Quantitative measures of gastrovascular flow in octocorals and hydroids: toward a comparative biology of transport systems in cnidarians

Abstract

AbstractUsing microscopy, the gastrovascular systems of four hydroids (Eirene viridula, Cordylophora lacustris, Hydractinia symbiolongicarpus, and Podocoryna carnea) and two distantly related alcyonacean octocorals (Acrossota amboinensis and Sarcothelia sp.) were examined and compared within a phylogenetic framework. Despite a range of stolon widths (means 53–160 μm), the hydroid species exhibited similar patterns of gastrovascular flow: sequentially bidirectional flow in the stolons, driven by myoepithelial contractions emanating from the center of the colony. Unlike the hydroids, the gastrovascular system of A. amboinensis (mean stolon widths for 5 colonies, 0.57–1.21 mm) exhibited simultaneously bidirectional flow with incomplete, medial baffles (width 4–20 μm) separating the flow. Baffles visualized with transmission electron microscopy consisted of endoderm, mesoglea, and occasionally another layer of tissue. Mean flow rates of the gastrovascular fluid for seven stolons ranged from 125 to 275 μm s−1, with maximum rates of 225–700 μm s−1. In Sarcothelia sp., stolons were of comparable width (means for 13 colonies 0.55–1.4 mm) to those of A. amboinensis. These stolons, however, were divided by several partitions (width 8–25 μm), both complete and incomplete, which were spaced every 100.5±5.1 μm (mean±SE; range 27.1–283.7 μm) and appeared structurally similar to baffles. In lanes defined by these partitions, ciliary motion was visible in image sequences, and flow was unidirectional. Within a single stolon, flow moved in different directions in different lanes and changed direction by moving from lane to lane via occasional spaces between the partitions. Mean flow rates for 30 stolons ranged from 75 to 475 μm s−1, with maximum rates of 85–775 μm s−1. For both octocorals, flow rates of the gastrovascular fluid were not correlated with the width of the stolon lumen. While octocoral gastrovascular systems probably exhibit differences based on phylogenetic affinities, in all species studied thus far, gastrovascular flow is entirely driven by cilia, in contrast to the hydroid taxa.