The energetics of the brackish water serpulid polychaete Mercierella enigmatica Fauvel

Abstract

Investigations of the distribution of Mercierella enigmatica and the physical environment in the Thames estuary form the basis for a detailed laboratory study of the energy relations of this fouling organism. It was found that Mercierella's intertidal distribution is positively correlated with that of the surface water at Greenhithe, which effectively buffers the animal against extremes in oxygen tension, salinity, and temperature, whilst allowing unlimited feeding.

The following components of the energy budget under winter and summer conditions were investigated: particulate feeding; the integumentary uptake of dissolved organic compounds; egestion; respiration; nitrogenous excretion; somatic growth; reproduction; tube production; and mucus production. The flow of energy through the various channels was quantified using microbomb calorimetry, biochemical techniques, respirometry (a volumetric micro-respirometer was designed specifically for this purpose), and radioactive tracers.

A comprehensive investigation of the uptake kinetics, effects of endogenous and exogenous factors, and the fate of absorbed organic molecules, shows that these are apparently of little value as a supplementary nutritional source. An alternative hypothesis is proposed for the role of exogenous dissolved organic molecules (DOM) in Mercierella's energy budget.

The oxygenational properties of the blood pigment were investigated, and the relative significance of the potential respiratory surfaces during the different phases of activity are estimated based on histological evidence. In common with the majority of chlorocruorins, the blood pigment has a high P50 and is incompletely saturated when in equilibrium with air. The branchial crown is the major surface for gaseous exchange when it is extended, whereas the crown and rest of the body function as separate systems when the worm withdraws inside its tube.

The changes that occur in the gonadial tissues, coelomic cells, and population during the annual reproductive cycle were investigated, and it was found that the Greenhithe population is composed of males, females, and protandrous hermaphrodite individuals. Although the latter are a means by which the energy expended in reproduction by young individuals is reduced, whilst maximising their potential contribution to the population's reproductive output, it is shown that these are of no apparent value in temperate localities.

An important outcome of this study is the discovery that mucus production and respiration are the two major pathways through which energy leaves the body of the worm. These are followed by tube production, somatic growth, reproduction, and nitrogenous excretion, in decreasing order of magnitude. These results are compared and discussed in relation to published values for other aquatic invertebrates.