Food of the grouper Caprodon longimanus from Alejandro Seikirk Island, Chile (Perdformes: Serranidae)

Abstraet: Diel and food preference of Caprodon longimanus were 'studied at Alejandro Selkirk Island, Chile. The fishes (N=55) were collected in November and December 1996 using long line and lhe sizes ranged from 176-286 mm in SL. This grouper is a pelagic opportunistic zooplanktivorous predator. Thalia sp. (Tunicata: Salpida) was the most frequent and abundant prey accounting for 93.3% occurrence based on 1655 individuals. A total of 17 genera of crustaceans as well as pteropods, chaetognaths and polychaete worms were found in the stomach contents. A total of 14.3% of al! stomachs were empty.

Groupers are carnivorous fishes that live near the bottom in littoral and sublittoral zones of tropical and subtropical seas.Most species occur on coral reefs, but some live in estuaries and are near or al the top trophic levels of the food chains.This group includes over 400 species within five subfamilies and most of them are of considerable economic value (Shpigel andFishelson 1989, Heemstra andRandall 1993).
These fishes play an important role in coastal marine ecosystems where they are active predators of a number of organisms.Most groupers feed mainly on a variety of fish es, large crustaceans and cephalopods (Randall and Brock 1960, Randall 1967, Harmelin Vivien and Bouchon 1976, Nagelkerken 1979, Diamant and Shpigel 1985, Parrish 1987, Shpigel and Fishelson 1989, Bullock and Smith 1991).Some species may even occasion ally prey on juvenile turtles (Witzell 1981).
A few species, however, stand apart from this pattern eating small prey and showing spe cific adaptations for this feeding mode.They have long and numerous gillrakers that allow them to filter planktoníc organisms (Parr ish 1987, Bullock andSmith 1991, Heemstra andRanda111993).
The genus Caprodon is a serranid that belongs to the subfamily Anthiinae and is com posed by three species (K.harin and Dudarev 1983), Although there is abundant information in the literature regarding feeding habits ofser ranid species, groupers, sandbasses, etc., there is almost no information for any species of Caprodon.
The species C. longimanus (Günther, 1859) has an usual and interesting distribution, ranging from the Indopacífic (Kharin and Dudarev 1983) to the southeastern Pacific islands (Sepúlveda andPequeño 1985, Rojas andPequeño 1997 in prep.), at depths from 3 to 200 m.The purpose of the present study is to pro vide the first information on the.diet of C. longimanus at Alejandro Selkirk ' Island, Juan Fernández Archipelago, Chile.

MATERIALS AND METHODS
Fifty-five specimens of C. longimanus were collected on November 21 and December 4, 1996 at Alejandro Selkirk (33°46'S, 80046'W), the westernmost island of the Juan Fernández Archipelago.The fishes were caught by artisanal fishermen using long-Iines .. Their weights (± 0.5 g) and standard lengths (SL, ± 0.5 cm), were recorded and the sto m achs were removed and preserved in 10% formaldehyde.The specimens were deposited in the Division of Marine Fishes, Institute of Zoology of University Austral ofChile (IZUA PM 2016-2020).
In the laboratory, the stomachs were opened and the contents analyzed.The prey items were identifíed to the lowest taxonomic level following Palma (1985) and Palma and Kaiser (1993).Due to the nature of the fQod, the digestion rate, ineffective preservatíon, an important fraction of the samples contained material unidentifiable that was c1assified as "unidentified prey".The number of empty stomachs was also recorded.
The numeric frequency and the frequency of occurrence were ca1culated according to Hyslop (1980) and Starck and Schroeder (1970) respectively.The most important prey items were determined according to index of dietary importance (DI) (Hureau 1969).The biomass of prey items could not be determined due to the high degree of digestion and the soft and delicate nature of the dominant prey items.Diversity, evenness, and dominance of prey were calculated using the computer package ACOM (Navarro 1984).

RESULTS
Caprodon longimanus has a long oblique mouth, with the upper jaw extending past the lower jaw when c1osed.The jaw has two rows of villiforms teeth, with two small canines at the end of each row.The premaxilla has two rows of small conica!teeth.Band broadest anterior end of jaws near symphysial diastem.A rhombus-shaped patch of about two rows are found on the vomer and on a band of two rows on the palatines.The tongue is slender, without teeth.The stomach cavíty is narr ow, short, and not heavily muscled.
The numbers of individuals and frequen cy of occurrence in each food category found in the stomachs are shown in Table l.Based on the presence of mature gonads, all indivíd uals were considered to be adults.A total of 14.3% of all the stomachs were empty and they correspond to índividuals captured deep er than 150 m in the water column.The food items consumed by C. longimanus included salps, radioJarians, siphonophores, poly chaets, pteropods, chaetognaths, tunicates and crustaceans.Five orders (Amphipoda, Calanoida, Decapoda, Euphausiacea, and Ostracoda) with 17 genera of crustaceans were represented (Table 1).
Thalia democratica (Van Soest 1973) (Tunicata: Thaliacea), was the main prey item (by number of individuals recorded 1655 and frequency of occurrence 93.3%).CaJanoids copepods (mainly Pleuromamma sp. with 43.3% frequence of occurrence) were the sec ond most important food ítem.Radiolaria, Copepoda Cyclopoida and Amphípoda Hyperiidea as a group were the third prey ítem.Pisces (eggs and larvae), pteropods, chaetog naths, euphausids, decapods, doliolids and appendicularians wete found only occasional Iy' in the stomach content.Fishes might be more impOltant in the diet of C. longimanus than our data suggest.Their identification proved difficult because of the advanced state of digestion.The diversity, dominance and evenness índices of the items prey was 0.89, 0.82, and 0.18 respectively.A high number of stomachs had unidenti fiable remains (85.7%) that contained scales, crustaceans appendages, destroyed eggs and components digested beyond recognition.

DISCUSSION
The serranid fishes show a correlation .betweengillraker configuration and feeding habits.Those taxa with fewer and shorter gill rakers are generally carnivores, whereas the opposite holds for planktivores (Parri sh 1987, Bullock andSmith 1991, Heemstra andRandall 1993) (Table 2).The condition of more developed and abundant gillrakers is present in C. longimanus, which suggests planktivorous feeding habits.In fact, this species has more gillrakers than any other.ser ranid in the southeastem Pacific off Chile (Rojas and Pequeño 1997b in prep.).
Based on the type of prey found, C. longimanus can be characterized as a oppor tunistic pelagic polyphagic-zooplanktivorous predator.Salps, copepods, .andamphipods are the main components of the diet of this  Bullock and Smith 1991, Shapiro and Genin 1993, Heemstra 1995).
The absence of food items in sorne speci mens can be attributed to the regurgitation (by contraction of esophagic muscles) and/or by eversion of the stomachs caused by changes in pressure during capture.Specimens collected at depths between 150 and 190 m presented everted guts with food remnants scattered in the oropharyngeal cavity.This was not observed in specimens captured at depths les s than 150 m.
Although no plankton samples were col lected to study food preference, the high num bers of T. democratica in the stomach contents can be attributed more to an eventual seasonal abundance of this resource than to prey selec tion.During summer months, T. democratica is found in high concentrations, associated with phytoplankton abundance, in the Juan Femández Archipelago (Palma 1985, Palma & Kaiser 1993).This means that the diet during other times of the year is expected to be dif ferent, but containing mainly planktonic prey.The feeding strategy of C. longimanus would involve specialized predation and energetic benefits, i.e. reduction of search time by utiliz ing the most abundant resource, increasing feeding efficiency.
The low diversity and evenness of prey items were related to the dorninance of large quantities of blastozoids of salps in the stomach contents.These results could reveal a certain degree of specialization in feeding habits.
According to Berg (1979), high diversity index values characterize euryphagous fishes, while low values (as in the case of C. longimanus) are indicative of stenophagous fishes.Furthermore the buccal morphology, gillraker configuration, and the diet are all consistent with a zooplank tivorous mode of feeding, a rather unusual find ing among serranids, which are characterized as being typically camivorous.
The results provide a basis for further studies and comparisons of the role of this species along íts dístributional range that encompasses biotopes as distant and different as those of Australia, Japan, Korea, Hawaii and of course the Eastern Pacific Islands.

ACKNOWLEDGMENTS
Special thanks are due to the fishermen from Alejandro Selkirk Istand for collecting the specimens used in this study.This manu script benefited substantially from critical reviews by Guillermo Herrera (Los Angeles County Museum, USA), Carlos Jara and Roberto Schlatter (IZUA, Chile).We also acknowledge the Programa Oceanopolítico Integrado (POI) of the Chilean Navy and the Corporación Nacional Forestal (CONAF), for transportation, use of facilities, and lodging in Juan Fernández National Park.This research was partíally supported by the Deutscher Akademischer Austauschdienst (DAAD) through a scholarship for graduate' studies to the first author, by the Dirección de Investigación, Universidad Austral de Chile (Proyecto S 96-04), and by the National Geographic Society (Grant 5257-96).

TABLE 1
Composition of diet of C. longimanus A B DI A B DI I 3.3 III CHAETOGNATHA Larvae of Euphausia sp.(A) number ofspecimens, (B) percentage of ocurrence, (DI) dietary importance.Percentages given are based upon total number of prey iterns.