CHELAL MORPHOMETRY, PREY-SIZE SELECTION AND AGGRESSIVE COMPETITION IN GREEN AND RED FORMS OF CARCINUS-MAENAS (L)

Carcinus maenas exhibits a range of adult colour forms, from green to red. Coloration is at least partially correlated with the length of intermoult. There are no external morphometric differences between colour forms, but red crabs have a greater chelal dry mass per unit chelal size, shown particularly in the chelal integument, apodemes, and to a lesser extent the closer muscle. Sarcomere length of the closer muscle of the crusher chela is not significantly different in the two colour forms. Strain gauges mounted in a model mussel showed that red crabs, of ≈21 mm chelal height, exerted a mean force of 21.5 N and green crabs, of similar size, 16.7 N. Red crabs exerted a greater proportion of slow chelal contractions and maintained them for longer than green crabs. Red crabs preferred larger mussels and usually dominated green crabs in aggressive disputes over prey. The wider range of potential prey and aggressive dominance of the red crabs probably places them at a competitive advantage over the green forms, which find refuge in marginal habitats, such as high shore-levels and estuaries. © 1990.