The tissues of articulate brachiopods and their value to predators
The punctate brachiopods Liothyrella neozelanica and Neothyris lenticularis were compared with the impunctate Notosaria nigricans and the bivalve molluscs Mytilus edulis and Lima colorata in terms of the force needed to crush the shells, the time needed to drill through shells by a standard apparatus, the organic content of the internal tissues (ash-free dry mass, AFDM), the tissue density (AFDM per unit volume between the shell valves) and the inorganic content of the internal tissues and whole animals. At one site M. edulis needed greater force to fracture its shell ( x 3.5), more time to drill through the shell ( x 10.8) and contained more AFDM ( x 6.2) than the brachiopod L. neozelanica. At a second site there was no significant difference between the brachiopods N. lenticularis and N. nigricans and the bivalve L. colorata in terms of force needed to crush the shells, but the bivalve was x 2 to x 30 harder to drill through and contained 10.7 times as much tissue AFDM as N. lenticularis and 4.2 times as much as N. nigricans. The impunctate N. nigricans had 2.6 times as much AFDM in its internal tissue as the punctate N. lenticularis. Cost-benefit ratios showed neither group to be consistently better prey items for predators in terms of reward (AFDM available) per unit effort. The punctate brachiopods had the lowest tissue densities (AFDM per internal volume) with values of 5-8 mg cm-3, the impunctate N. nigricans was next at 12-16 mg cm-3 and the bivalve molluscs were highest with values of 45-120 mg cm-3. Tissue inorganic contents were also markedly different. The punctate brachiopods had values of 47.3% (N. lenticularis) and 56.2% (L. neozelanica), whereas the figure for the impunctate N. nigricans was 30.2%, and the bivalves were much lower at 12.8% (M .edulis) and 14.2% (L. colorata). These results suggest that predation pressures may be low on articulate brachiopods because of the very low tissue density and high inorganic content (because of the presence of spicules) of the internal tissues, both of which are characteristics which are not consistent with the possession of potent chemical defences.