Looking for skin and gill parasites as biological tags for Atlantic bluefin tuna (Thunnus thynnus)

Abstract

-Skin and gill parasites found in juvenile Atlantic bluefin tuna (Thunnus thynnus) caught in the Bay of Biscay, northeast Atlantic, were examined with the aim of finding biological tags. The degree of infection of different microhabitats was analyzed and the annual prevalence by species of parasite obtained. Of the specimens examined, 98% had parasites, and the gills were by far the most infected microhabitat, followed by the skin and the pectoral fin. Within the gill cavity, parasites were most commonly found in the afferent margins of the primary lamellae. Three species of Copepoda were identified, two species of Monogenea, and eleven Digenea. Thus, ten species (Digenea: Didymozoidae) and a new host record for Copiatestes thyrsitae (Digenea: Syncoeliidae) were recorded in Thunnus thynnus. The suitability of the different external parasites found is discussed and gill didymozoids are put forward as useful biological tags. Confusing taxonomy within this digenean family makes species identification of the different morphotypes difficult. However, their habitat specificity in hosts can be a good tool to differentiate morphotypes and characterise host individuals.

Metazoan gill parasites of wild albacore Thunnus alalunga (Bonaterre, 1788) from the Balearic Sea (western Mediterranean) and their use as biological tags

“…The great species richness and high infection levels of didymozoids reported on the gills of albacore, and confirmed in the present study, indicate that these parasites are the principal component of the gill parasites of this host species. This pattern was also observed in K. pelamis (Lester et al, 1985), T. albacares (Lardeaux, 1982) and T. thynnus Mladineo et al, 2008;Rodríguez-Marín et al, 2008), suggesting that tunas are among the preferred hosts for didymozoids (Nikolaeva, 1985).…”

“…Didymozoids are considered as semi-permanent parasites, because their cysts are easily detectable in their specific location, and their remains are recognizable in the host tissues well after their death (Rodríguez-Marín et al, 2008;Speare, 1995). Moreover, the significant site selection showed by these parasites facilitates their detection and could be useful for species identification (Culurgioni et al, 2007;Rodríguez-Marín et al, 2008).…”

“…Moreover, the significant site selection showed by these parasites facilitates their detection and could be useful for species identification (Culurgioni et al, 2007;Rodríguez-Marín et al, 2008). Among didymozoids, D. pretiosus and D. longicolle could be used to discriminate Mediterranean and northeastern Atlantic albacore, since they have been found only on tunas from the Mediterranean Sea, although D. longicolle has also been described on tunas from the Gulf of Mexico (Nikolaeva and Parukhin, 1968) and the Pacific Ocean (Ishii, 1935).…”

“…Nevertheless, taking into account the lack of biological data on their life cycle, the record of many species in separate geographical areas, and of similar species on sympatric hosts, their use would require some caution. In this sense, molecular techniques should be applied to assess the genetic differences among and within the species of this group Rodríguez-Marín et al, 2008;Žilic et al, 2007). Moreover, considering that some of them may also infect the opercula and buccal cavity, the analysis of the whole head would probably give more complete information on the parasite fauna of the host.…”

“…The parasite fauna of albacore has been investigated by Jones (1991), Pozdnyakov (1990) and Schwartz (1939) in the Pacific Ocean, and by Aloncle and Delaporte (1974), Dollfus (1952), Guiart (1938), Legendre (1940), Postel (1963Postel ( , 1964 and Priol (1944) in the Atlantic Ocean, while no parasitological data are available for the Mediterranean Sea. Gill parasites of tunas are often used as biological tags because gills are not affected by handling manipulation, can be easily dissected during the evisceration and do not have any commercial value (Lester et al, 1985;Rodríguez-Marín et al, 2008).…”