Feeding mechanisms of the solitary ascidian, Ascidia paratropa (Tunicata, Ascidiacea)
Date
1985
Authors
Pennachetti, Catherine Anne
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Abstract
I studied factors controlling variations in filter-feeding mechanisms of Ascidia paratropa. The feeding basket consists of two adjacent and connected gill screens. The outer (stigmatal) screen is loosely folded and the inner screen is composed of ciliated longitudinal vessels and primary and secondary papillae (SEM). The endostyle continuously produces a net with pore dimensions of approximately 0.6 X 0.8 µm (TEM) and with elastic and adhesive properties. Light microscopy shows that it is usually supported by and transported across the inner gill screen, but can also be moved by localized muscular activity of the dorsal laminar fold and longitudinal vessels.
Thermistor flow probe recordings in a laminar flow tank show that A. paratropa pumps water through the branchial basket at a constant rate while morphological and environmental parameters are constant. Water follows distinct and predictable laminar pathways through the basket, as shown by fluorescent dye studies. However, constant pumping activity is frequently interrupted by partial contractions of one or both siphons and by squirts. Partial contractions result in transient changes in the speed of water flow while squirts involve momentary arrests of current flow. Light microscopy shows that during squirts the net is transported as a flat sheet, independent of the inner gill screen, and that production of the mucous net is not interrupted. Squirts appear to serve multiple functions. Sometimes they are associated with specific events such as defecation or disturbance; however, most often they appear to be spontaneous.
A. paratropa occurs in two forms (Canadian and American), which occur in different environments. Although the forms possess similar feeding mechanisms, the frequency of partial contractions and squirts is different in the two forms. For example, siphon activity is always more frequent in Canadian vs comparable American individuals. Environmental factors also affect the frequency of activities of this species. For example, siphon activity is more frequent under low vs high ambient current, and at very low and high concentrations of suspended particles. On the other hand, changes in orientation and concentration of food particles do not have an obvious effect on the frequency of siphon activity, but do affect flow rate. Changes of long duration in siphon shape, diameter and orientation are also common, but are difficult to quantify. It appears that the food collecting habits of the two forms of A. paratropa differ, and that both forms use behaviour to affect portions of their feeding mechanisms.
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UN SDG 6: Clean Water and Sanitation