Competition between Cellana tramoserica (Sowerby) (gastropoda) and Patiriella exigua (Lamarck) (asteroidea), and their influence on algal standing stocks

Caging experiments were undertaken in the field to test if competition occurs within and between the limpet Cellana tramoserica (Sowerby) and the starfish Patiriella exigua (Lamarck). Both species suffer from intraspecific competition, even at natural densities, and increased density reduces body weight and survival in Cellana tramoserica and growth rate in Patiriella exigua. Cellana tramoserica depresses the growth of Patiriella exigua, while the starfish unexpectedly seems beneficial to Cellana tramoserica, increasing its body weight. The difference in these effects may be due to the divergent methods of feeding used by the two species. C. tramoserica digs into the substratum and removes most available microflora, while Patiriella exigua everts its stomach onto the rock face and can only remove superficial or loosely attached microflora and detritus, and leaves a mucous web which may supplement the diet of Cellana tramoserica. Even at low densities C. tramoserica can completely prevent macroalgae from developing, while Patiriella exigua is unable to do so, although it shows the rate of development. Macroalgae grew in all the cages which contained only P. exigua, but in spite of this the starfish still suffered from intraspecific competition, probably because they cannot feed on macroalgae. Cellana tramoserica also competes by interference, and on contact with Patiriella exigua it extends its mantle and pallial tentacles, both of which may release mucus. P. exigua retreats from Cellana tramoserica, and those parts of its body that have been touched by the limpet may temporarily be immobilised and become wrinkled up. It is suggested that this interference behaviour by C. tramoserica is derived from a similar but more intense reaction that it has to invertebrate predators. Patiriella exigua lacks a pelagic larva and does not have a refuge outside the range of Cellana tramoserica. Other factors must thus be sought to explain the continued coexistence of Patiriella exigua with Cellana tramoserica. It is suggested that although C. tramoserica has a competitive effect on Patiriella exigua, it (and other grazers) may be necessary for P. exigua, preventing macroalgal growth and maintaining a surface suitable for P. exigua to feed on.