ON THE HEART OF THE ORANGE TUNICATE ECTEINASCIDIA TURBINATA HERDMAN

¹ Department of Entomology, University of Maryland, College Park, Maryland 20742 and the Bermuda Biological Station for Research, St. George's West, Bermuda

1. The non-chambered tubular heart of the colonial ascidian Ecteinascidia turbinata is made up of a single layer of very long, thin, cross-striated and spirally-wrapped muscles. Enclosed within a large non-contractile pericardial sac, the heart and pericardium are innervated by a fine non-ganglionated network. The channels and vessels through which hemolymph circulates are non-contractile.

2. In recently captured, intact, and healthy-looking tunicates over a 5 year period, the heart exhibited either prolonged diastolic arrest, prolonged highly erratic (arrhythmic) beating, or fairly rhythmical but highly variable beating with periodic reversals in beat direction. No significant differences were found in contraction rates originating from either pole of the heart.

3. The heart begins to beat and reverse in the embryo and reversals continue throughout life. Synchronous reversals were not observed either between embryos from the same mother or between two or more attached individuals.

4. The extremely variable character of rhythmical heart beating involves both the number and duration of complete advisceral and abvisceral cycles. The variability is not due to (a) developmental stage (embryo, tadpole, mature individuals), (b) body length, (c) time in sea table after captivity, or (d) amount of sea water in examination chamber.

5. The great variability of heart rhythmicity in intact animals diminishes markedly within the first hour after isolation, if the tunicates are left undisturbed. The heart is capable of beating and reversing with remarkable uniformity for long periods both in unchanged and in constantly changing sea water in small chambers. Simple manipulation of intact tunicates can readily and quickly induce renewed and extreme cardiac variability.

6. Pressing externally directly over the anterior (advisceral) pole of the heart or over the center of the heart shortly after the beginning of either the advisceral or abvisceral cycle significantly increases the rate of heart reversal.

7. Hearts continue to beat and reverse for a long time after both siphons (including the "brains") are removed. When the pericardium alone of the heart itself is punctured with a fine needle or cut with scissors, the heart always stops instantly in diastole for variable periods of time. The two halves of a centrally-severed heart are capable of reversing. Heart reversals were observed after puncturing small portions of the anterior and posterior poles of the heart. When the anterior pole was completely ablated, heart beats were observed only from the posterior (abvisceral) pole. Semi-isolated hearts were never observed to beat normally and rhythmically.