Patterns of shell repair in articulate brachiopods indicate size constitutes a refuge from predation

The cost of overcoming prey defenses relative to the value of internal tissues is a key criterion in predator/prey interactions. Optimal foraging theory predicts: (1) specific sizes of prey will result in the best returns to predators, and (2) there will often be a size at which the cost/benefit balance is low enough to effectively exclude predation. Data presented here on styles of repaired shell damage and size at which injury had been sustained was collected from samples of terebratulide brachiopods from the Antarctic Peninisula (Liothyrella uva), Falkland Islands (Magellania venosa and Terebratella dorsata) and Chile (M. venosa). The predominant form of damage on shells was indicative of predators attacking the valve margins. The modal size for repaired damage was more than 10 mm smaller than the modal size for the overall size distribution in each species and there were no repaired attacks in the largest size classes of any species. These data suggest that size forms a refuge from predation, as would be predicted by optimal foraging theory. The optimal sizes that predators appeared to attack vary between species, as do the sizes that provided a refuge from predation. High levels of multiple repairs (19% of the M. venosa population from the Falkland Islands sampled had 2 or more repairs) suggest that the mortality following attack is low, suggesting that many predators abandon their attacks

Predation on two brachiopods, Joania cordata and Argyrotheca cuneata, from an offshore reef in the Tyrrhenian Sea

Predator holes in empty shells of Joania cordata and Argyrotheca cuneata (Brachiopoda: Megathyrididae) collected in the marine protected area ‘‘Secche di Tor Paterno’’, central Tyrrhenian Sea, Italy (41350N– 12200E, at depths of 20–28 m), were analyzed. Predation intensity was low but appreciable, with the more common species J. cordata preyed on more frequently (6.7 %) than A. cuneata (3.8 %). Three main types of holes were recognized: (1) cylindrical drill holes with a circular outline, (2) larger irregular holes with a jagged outline, and (3) small holes at the bottom of depressions in the shell. They were probably produced by muricid gastropods, crabs, and Foraminifera, respectively. The large, irregular holes were the most common type in both brachiopod species. Evidence for predator selectivity with respect to which valve, the position of the hole on the valve, and the size of the brachiopod with respect to those available was assessed. The ventral valve, the postero-medial portions of both valves, and larger (J. cordata) or medium-sized (A. cuneata) shells were more frequently holed