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Focuses on a study which demonstrated that primitive germ cell and somatic cell lineages in chimeric protochordates have traits that also make them likely units of natural selection. Methodology of the study; Results and discussion; Conclusions.

Discloses that both somatic cell and germ-line parasitism are a common occurrence in fused chimeras of the colonial tunicate, Botryllus schlosseri. Laboratory colony fusions of somatic chimerism; Germ line parasitism; Field chimeras; Analysis of adjacent unfused field colonies.

The relative importance of pre-settlement and post-settlement processes in limiting recruitment rates of the colonial ascidian Diplosoma similis was established through (1) direct observations of larval dispersal, (2) comparison of pre-settlement and post-settlement mortality rates, and (3) determination of whether larval settlement rates limit the areal coverage of adults. During a 1-5 min planktonic larval phase most larvae swam 0-4 m, usually in the direction of the reef, and settled at a depth less than or equal to that of the parent colony. Between larval release and settlement, benthic and planktonic mortality sources killed 41-47% of the larvae, while at least 90% of all newly settled colonies died within 1 mo post-settlement. The larval settlement rate was greater than 4600 larvae. m-2°yr-1, which in the absence of growth, was too low to account for the percent cover of adults. However, since the annual supply of recruits would occupy >40% of the space normally inhabited by potentially long-lived adults, it seems unlikely that larval settlement rates limit the percent cover of this ascidian. These results suggest that larval habitat selection determines the vertical zonation of recruits and post-settlement mortality determines the density of juvenile colonies. [ABSTRACT FROM AUTHOR]