Multilocus phylogenetic analyses reveal that habitat selection drives the speciation of Didymozoidae (Digenea) parasitizing Pacific and Atlantic bluefin tunas.

Parasite communities of wild and reared bluefin tuna display remarkable diversity. Among these, the most prevalent and abundant are the Didymozoidae (Monticelli, 1888) (Trematoda, Digenea), considered one of the most taxonomically complex digenean families. The aim of this study was to evaluate phylogenetic structure of Didymozoidae occurring in Pacific (Thunnus orientalis) and Atlantic bluefin tuna (T. thynnus) in order to increase our knowledge of didymozoid zoogeography and identify species that could successfully be employed as biological tags for stock assessment studies. For the present analyses we used 2 nuclear ribosomal DNA loci, part of the 28S gene and the second internal transcribed spacer (ITS-2) as well as a portion of the mitochondrial cytochrome c oxidase subunit 1 gene (cox1). In most parasitic groups, morphology is the primary factor in the structuring of phylogenetic relationships. In rare examples, however, habitat has been suggested as a primary factor affecting parasite evolution. During their evolution, didymozoids have spread and inhabited a remarkable number of different sites in their hosts, colonizing exterior as well as strictly interior niches. Our data suggest that habitat selection has been the leading force in shaping didymozoid phylogenetic relationships. For 2 didymozoid species (D. wedli and D. palati), cox1 sequences indicate intraspecific differences between Mexican and Adriatic populations.

Seasonal epizootics of sea lice, Caligus spp., on southern bluefin tuna, Thunnus maccoyii (Castelnau), in a long-term farming trial.

Within the typical 2-8 month (January to August inclusive) farming cycle for southern bluefin tuna, Thunnus maccoyii (Castelnau), in Spencer Gulf, South Australia, counts of a sea louse, Caligus chiastos Lin et Ho, 2003, were strongly statistically associated with both fish condition and severity of eye damage. During a trial examining the feasibility of maintaining T. maccoyii in farms for more than 1 year, including over the summer season when temperatures may exceed 24 degrees C, we collected additional epidemiological data on burdens of sea lice over a 17-month period (April 2005 to August 2006 inclusive), on a total of 200 T. maccoyii and 40 'control'T. maccoyii farmed and harvested within 2006. In the first farming season, an epizootic of C. chiastos was characterized by a significant increase in prevalence from 0% to 55% in the first 6 weeks after transfer to farms from the wild, which was followed by a significant decline to zero over the next 12 weeks. A single specimen of a second species of Caligus was also detected within this 4.5-month period. In the second farming season, we recorded a third species of sea louse, C. amblygenitalis Tripathi, 1961. In March 2006, a second epizootic peak occurred, this time with mixed infections of C. chiastos and C. amblygenitalis, with a combined prevalence of 100%. The prevalence of both sea lice species then declined significantly over the second winter period (June to August inclusive). On all but one date that sea lice were detected, sea lice counts were significantly associated with the severity of gross eye damage. Because both peaks in infection occurred in summer months (December to February inclusive), we conclude that infections of sea lice pose a risk to the farming of T. maccoyii under certain summer conditions within Spencer Gulf.