J. Fish zyxwvut Biol. zyxwvutsr (1 989) 35,599-601 zyxwvu Relationship between host size of zyxw Anguilla anguilla and the infection level of the monogeneans Pseudodactylogyrus SPP* K. BUCHMANN Department of Hygiene and Microbiology, Royal Veterinary and Agricultural University, 13 Biilowsvej, DK-1870 Frederiksberg C., Denmark (Received 26 May 1989, Accepted4 July 1989) Key words: Pseudodactylogyrus; eels; infection level; host size In ecological studies of monogenean infestations of fish little attention has been paid to the importance of the host size. However, Frankland (1955) and Paperna et zyx al. (1984) provided evidence of a positive correlation between host size and monogenean parasitization level, and the present paper reports on this subject in Anguilla anguilla infected by the gill- parasitic monogeneans Pseudodactylogyrus spp. The study was conducted in the commercial eel-culture system described by Buchmann (1988~). On two occasions (experiments 1 and 2) small pigmented eels (total length 74-12.5 cm) parasitized only by adult parasites (prevalence 50%, abundance zy 1-1) were placed for 7 days in a fish tank with oncomiracidia present, after which 27 of the eels in each experiment were examined for post-larvae of Pseudodactylogyrus spp. zyx as described by Buchmann (1988~). The relationships between body length of eels and number of post- larvae recorded on the gills is shown in Fig. I(a), (b). On a third occasion larger eels (total length 164-25.5 cm) infected only by adult parasites (prevalence 45%, abundance 1 zy .O) were likewise subjected to infection in the fish tank, but for 3 weeks, after which the total number of parasites (post-larvae and adults) were recorded in 30 eels [Fig. l(c)]. In all three cases a positive significant correlation (P< 0.01) between total body length and parasite load was recorded (Fig. 1). A similar association has been recorded in Gadus virens infected by Diclidophora denticulata (Frankland, 1955) and Liza carinata infected by Benedenia monticelli (Paperna et al., 1984). Paperna (1963), Prost (1963) and Molnar (1971) focussed on the importance of fry gill differentiation for the susceptibility to monogenean infection. However, as was shown by Buchmann (19886) even the earliest stage of eel used for commercial cultivation (glass-eel) is susceptible to infection, and other factors must cause the increase in infection level after short-term exposure in larger eels. A positive correlation exists between fish length and gill surface (Hufhes; 1966), and Gray (1954) found that the gill surface of Anguilla rostrata was 302 mm g fish or 193 mm' cmP2 fish surface. Thus, larger fish will provide larger attachment space for parasite larvae. The importance of the total gill surface for the maximum number of gill parasites able to settle on the gills is also reflected by the present author's unpub- lished studies on heavily infected moribund eels: about 200 Pseudodactylogyrus parasites covered the total gill surface in small pigmented eels (10-12cm, 1-2g) whereas more than 1000 adult parasites have been recorded on gills of larger eels (40cm, 90g). The increased volume of water passing over the gills of larger eels would also increase the probability of gill/parasite larva contact, resulting in a higher infection level. However, this will be observed particularly in short-term studies, as the host-response (tissue- reactions on the gills) is liable to influence the parasitization level during the post- infection period (Buchmann, 19886). The present findings accentuate the need for proper definition of host size in ecological studies and medical treatment experiments of the A. anguillal Pseudodactylogyrus system. 599 0021-1 112:89/010599+03 S03.00jO 0 1989 The Fisheries Society of the British Isles