On the Biology of Sagitta. III. A Further Observation on the Growth and Breeding of Sagitta setosa in the Plymouth Area

Extract

In a previous paper an account was given of the breeding and growth of Sagitta setosa throughout a year (2). The results appeared to show that the last spawning took place in October and the resulting offspring lived through November and December without maturing and that by February there were indications that a number were spawning. At the end of the following April and beginning of May a complete spawning population was formed. Owing to the large size at which these matured it was assumed (2, p. 148) that they had been the offspring of adults spawning in February. The true trend of events was however by no means clear as the increase in average size from January to May was very even (2, Plate II). There was a distinct difference at this period between S. elegans and S. setosa. In S. elegans a spawning population of individuals all about the same length was produced in February which apparently gave rise to a brood which had grown to a large size by April and May, and the change in size from February to May was much more abrupt than in S. setosa (1, Plate I). It was clear that while the February curve for S. elegans showed very little variation in size for the majority of the population caught, the population in May tended to be very widely dispersed. In S. setosa, on the other hand, there was a gradual narrowing down of the dispersion until a spawning population with little variation in length was produced by May.