Video Microscopical Measures of Gastrovascular Flow in Colonial Hydroids

Abstract

Measures of internal fluid flow have the potential to illuminate energy allocation and life-history variation in colonial invertebrates. Using video microscopy and digital image analysis, fluid flow was characterized in hydractiniid hydroids at three regions of the dead end stolons located at the periphery of colonies. Outflow begins at an initially high rate (range 50-800 jLm/s). As the stolon fills, the outflow slows and then stops, and a period of backflow (<100 s), having a relatively low initial rate, follows. Backflow rate increases (less than or equal to outflow rate) as the stolon contracts. The cycle ends with the stolon lumen fully closed and the flow rate again equal to zero. A proximal-distal flow rate gradient is apparent over the 3 regions examined along peripheral stolons. Comparisons of peripheral stolons over several contraction cycles show that Hydractinia symbiolongicarpus has shorter and wider stolons, wider lumen diameters, shorter flow-cycle periods, and similar flow rates relative to Podocoryne carnea. In H. symbiolongicarpus, a large portion of the variation in stolon length correlates with the examined flow parameters. Surprisingly, this is not the case in P. carnea. Further, the generally larger lumen diameters of H. symbiolongicarpus are not associated with higher flow rates as would be expected if pressure and tissue resistance is similar between the species. This suggests that these hydroids may regulate fluid flow at different magnitudes, or in accordance with their distinct physiological optimums, or both. Additional key words: clonal, Cnidaria, Hydrozoa, Hydractinia, Podocoryne, image analysis Great insight into invertebrate biology has been gained by visualizing and measuring fluid flow at low Reynolds (Re) numbers (e.g., Rubenstein & Koehl 1977; Koehl & Strickler 1981; LaBarbera 1984; Loudon et al. 1994). Such analyses have helped to elucidate methods of food capture, movement, and other environmental interactions in animals that function at low Re. Curiously, despite the possibility of similar insight, fluid movements within organisms have been largely ignored. Particularly in colonial animals, in vivo analysis of fluid flow within a colony may help to illuminate how the colony allocates energy and regulates development and life history, and the extent to which physical factors govern these functions. For instance, a colony with vigorous flow to peripheral areas suggests an energy allocation regime and a complement of life-history traits different from that of a colony with sluggish flow to peripheral areas and vigorous flow in central areas (e.g., runner vs. sheet, see Buss & Blackstone 1991). Recently, video microscopy has made possible studies ranging from the cellular level (Tanner & McAteer a Author with whom to correspond. 1991; Karnaky et al. 1992; Bohrmann & Biber 1994) to the organismal level (Boraas et al. 1992; Wetherbee & Andersen 1992; Hoppin et al. 1994). Nevertheless, relatively few of such studies focus on organismal physiology in vivo (but see Colmorgen & Paul 1995; Hudetz et al. 1995). It is largely the technical challenges of these studies that have prevented their application. Our technique combines aspects of video microscopy and digital image analysis (see Rohlf & Bookstein 1990) in measures of internal fluid flow, with particular relevance to the physiology of colonial organisms, e.g., hydroids, bryozoans, kamptozoans (entoprocts), ascidians, and pterobranchs. This work provides another aspect of the general optophysiology of Paul (see Colmorgen & Paul 1995). We present experimental results obtained by this method from two species of colonial hydroids.