A Contribution to the Study of Relative Growth of Parts in Inachus Dorsettensis

Abstract

ABSTRACT The relative growth-rate of the various parts of the body was investigated by analysis of linear measurements carried out on the carapace, appendages, and abdomen. The results were as follows: (a) The chelar propus shows strong positive heterogony in the 3 and very slight positive heterogony in the ♀. The ♂ chela shows dimorphism, the ♂♂ being differentiated into “high” and “low” forms and the dimorphism in all probability being due to the fact that the ♂ chela assumes the ♀ type of growth to a greater or less extent in the non-breeding season. (b) The pereiopods are more positively heterogonic in the ♂ than the ♀ and in both ♂ and ♀ there is a graded k series, but whereas in the ♂ k increases from P1—P4, in the ♀ the series is reversed k being greatest for P1. In both sexes the heterogony in the pereiopods is not so marked as that of the chelar propus. In the 3 the pereiopods suffer an actual decrease in absolute size at the time when the relative growth-rate is least for the chelar propus and after this period never again attain to their original relative size. (c) The third maxilliped is negatively heterogonic in both sexes and slightly more so in the ♂ than the ♀, although this difference may not be significant. (d) The abdomen in the ♀ shows marked positive heterogony in young crabs but is isogonic after the attainment of sexual maturity; in the ♂ the abdomen is isogonic in young crabs but becomes slightly negatively heterogonic in old animals. The abdomen in the ♀ is dimorphic, the “Tow” type (characteristic of the adolescent crab) being separated from the “high “type (characteristic of adult crab) by a single moult. There is considerable variation in the relative abdominal growth-rate in adolescent crabs and consequently in the time of attainment of sexual maturity and the full relative abdominal width. During the period of adolescence the increase in relative length and breadth is the same for all segments except the 5th, which forms a growth-centre for length. At the period of rapid conversion of the “low” type of abdomen into the “high” the 5th segment remains the growth-centre for length but a growth-centre for breadth is established in the 6th segment, and increase in relative breadth decreases from the distal to the more proximal segments. A comparison of the ♂ and ♀ as regards relative growth of parts was made and brought to light certain facts. All the pereiopods are relatively longer in the ♂ than the ♀. This is a graded effect—the actual difference in relative length (in ♂ and ♀) decreasing from P1—P4, but the increase in relative length relative to the actual size of the pereiopods increasing from P1—P4. These facts are interpreted as meaning that there is a common stimulating effect in the ♂, and also a retarding effect of the ♂ chela on the appendages posterior to it. It is tentatively suggested that the facts relating to the 3rd maxilliped are explained if we assume that the 3 chela has the same retarding effect on the anterior appendages as on those posterior to it. Certain general conclusions were drawn : (a) There is a different distribution of growth-potential in the two sexes, and strong positive heterogony is found associated with the development of the secondary sexual characters. (b) The results on the relative growth-rates of the appendages in ♂ and ♀ and on the growth-rates of the individual segments of the ♀ abdomen, indicate the presence of definite gradients in the body, in relation to which growth takes place. (c) The growth-centre for the heterogonic organs is situated towards the distal end of the organ; this is in agreement with results obtained for other Crustacea.