Gastrovascular Flow and Colony Development in Two Colonial Hydroids

Using field-collected Podocoryne carnea and Hydractinia symbiolongicarpus, ten colonies of each species were produced by sexual crosses and grown from primary polyps to sexual maturity. At comparable stages in the life history, each colony's morphology was measured using digital image analysis, and each colony's gastrovascular flow to three peripheral stolon tips was recorded using video microscopy. Gastrovascular flow to peripheral stolon tips shows very different patterns in the two species. H. symbiolongicarpus exhibits maximal flow as a primary polyp. Both the mean and the variance of the flow rate decline by the time of stolonal mat formation. When covering the substratum, gastrovascular flow is low and somewhat more variable than at the time of stolonal mat formation. On the other hand, P. carnea exhibits minimal flow as a primary polyp. At subsequent stages, flow increases monotonically. Thus at the primary polyp stage, H. symbiolongicarpus exhibits a greater rate of flow to peripheral stolon tips than P. carnea. The reverse is true at subsequent stages. In general, these results support the hypothesis that higher rates of gastrovascular flow produce runner-like colonies, while lower rates of flow produce sheet-like colonies.