Notes on the Biology of Cirripedes

In the course of some 4 years′ work on the biology of fouling organisms, observations have been made on aspects of the general biology of Balanus balanoides, B. crenatus and Verruca stroemia . Observations on the state of development of the ova within the mantle cavity of B. balanoides and of the occurrence of the larvae in the plankton indicate that hatching takes place during the first fortnight in March, that the later-stage nauplii are abundant during the second half of that month and that the cyprid larvae may become abundant during April. The larval sequence is not always completed successfully; conditions which seem inimical are discussed. Factors which seem to be of importance in affecting settlement of B. balanoides are discussed. The presence of other organisms is an adverse factor, though the potency of this factor is probably not the same for all organisms; the presence of slime can prevent settlement and, though evidence can be quoted in support of heavier settlement on shaded surfaces, it is emphasized that the operation of other factors may have brought about this result. The proportion of the tidal cycle during which current speeds are not too high to prevent settlement seems likely to be important. Provided that other conditions are favourable B. balanoides settles readily on surfaces continuously immersed and individuals which settle under these conditions grow more rapidly, at least for the first few months after settlement, than their contemporaries on the shore. The larvae of B. crenatus appear in the plankton rather earlier than those of B. balanoides , but the time taken to complete the larval sequence from the 1st stage nauplius to the cyprid is roughly the same as that for the latter species, namely 1 month. In B. crenatus , however, hatching is not limited to a short period during March but continues at intervals through the summer. There are indications that the successful completion of the larval sequence of this species is less affected by an abundance of phytoplankton than is that of B. balanoides . Settlement of B. crenatus is periodic and individuals which settle early in the spring can attain practically their full size (28–30 mm. basal length) during the same season. Growth during the summer takes place at average rates which vary from 0·1 to 0·2 mm. increase in basal length per day. Growth is negligible during the winter months. Specimens of B. crenatus which settle early in April contain developing ova by the beginning of July, so that spring-settled forms can produce larvae which themselves may settle before the end of the summer. Observations on Verruca stroemia have been limited to the occurrence of the larvae in the plankton. Like those of Balanus crenatus , the larvae of Verruca stroemia are present at intervals through the summer. Early-stage nauplii of this species may dominate the plankton early in the year, but no correspondingly heavy hauls of later-stage nauplii have been recorded. Possible explanations of this discrepancy are discussed. The cyprid never occurs in any numbers in surface hauls. Incidental observations on the planktonic occurrence of the nauplii of Sacculina (probably S. carcini ) and of Peltogaster paguri are given. Details are given of the abundance of all Cirripede larvae in the plankton for the years 1944–47 which emphasize the extent of variation that can occur from year to year.