Haemolymph Pressures in Barnacles: A Comparative Study

Haemolymph pressures have been measured for several Australasian thoracican barnacles. The list included six lepadomorphs and four balanomorphs, with particular interest centred on Catomerus polymerus, a primitive chthamaloid in which certain lepadomorph features are vestigially retained. The results were incorporated with those from the literature to present a more comprehensive data set for thoracicans. In general, lepadomorphs have higher haemolymph pressures than balanomorphs and their maximum pressures are sustainable. Lepadomorphs with high absolute pressures (range, 38-350 cm H₂O) were all intertidal species from exposed shores, highly armoured in both capitulum and stalk, with few haemolymph spaces in the stalk. These animals exhibited limited stalk elongation/shortening and bending. The other lepadomorphs were less well armoured, their peduncular cuticle was not reinforced and there were many haemolymph spaces interspersed amongst the peduncular tissues. Much lower absolute pressures were generated (range, 6-116 cm H₂O) and these animals exhibited considerable elongation/contraction or bending (or both) of the stalk. C. polymerus has reasonably high haemolymph pressures, which overlap with those measured in some of the lepadomorphs. This chthamaloid has a low haemolymph volume, most of the available space enclosed by the wall plates and basis being occupied by tissue. C. polymerus generated very high maximum pressures, which were sustainable. This balanomorph, retaining evidence of scalpelloid ancestry, is adapted to survive in the high intertidal pounded by surf. Tesseropora rosea, another surf barnacle of a more advanced balanomorph form, also has high maximum pressure, although lower than that of C. polymerus. It too has low haemolymph volume. The remaining balanomorphs examined contain significant haemolymph volume, exhibited low absolute pressures (range, 3-19 cm H₂O) and, except for Balanus hameri, non-sustainable maximum pressures. The implications of an expansive haemolymph system and low absolute pressures in balanomorph evolution are discussed.