COMMUNITY ECOLOGY OF METAZOAN PARASITES OF THE LATER JUVENILE COMMON SNOOK Centropomus undecimalis ( OSTEICHTHYES : CENTROPOMIDAE ) FROM THE COASTAL ZONE OF THE STATE OF RIO DE JANEIRO , BRAZIL

Between April and December 2000, seventy-nine specimens of Centropomus undecimalis from Angra dos Reis, coastal zone of the State of Rio de Janeiro (2301’S, 4419’W), Brazil, were necropsied to study their infracommunities of metazoan parasites. Nine species of metazoan parasites were collected: 1 digenean, 1 monogenean, 1 acantocephalan, 1 nematode, 4 copepods, and 1 isopod, and 96.2% of the fishes were parasitized by one or more metazoan, with mean of 85.3 ± 122.9 parasite/fish. The digenean Acanthocollaritrema umbilicatum Travassos, Freitas & Bührnheim represented the majority of the parasites specimens collected, totaling, 94.7%. This species was the most abundant, prevalent, and dominant, showing positive correlation with the host's total body length and parasite abundance. The copepod species Acantholochus unisagittatus Tavares & Luque presented differences in abundance in relation to sex of host. The mean diversity in the infracommunities of C. undecimalis was H = 0.095 ± 0.116, with no correlation with the host's total body length but correlated with the host's sex. No pair of parasite species showed positive or negative association or covariation. The dominance of digenean A. umbilicatum in the later juvenile common snook parasite community could be related with the predatory food habits of common snook and an apparent feeding transition period which might occasion great exposure to infective forms.


INTRODUCTION
The common snook, Centropomus undecimalis (Bloch, 1792) is a demersal species that spends much of its life in estuaries, but migrates to ocean inlets or just offshore to spawn (Tucker & Campbell, 1988).This is a euryhaline and diadromous fish, with known distribution from Pamlico Sound, North Carolina, USA, southward to Rio de Janeiro, Brazil (Rivas, 1986).Centropomus undecimalis is predaceous and occupies high levels in the trophic web (Figueiredo & Menezes, 1980).Centropomid fishes are commercially important and widely used in aquaculture as a food fish and also as recreational fish (Vasconcelos-Filho & Galiza, 1980;Bórquez & Cerqueira, 1998;Kennedy et al., 1998;Tolley & Peebles, 1998).
Studies on quantitative aspects of the parasites of C. undecimalis from the Brazilian coastal zone were restricted to the digenean A. umbilicatum (Tavares & Luque, 2001a) and to the copepod Caligus praetextus Bere, 1936(Tavares & Luque, 2001b).Studies on ecological aspects of parasite infracommunities of C. undecimalis are unknown.
In this report, we analyzed at the component and infra-community levels the metazoan parasite community of C. undecimalis from the coastal zone of the State of Rio de Janeiro.

MATERIALS AND METHODS
From April to December 2000, 79 specimens of C. undecimalis were examined from Angra dos Reis, coastal zone of the State of Rio de Janeiro, Brazil (23 o 01'S, 44 o 19'W).Fishes, identified according to Rivas (1986), measured 18.5-47.0cm (mean = 35.2± 3.4 cm) in total length.The average total length of male (35.8 ± 4.1 cm, n = 39) and female (35.3 ± 1.9 cm, n = 27) fishes in the study sample were not significantly different (t = 0.569, p = 0.575); thirteen specimens measuring 33.7 ± 5.1 cm were not differentiated as male or female and not included in this analysis.The term "later juvenile common snook" was used according to Peters et al. (1998) to classify the common snook specimens with body size similar to the sample studied in the present report.
The analysis included only parasite species with prevalence higher than 10% (Bush et al., 1990).The quotient between variance and mean of parasite abundance (index of dispersion) was used to determine distribution patterns, and its significance was tested using d statistical test.The dominance frequency and the relative dominance (number of specimens of one species/total number of specimens of all species in the infracommunity) of each parasite species were calculated according to Rohde et al. (1995).Spearman's rank correlation coefficient r s was calculated to determine possible correlations between total length of hosts and abundance of parasites.Pearson's correlation coefficient r was used as an indication of the relationship between the host's total length and the prevalence of parasites, with previous arcsine transformation of the prevalence data (Zar, 1999) and partition of host samples into four 5 cm length intervals.The effect of host sex on abundance and prevalence of parasites was tested using the Z c normal approximation to the Mann-Whitney test and the Fisher exact test, respectively.Parasite species diversity was calculated using the Brillouin index (H), because each fish analyzed corresponded to a fully censused community (Zar, 1999).The probable variation of diversity in relation to host sex (Mann-Whitney test) and to host total length (Spearman's rank correlation coefficient) was tested.For each infracommunity, evenness (Brillouinbased evenness index) was calculated.The possible interspecific association between concurrent species was analyzed using the chi-square test.Possible covariation among the abundance of concurrent species was analyzed using the Spearman rank correlation coefficient.Ecological terminology follows Bush et al. (1997).Statistical significance level was evaluated at p ≤ 0.05.
Voucher specimens from representative species of helminths were deposited in the Coleção Helmintológica do Instituto Oswaldo Cruz (CHIOC), Rio de Janeiro, Brazil; copepods and isopods were deposited in the Coleção de Crustacea do Museu Nacional (MNRJ), Rio de Janeiro, Brazil.

Component community
Nine species of metazoan parasites were collected (Table 1).The digenean A. umbilicatum, the most abundant and dominant species, accounted for 94.7% of total parasites collected and showed the highest values of mean relative dominance and frequency of dominance (Table 2).All parasites of C. undecimalis had the typical aggregated pattern of distribution observed in many parasite systems.Acanthocollaritrema umbilicatum showed the highest values of dispersion indices (Table 3).Abundance of A. umbilicatum was positively correlated with host total length, although prevalence of no specieswassignificantly correlated with host's total length (Table 4).The mean abundance and prevalence of A. unisagittatus were significant higher in the male (95.7 and 81.5%) than in the female (56.3 and 76.9%) hosts (Z c = -2.15,p = 0.03; F = 0.05).
Only one group, ectoparasites (copepods), was used to determine possible interspecific associations.Adult endoparasites and larval stages were not included in this analysis because only one species of each group showed prevalence higher than 10% (A.umbilicatum as an adult and Contracaecum sp. in the larval stage).The copepod species pair A. unisagittatus and C. praetextus did not share significant association and covariation (χ 2 = 2.47, p = 0.116; r s = 0.130, p = 0.253).

DISCUSSION
We detected some patterns in the structure and composition of the community of metazoan parasites of C. undecimalis from Brazil: (1) endoparasite dominance; (2) correlation of parasite abundance with host size; and (3) lack of parasite interpecific relationships.
The dominance of digenean endoparasites has been described for several parasite communities of marine fishes from the coastal zone of southeastern Brazil (Luque et al., 1996;Takemoto et al., 1996;Knoff et al., 1997;Luque & Chaves, 1999;Silva et al., 2000;Luque & Alves, 2001).Feeding habits and broad diet spectrum of demersal fishes, which bring them in contact with several potential intermediate hosts of marine acanthocephalan, digenean, and nematodes, might increase the presence of endoparasites in these fishes (Alves & Luque, 2001).The later juvenile Centropomus undecimalis shows predatory habits and an apparent feeding transition period (Vasconcelos-Filho & Galiza, 1980;Teixeira, 1997), thus, great exposure to infective stages of parasites trophically transmitted is possible.
According Sasal et al. (1999), the diet of the host species is the main factor affecting parasite community structure, specially for digenean trematodes that are transmitted to their final host through a predator-prey relationship.These authors proposed that hosts with a more diversified diet should encounter more intermediate host species and, consequently, harbor more parasite species.Despite parasite abundance and prevalence shown in the common snook, in the present report parasite species richness was lower than expected.Many authors have reported diversified predatory feeding habits for this fish species (Harrington & Harrington, 1961;Vasconcelos-Filho & Galiza, 1980;Lau & Shafland, 1982;Gilmore et al., 1983;McMichael et al., 1989;Teixeira, 1997;Peters et al., 1998).However, as reported by Teixeira (1997) and Peters et al. (1998), although as nursery grounds estuaries represent the most important habitat for common snook, because they are near urban areas these ecosystems suffer great anthropic pressure resulting from organic and industrial discharges.This leads to eutrophication, followed by increased biological oxygen demand which subsequently produces mortalities of many forage species (Khan & Thulin, 1991).Also, the schooling behavior related for juvenile and adult common snook (Peters et al., 1998) might favor direct ectoparasite transmission.Some ectoparasite species showed prevalence higher than 10% on C. undecimalis from Rio de Janeiro.In mariculture activities with high fish population density, ectoparasite prevalence and abundance increase is possible (Euzet & Raibaut, 1985).The ectoparasite fauna of common snook from Rio de Janeiro is composed by species with high pathogenic potential such as caligid and ergasilid copepods.We emphasize sea lice occurrence since Caligus species are responsible for great economic losses in world fish farming Caligus praetextus (Tavares & Luque, 2001b).

Parasites
The correlation among total length of C. undecimalis and abundance of A. umbilicatum, might originate in accumulative infection.Sasal et al. (1999) observed that large fish are supposed to eat more and, therefore, ingest more intermediate hosts.This is a pattern previously found in other marine fishes from Rio de Janeiro (Luque et al., 1996;Knoff et al., 1997;Luque & Chaves, 1999;Luque & Alves, 2001;Alves & Luque, 2001;Tavares et al., 2001).According to Polyanski (1961), quantitative and qualitative changes in parasitism are expected with fish growth.In the case of digeneans, this relationship is strongly influenced by changes in feeding habits of the fish correlated with age (Saad-Fares & Combes, 1992).Juvenile common snook have a preliminary pelagic stage followed by a longer demersal stage (Peters et al., 1998) and show an apparent feeding transition period associated to this ontogenetic change in habitat.Early juvenile common snook feed mainly on copepods, other microcrustaceans, and insect larvae, while later juvenile and adult common snook feed on a wider range of food items, specially finfishes and shrimps (Harrington & Harrington, 1961;Vasconcelos-Filho & Galiza, 1980;Teixeira, 1997), which can be intermediate hosts of digenean parasites in marine fishes.
The correlation of the sex of C. undecimalis with abundance and prevalence of copepod A. unisagittatus was surprising because biological differences in male and female common snook are unclear.Snooks are protandrous and size at maturity is somewhat confusing (Peters et al., 1998).Moreover, Poulin (1996) stated that high testosterone levels can cause immunosuppression in males and could lead in some cases to males suffering more from parasites than do females, although many parasite surveys have reported no significant differences in infection abun- dance and prevalence between female and male hosts.Quantitative relationships of the sex of the host with infection levels of some components of the parasite communities were also detected in other marine fishes from Rio de Janeiro (Luque et al., 1996;Knoff et al., 1997;Alves & Luque, 2001) and may suggest ecological and behavioral differences between female and male hosts.The lack of parasite species associated pairs shown in C. undecimalis is in agreement with the data obtained on other marine fishes, where the presence of a low number of associated species is a common pattern (Rohde et al., 1995).However, these data from quantitative associations between parasite species could be used with caution to explain the parasite community structure.According to Rohde et al. (1995) and Poulin (2001), interspecific relationships can only be considered valid when tested under experimental conditions.These results reinforce the postulate of Rohde et al. (1995) according to which the parasite community structure in marine fishes constitutes a confused and unsaturated species complex, unlike the interactive patterns related for other host groups.