Aspects of the physiology, biology and ecology of thalassinidean shrimps in relation to their burrow environment

This paper examines the burrow environment of thalassinidean shrimps (commonly called mud shrimps), drawing on our experience of a range of U.K. species with differing lifestyles (Calocaris macandreae, Jaxea nocturna, Callianassa subterranea, Upogebia stellata, U. deltaura) and makes comparisons with published work on a diversity of species elsewhere. Information on thalassinidean ecology and burrow structure is used, together with measurements of physicochemical conditions, to illustrate the range of conditions which thalassinideans may experience within their burrows, where conditions may be potentially hostile (hypoxic, hypercapnic, high in sulphide). Behavioural and physiological adaptations to the thalassinidean burrow-dwelling lifestyle are considered, particularly those that relate to survival in hypoxic and sulphidic conditions. Mud shrimps actively irrigate their burrows by pleopod beating, this often being intermittent: burrow irrigation is most intense in suspension-feeding species. Passive irrigation of burrows may also occur, generated by current flow at the plane of the mud surface. Thalassinideans spend progressively more time irrigating their burrows during hypoxia, but such activity is often not pronounced until the oxygen partial pressure of the water has declined to low levels. The shrimps are highly tolerant of hypoxia and are able to maintain aerobic metabolism down to very low oxygen partial pressures. Burrow water is pumped across the gills by the scaphognathites and their activity increases under hypoxia, thus maintaining a supply of oxygen to the gills. Rates of oxygen consumption are lower in thalassinideans than in non-burrowing decapods. The haemocyanins of thalassinideans have high oxygen affinities and have moderate Bohr values that facilitate oxygen uptake during hypoxia. When aerobic respiration can no longer be maintained, thalassinideans resort to anaerobic respiration and show a high tolerance of anoxia, with some species being able to survive anoxic conditions for several days. Thalassinideans may encounter high concentrations of sulphide as a result of their fossorial lifestyle and have a high tolerance to sulphide exposure, oxidising sulphide to thiosulphate. Relationships are explored between mud-shrimp activity, including feeding, burrow structure and burrow physicochemistry. For most mud shrimps the burrow is the feeding environment. Food is extracted either from the burrow water, or the burrow wall, or from macroscopic material accumulated in the burrow lumen. Microorganisms are important in decomposition processes within the burrow, in burrow geochemistry and in mud-shrimp nutrition. Interrelationships between the burrow environment and the wider environment are also considered. The irrigatory activity of mud shrimps not only introduces oxygenated conditions into the sediment column, but also exports nutrients to the overlying water. Burrow walls may act as sinks for trace metals, radionuclides and other chemicals and are important loci for diagenetic processes. Various burrow associates share the burrow environment with thalassinideans and some of the implications of such associations are discussed briefly. Throughout, various topics requiring further research are identified.