Growth rates and growth strategy in a clonal marine invertebrate, the Caribbean octocoral Briareum asbestinum

Abstract

Colony form directly effects colony reproductive output among colonial benthic invertebrates. The relationship between reproductive effort, colony form, and growth rate in colonies of the Caribbean octocoral Briareum asbestinum were examined by measuring the growth rates of 118 tagged colonies on Pinnacles and House Reef in the San Blas Islands, Panama. Colony growth rates, individual branch growth rates, and branch addition rates were measured over six month intervals from July 1986 to July 1988. Colonies grew at a net rate of 16.6 cm/year and added 1.2 branches/year. Individual branch measurements yielded a net rate of growth of 2.02 cm/year/branch. Positive growth rates, which provide a measure of minimum potential growth, were 71.3 cm/year, and 8.7 branches/year for colonies and 6.21 cm/year for individual branches. Net growth rates and branch addition rates were 76 and 85% less than potential growth rates indicating large losses to fragmentation and predation. Calculated mean longevity of individual colonies (ramets) is short (10.6 years), given observed rates of growth and loss. However, since fragmented branches can reattach and initiate new colonies, losses due to fragmentation contribute to the asexual expansion of the genet. In July 1987, 783 branches on the tagged colonies were individually mapped in order to characterize the relationship between branch size and the number of bifurcations (tips), and growth rates. The best predictor of growth rate was the number of tips per branch (r2 = 0.46; P < .0001). No relationship was found between branch growth and branch size, consequently branch growth per unit length decreased as branch size increased. The independence of branch growth and size may reflect the diversion of energy away from growth toward reproduction. As a consequence of this pattern, bifurcating growth forms will, on a colony basis, grow more rapidly and amass more reproductive tissue than simple linear extension. However, the short longevity of ramets of Briareum asbestinum limits the advantage to be gained from accumulating reproductive tissue by rapid branch bifurcation.