There is very little information on the genus Branchiobdella and the species relationships with their freshwater crayfish hosts inCroatia. Therefore, a base-line study was needed so that future changes in available habitat brought about by urban developmentand the probable introduction of non-native species can be accurately assessed. This investigation used preserved freshwatercrayfish collected from across Croatia between 1995 and 2005 as its source of the ectosymbionts. Crayfish species includedAstacus astacus, A. leptodactylus, Austropotamobius pallipes, A. torrentium, and the allochtonous North American species,Orconectes limosus. Only native European species of branchiobdellids were found: Branchiobdella astaci, B. parasita, B.pentodonta, B. hexodonta, B. italica, and B. balcanica, and this is the first report on the occurrence of these species, apart from B.italica, in Croatia. The distribution of these branchiobdellidans is compared with climatic and river drainage systems, andliterature reports of populations in other countries in the region.

The Australian freshwater crayfish, Cherax destructor is cultured commercially and has been translocated throughout much of Australia. Previous investigation on C. destructor using 16S rRNA sequences of samples collected from natural environments has revealed a significant phylogeographic structure in this species with three well supported geographically non-overlapping clades, namely ‘northern’ C. d. destructor, ‘southern’ C. d. destructor and C. d. albidus. Movement of individuals beyond their natural range of distribution may have adverse effects on genetic integrity of the species. In the present study, aspects of translocations of the species were genetically investigated. Sequences of the 16S rRNA gene region of the mitochondrial DNA (mtDNA) were obtained from samples collected in nine quasi-natural waterbodies, supplemented with sequences of samples obtained from 31 natural waterbodies examined in a previous study. Results of phylogeographic analysis provide evidence that certain haplotypes from major clades of C. destructor have been translocated. The findings of this study have important implications for the conservation and management of genetic diversity within C. destructor.

Signal crayfish (Pacifastacus leniusculus) originating from Lake Tahoe, California, were introduced into Karisjärvi, a small (11 ha) lake in central Finland (61° 58'N, 25° 32'E), in 1968 and 1969. Since then, the population has been monitored regularly by trap catches. The stocked signals (3–5 years old) were caught until 1973, by which time they had reached ages of 7–9 years. Catches have increased slowly since the early 1980s, the peak occurring in 1991 at 2.0 individuals per trap per night. The population size was estimated as 60 trappable specimens in 1974, 95 in 1981 and 420 in 1988. The mean density of adult population in suitable biotopes was low, 0.07 specimens per m–2, or 0.3 per shore metre, in 1988. The slow development of the P. leniusculus population has been attributed to environmental factors, mainly the limited area of good crayfish habitat. More than 90 % of all signal crayfish were caught in one-third of the shore area, i.e. in steeply sloping lake beds suitable for burrowing or rich in shelters such as rocks and submerged trees. The signals avoided flat, soft bottoms. Fifty per cent of the female P. leniusculus matured at 90 mm TL (smallest 64 mm), i.e. half of the females entered the breeding population in the autumn of their fourth year. The mean size of newly hatched (stage 2) juveniles was 9.7 mm TL and of one-summer olds 30.3 mm. The largest specimen trapped measured 159 mm. The signal crayfish imported into Finland and stocked were infected with crayfish plague (Aphanomyces astaci), but no mortality has been recorded. Two Branchiobdellidae (Annelida, oligochaeta) epibiont species new to Finland were imported from North America with P. leniusculus. The continuous occurrence of these commensals in the signal crayfish population indicates that they have adapted to Finnish conditions.