Commensal copepods were found abundantly in samples of the ascidian Molgula complanata Alder and Hancock taken from experimental panels for histological examinations (see Schmidt, 1982). As little data exists on the possible ecological importance of such association to the ascidian host (Gotto,1979 for review), the incidence of‘ infection’ was quantified in the present study over a period of 8 months.

Between 31 October 1980 and 3 June 1981 specimens ofM. complanata( 6 mm maximum diameter) were removed from panels suspended from a raft in Langstone Harbour, Hampshire; 50–125 individuals per sample. Associated copepods found in the branchial basket were:Notodelphys agilis Thorell,Botryllophilus macropus Canu, andDoropygus pulex Thorell* (cf. fig. 2 in Schmidt, 1982). The former two species occurred sporadically, but the latter consistently. Owing to the relatively large, globular size ofD. pulex (caused by very inflated brood pouches), it almost filled the space provided by the branchial basket of smallM. complanta. Hence,D. pulex was found onlyone per ascidian, although 1–3 juveniles (several copepodid stages) were often present with the adult.

On 31 October 1980,D. pulex was present in 64% of 115 animals examined. It is interesting to note that, despite such abundance,D. pulex has not been reported previously in association withM. complanata (Gotto, i960). On 28 November 1980, the incidence of ‘infection’ was still high (62 %) but thereafter declined (14 January 1981: 35 %; 10 February 1981: 34 %; 11 March 1981: 42 %) to a value of 13 % by 8 April 1981, around which it stayed until the end of sampling (3 June 1981).