BORN TO LOSE .1. MEASURES OF TISSUE LOSS AND REGENERATION BY THE BRITTLESTAR MICROPHIOPHOLIS-GRACILLIMA (ECHINODERMATA, OPHIUROIDEA)

To measure amounts of tissue lost in natural populations of the burrow ing amphiurid ophiuroid Microphiopholis gracillima (Stimpson), individ uals were collected from subtidal mud flats in North Inlet, South Caro lina, USA, at monthly intervals between February 1985-February 1987 an d December 1989-November 1990. Between 20 and 70% of all individuals w ere regenerating the disc, and 85% of the 2045 arms examined had regen eration scars; >50% had one scar and some arms had up to 4 scars. Fewe r individuals were regenerating discs in warmer months, but there was no seasonality in arm-loss frequency. To quantify rates of arm regener ation in the field, individuals which had 1, 2, or 3 arms removed were placed in mud-filled cores in the field in late July and November 198 8 and in March and May 1989, and recovered after periods of about one month. Another set of cores was held in a running seawater laboratory during the May 1989 experiment. No regeneration occurred during the co oler times of year (November and March), and rates of regeneration wer e slower in May (total: 0.13 mg/d; tissue: 0.03 mg/d) than July (total : 0.17 mg/d; tissue: 0.05 mg/d). These rates indicate complete replace ment of lost tissue in 100 to 120 d during the growth season. Within e xperiments, per arm regeneration rates were similar regardless of the number of arms removed. This finding is complicated by small sample si ze, high variability and low statistical power, but in general individ uals which lost 2 or 3 arms regenerated proportionally more tissue tha n individuals which lost 1 arm. Individuals held in the laboratory reg enerated the same amount of tissue but 30% less skeleton than individu als in the field. Sublethal tissue loss is common in this population, and M. gracillima is capable of regenerating at least 50% (each arm = 17% of total body weight x 3) of its standing crop in a single growing season. Burrowing brittlestars probably constitute a significant rene wable energy source for higher trophic levels in areas where they occu r in dense populations.