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The growth and development of colonial hydroids is dependent upon a flow of gastrovascular fluid through a common vasculature. Gastrovascular flow is believed to be driven by polyp contractions, stolonal contractions and endodermal ciliation. In the athecate hydroid Hydractinia symbiolongicarpus we found that polyp contractions, stolonal contractions and gastrovascular flow were only weakly correlated on a local scale. Stolons isolated from vascular continuity to polyps were found to display contraction frequencies indistinguishable from those observed in intact colonies. Transmission electron micrographs reveal a unique organelle, with characteristics of a contractile vacuole, in the apical stolonal endoderm. The activity of this organelle, inferred from static observations, correlates with the observed pattern of stolonal contractions. This is the first record of a contractile vacuole in an eumetazoan animal. Its description calls for a revision of existing interpretations of the mechanism of stolonal contractions in hydroids.

For one common cosmopolitan naidid worm, Stylaria lacustris, we studied the effects of different environmental factors upon (1) the alternation of reproductive modes, (2) the rates of population increase, and (3) the combination of each of (1) and (2). While age, temperature, population density, or rate of feeding did not affect the mode of reproduction, photoperiod had a dominant effect. Under long-day conditions (LD > 12: 12), all worms reproduced exclusively by paratomic fission, theoretically ad infinitum. When transferred to short-day conditions (LD {le} 12:12) the worms ceased vegetative reproduction, and within 2 to 4 weeks developed the hermaphroditic genital apparatus and a clitellum. After an additional two weeks, the first cocoons were produced. The switch to the bisexual mode of reproduction was cum grano salis irreversible. These findings are consistent with observations of field samplings, and allow one to predict the annual life-cycle strategy of S. lacustris. This is the first example of a photoperiod determined life-cycle within the oligochaete worms. The vegetative mode of reproduction led to extremely high rates of population increase, whereas with the bisexual mode of reproduction the number of individuals was roughly stable. However, because S. lacustris could not withstand temperatures of 5{deg}C or lower, the switch to sexual reproduction and the formation of diapausing cocoons appear to be the only mechanism of overwintering. Nevertheless, some `asexual' clones never switch to sexual reproduction, whereas a loss of the asexual vegetative mode of reproduction did not occur. In contrast to some general predictions from life-history theories, the reproductive strategy of S. lacustris is highly preprogrammed and cannot respond to sudden and unexpected environmental changes.

Many aquatic invertebrates possess complex life cycles, which include a switch from a vegetative to a sexual mode of reproduction. While sexual reproduction is usually linked to slow rates of propagation, vegetative reproduction leads to high rates of clonal propagation and a fast increase in population size. I have attempted to identify developmental genes which are differentially expressed between sexually and vegetatively reproducing individuals. Quantitative assays of the expression of those diagnostic genes could be applied to field samples to gain information on the reproductive status of the sample. Here I outline a general strategy for identifying and testing the usefulness of homeobox genes as candidates for regulatory genes whose expression patterns are indicative of a sexual or vegetative developmental phase.

The phylogenetic relationships of the Recent cnidarian classes remain one of the classic problems in invertebrate zoology. We survey the structure of the mitochondrial genome in representatives of the four extant cnidarian classes and in the phylum Ctenophora. We find that all anthozoan species tested possess mtDNA in the form of circular molecules, whereas all scyphozoan, cubozoan, and hydrozoan species tested display mtDNA in the form of linear molecules. Because ctenophore and all other known metazoan mtDNA is circular, the shared occurrence of linear mtDNA in three of the four cnidarian classes suggests a basal position for the Anthozoa within the phylum.