Seasonal Changes in a Population of Carcinus maenas (L.) in the Littoral Zone

Abstract

In previous work it has been shown that Carcinus maenas (L.) exhibits rhythmic patterns of locomotor activity with periods of greatest activity coinciding with the times of high tide and with the hours of darkness (Naylor 1958, 1960). During those experiments it seemed worth while to relate laboratory observations to observed behaviour in the field. Edwards (1958) marked and recaptured Carcinus during successive periods of low tide and showed that there was a rapid migration up and down the shore in which the majority of large crabs took part, but those experiments were carried out only in the summer months and included no sampling of crabs at high tide. In the present investigation an attempt was made to determine the abundance of Carcinus in the intertidal zone at both high and low tide in all months of the year. Specimens were obtained at low tide by turning stones, and high tide by setting baited creels from a pier which traversed the tidal zone. A particularly striking feature of the present results was a fall in numbers of crabs collected between tidemarks at low tide during the winter months. The phenomenon was briefly mentioned by Edwards (1958) who interpreted it as indicating a winter off-shore migration in Carcinus similar to that which has been described for Cancer pagurus L. (Williamson 1900, Meek 1913). However, in the present investigation there seemed to be no complete off-shore migration since it proved possible to obtain Carcinus in creels at high tide in the intertidal zone in at least some of the winter months, when specimens were normally absent from the shore at low tide. These extensive collections therefore permit a fairly full discussion of the tidal and seasonal migratory behaviour of Careinus. The samples also give some information on seasonal changes in size composition and on the breeding biology of a shore crab population.