Body volumes and internal space constraints in articulate brachiopods
Brachiopods were once dominant in all the oceans of the world. but their distributions are non more restricted. There are few species which are found in shallow warm habitats and these are predominantly small. They have exceptionally low metabolic rates and exhibit low energy lifestyles. The majority of living articulate brachiopods are punctate (possessing mantle extensions. or caeca. which traverse the shell). Evidence produced hei-e suggests that the evolution of these phenomena may have been strongly affected by architectural constraints placed on articulate brachiopods by the use of the lophophore for feeding and respiration. They are essentially space limited because of the large volume needed for this organ. In some punctate brachiopods over 75% of their total body volume may be occupied by the lophophore and mantle cavity. This figure is only 60% in an impunctate (no caeca) species and may be only 20% in bivalve molluses. The implications are that caeca evolved to reduce pressure on space requirements, that maximum sizes may be set by the scaling patterns of space allocation and metabolic efficiency is a consequence of space constraints. Current distribution patterns may be strongly affected by the low metabolism and low energy lifestyles. The relative success of small brachiopods in warm shallow seas may have been facilitated by the scaling patterns of space allocations which show small specimens to have similar mantle cavity volumes to bivalve molluscs.