Frontiers in Marine Science (Jun 2014)
Distribution of <i>Opecoeloides furcatus</i> (Digenea, Opecoelidae) from the red mullet (<i>Mullus barbatus barbatus</i>) (L.1758) off the northern Sicilian coasts (Southern Tyrrhenian Sea)
Abstract
Opecoeloides furcatus (Bremser in Rudolphi, 1819) is an opecoeliid digenean infecting the digestive tract of the two mullidae Mullus barbatus barbatus and M. surmuletus (Carreras-Aubets et al., 2012). The Opecoelidae constitute a large and cosmopolitan family of digeneans characterised by a series of complex life-cycles involving two or three hosts and several developmental stages (Jousson et al., 1999). The specialist O. furcatus uses the gastropod Mitrella scripta (L.) as first intermediate host and, probably a decapod as second intermediate host (Jousson & Bartoli, 2000). The present study aims to assess the potential impact of O. furcatus on M. barbatus barbatus specimens inhabiting the seabed off the northern Sicilian coasts (Southern Tyrrhenian Sea). The distributional pattern as well as the influence on host growth have been evaluated. Samples have been collected from Cape Rasocolmo (Messina) to Cape S. Vito (Palermo) encompassing two Fishery Exclusion Zones: the Gulfs of Patti and the Gulf of Castellammare. A total of 10 hauls have been performed between 32 and 110 meters depth during an experimental otter trawl survey performed in May 2012. On board only red mullets of total length ranging from 110 and 150 mm were sorted and frozen. In the laboratory samples were defrosted, measured, weighed and dissected in order to macroscopically evaluate their sex. The entire digestive tract was removed and examined under a stereomicroscope for the presence of O. furcatus. The three epidemiological indices: prevalence of infestation (P%), mean intensity and mean abundance were calculated according to Bush et al. (1997). Fisher's exact test and Bootstrap t-test were applied to test for sex and sampling site factors (Reiczigel & Ròzsa, 2005). To test for correlation between prevalence and mean intensity vs depth the Spearman test was applied. The values of prevalence of infestation were additionally mapped by using GIS software. The parasitic effect on host growth was evaluated applying the length-weight relationship separately for infected and uninfected specimens. Differences between infected and uninfected specimens were tested using an analysis of covariance. The prevalence and intensity of infestation did not differ significantly between sexes (Table 1); consequently, the spatial distribution of this parasite in the study area was examined computing the parasitic prevalence in each haul for combined sexes. A total of 53 out of 251 M. barbatus barbatus were parasitized (Table 1). Each infected fish was parasitized by one to seventeen parasites. Generally the parasitic prevalence was 21.1 % and the mean intensity was 2.06. The parasite occurred throughout the study area with wide fluctuations of prevalence values ranging between 0 and 40 % (Figure 1). The parasitic prevalence values were significantly higher inside the Fishery Exclusion Zones (P = 32.3 %) than in the fishing zones (P = 14.2 %) (Fischer exact test, p value0.05). No significant correlation was found between prevalence and mean intensity vs. depth factor (rs = 0.22, p > 0.05; rs = 0.07, p > 0.5). The length-weight relationships showed a positive allometry for both uninfected and infected specimens, with a b value of uninfected specimens (b = 3.330) even lower than that of infected ones (b = 3.505); however the difference between infected and uninfected specimens was not significant (F = 0.903, p > 0.05). Although to date specimens of O. furcatus have been reported in M. barbatus barbatus from various localities in the Mediterranean Sea (Paggi et al., 1998; Martinez-Vicaria et al., 2000), the obtained value of prevalence of the parasite from our study area was lower than from the north African and from the Mediterranean Spanish coasts (Martinez-Vicaria et al., 2000; Carreras-Aubets et al., 2012; Derbel et al., 2012;). The comparison of length-weight relationships of infected and uninfected specimens and the low intensity of infection allow us to assume that, for the moment, there is no severe effect on the host population dynamic: O. furcatus does not affect the general-being of its host. Interestingly the spatial distribution on O. furcatus along the study area is not related to the depth appearing rather influenced by the presence of the two Exclusion Fishery Zones. Future studies will be performed in order to clarify this latter observation in relation to the seabed features and habitat integrity status among fished and un-fished areas as well as about the identification of intermediate crustaceans hosts (Mangano et al., 2014). Figure caption Figure 1. Prevalence of infestation (P%) of Opecoeloides furcatus in red mullets (Mullus barbatus barbatus) off the northern Sicilian coasts.
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