these scales and the pattern of calcite deposition within them was studied using the skeletal growth marker calcein, and the validity of using the radial shield scales as age markers was examined. Individual brittle stars, injected intra-coelomically with calcein solution in the field at 6-month intervals, were collected and transverse fluorescent growth marks were found on radial shield scales. Measurement of the amount of stereom laid down between and after injections indicated the amount of growth in successive 6-month periods. Growth rate, as determined by measurement of the length of the radial shields, was faster during winter/spring, but the number of new radial shield scales increased during summer/autumn. Deposition of calcite within the scales also varied seasonally, greater amounts of large-pored stereom being deposited in winter. Growth bands were not laid down annually; as many as 11 bands appeared in a single year. Larger brittle stars did not appear to deposit new radial shield scales; 23 mm may be the maximum disc diameter for A. constrictum. Additional key words: Calcein, stereom Astrobrachion constrictum FARQUHAR 1900 is a euryalinid ophiuroid that lives in a mutualistic relationship on black coral colonies of the genus Antipathes (Grange 1991). Both the brittle star and black coral are normally found in deep water around southeast Australia and New Zealand (Baker 1980), but in New Zealand's Fiordland, a habitat with little current or wave action and low light intensity, as in far deeper waters, accommodates both organisms within SCUBA diving depths. This provides a unique opportunity to study a member of the little-known ophiuroid order Euryalida. In many ophiuroids, skeletal elements are large relative to body size and, in some species, arm and disc ossicles exhibit bands that have been interpreted as annual growth markers (Gage 1990a,b; Dahm 1993; Dobson & Stancyk 1994; Law 1995; pers. obs.). Skeletochronometry, the measurement of growth bands in skeletal elements (Stump & Lucas 1990), has become an important tool in the aging of ophiuroids. Examination of the anatomy of Astrobrachion constrictum (unpubl. obs.) revealed no apparent growth bands in arm ossicles, the stereom being, using the nomenclature of Smith (1980), of a uniform, labyrinthic nature. However, transverse bands were present in the radial shields (Stewart 1995). The drugs tetracycline hydrochloride and calcein (2,4-bis-[N,N'-di-(carbomethyl)-aminomethyl] fluorescein) have both been used successfully to leave a fluorescent mark on the calcified elements of a variety of living organisms, including fishes (Wilson et al. 1987; Tsukamoto 1988), brachiopods (Rowley & McKinnon 1995), and echinoderms (Kobayashi & Taki 1969; Taki 1972a,b; Pearse & Pearse 1975; Ebert 1984, 1988; Stump & Lucas 1990; Dobson & Stancyk 1994; Medeiros-Bergen & Ebert 1995). Tetracycline incorporated at the site of calcification fluoresces yellow/green when viewed under ultraviolet (UV) light and calcein fluoresces green under UV (Suzuki & Mathews 1966; Kobayashi & Taki 1969; Seeto 1992). Calcein is reportedly less toxic than tetracycline (Wilson et al. 1987; Rowley & McKinnon 1995) and, after trials were conducted to establish if the chemical would be incorporated in the skeletal elements of A. constrictum (Stewart 1996), it was selected as the preferred marker for the present study. The purpose of this study, the first description of skeletal growth bands in a euryalid, was to examine patterns of growth and form in the radial shields of A. constrictum, determine the mechanism of band formation, provide information on the periodicity of the This content downloaded from 157.55.39.111 on Sat, 17 Sep 2016 05:15:37 UTC All use subject to http://about.jstor.org/terms