Effect of cloning rate on fitness-related traits in two marine hydroids

Hydractinia symbiolongicarpus and Podocoryna carnea are colonial marine hyd roids capable of reproducing both sexually and asexually. Asexual reproduct ion, by colony fragmentation, produces a genetic clone of the parent colony . This study examines the effect of very different cloning rates on colony growth rate, oxygen uptake rate. and colony morphology. Colonies of one clo ne of each species were maintained for an extended time in two treatments: in a state of constant vegetative growth by repeated cloning, and in a stat e restricted from vegetative growth (no cloning). For both species, tissue explants taken from the growing colonies grew more slowly than similar expl ants taken from the restricted colonies. For one species, tissue explants f rom the growing colonies used oxygen at a higher rate than similar explants from restricted colonies; for the other species, no difference was detecte d, although the sample size was small. For both species, tissue explants fr om restricted colonies formed more circular, "sheet-like" shapes, whereas t hose from their growing counterparts formed more irregular, "runner-like" s hapes. After these experiments, in the third winter of treatment, all colon ies experienced a severe tissue regression. Within 6 months after this even t, the colonies had regrown to their former sizes. A growth assay at this p oint revealed no difference in growth rate, possibly suggesting an epigenet ic basis for these results, Changes in clonal growth rates and morphology c orrelated with variation in fragmentation rate might affect the ecology of these and other clonal organisms.