Invest. Mar., Valparaíso, 35(2): 25-34, 2007

Trophic ecology of the lobster krill Munida gregaria in San Jorge Gulf, Argentina

Trophic ecology of the lobster krill Munida gregaria in San Jorge Gulf, Argentina

Julio H. Vinuesa¹ & Martin Varisco²

¹Centro de Desarrollo Costero, Facultad de Humanidades y Ciencias.Sociales, Universidad Nacional de la Patagonia San Juan Bosco, Consejo Nacional de Investigaciones Científicas y Técnicas.

²Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia San Juan Bosco, Consejo Nacional de Investigaciones Científicas y Técnicas. Campus Universitario, Ruta 1 Km 4, P.B. (9000), Comodoro Rivadavia, Chubut, Argentina.

ABSTRACT. The "langostilla", Munida gregaria, also called lobster krill or squat lobster, is a very common galatheid crustacean in San Jorge Gulf and around the southern tip of South America. Previous studies have shown that this species plays an important role in the trophic webs wherever it has been studied. In order to determine its natural food sources, we analyzed 10 samples (30-36 individuals each) taken from different sites in San Jorge Gulf. Moreover, stomach analyses were performed on 32 fish species, 4 mollusk species, and 7 crustacean species from the gulf. The lobster krill is primarily a detritivore or surface deposit-feeder and secondarily a predator and/or scavenger. Its main energy sources are particu-late organic matter and their associated bacteria, small live organisms on the surface of the sediment layer (ostracods, copepods, foraminifers, other protists), and animal debris. Polychaetes are the main prey of lobster krill in the study area. This dual complementary feeding behavior is common in the studied galatheids, making them a fundamental link between detritus and benthic and demersal top predators. Some species of these predators constitute important fisheries. Different life-cycle stages of the squat lobster were preyed on by 32 of the examined species. However, the spectrum of predators is still incomplete, with other species feeding on larvae and juveniles of the species.

Key words: feeding, predators, lobster krill, Munida gregaria, San Jorge Gulf, Patagonia.

Palabras clave: alimentación, depredadores, langostilla, Munida gregaria, golfo San Jorge, Patagonia.

INTRODUCTION

Three anomuran decapod crustaceans of the Family Galatheidae are known to live along the coasts and in the shelf waters of the southwestern Atlantic Ocean: Munida gregariaFabncius, 1793,M. subru-gosa White, 1847, andM. spinosa Henderson, 1855 (Boschi et al., 1992). The first two, considered to be one species in Australia (Williams, 1973,1980), live in sub-littoral waters generally no deeper than 200 m. M. spinosa, however, is found in deeper waters (100-1000 m deep) (Boschi et al, 1992).

In Argentina and Chile, M. gregaria and M. su-brugosa have almost always been considered to be different species due to morphological differences in adult specimens (Retamal, 1981, 2000, Boschi et al, 1992,Arntzeia/., 1999; Vinuesa, 2005). Recent studies again propose the existence of one only species, M. gregaria (Pérez-Barros et al, 2005), having two morphotypes or highly differentiated subspecies, with M. subrugosa being the morphotypes inhabiting the gulf.

Galatheids fisheries have developed due to the very abundant populations of these organisms. Longhurst (1968) estimated the potential galatheids production in the Pacific Ocean, including the two Pleuroncodes species (P. planipes, P. monodon), to be 30,000 to 300,000 tons per year. In Chile, catches of the squat lobster (P. monodon) ranged from 809 tons per year in 2003 to 1254 tons per year in 2005. Landings of another species in this family, the yellow squat lobster (Cervimunida johni), were recorded at 1,929 and 3,002 tons per year tons in the same respective years (SERNAPESCA, 2006). There is no fishery exploiting the lobsters in San Jorge Gulf, although over 2,000 tons are estimated to be lost per year (CEDEPESCA, 2005).

It is important to study the feeding habits of species within a community, as such studies provide important information for understanding the regulation of the populations in a specific area; feeding habits affect abundance, growth, mortality, and possible migrations. It is also important to collect information on the trophic relationships existing between species that are abundant and/or important fishing resources and the rest of the species living in a given area, as this allows us to infer possible future effects on the populations when fishing pressure is applied to the species in question or those that interact with it.

The squat lobster, Munida gregaria, known as bogavante or langostilla in Argentina and langostino in the Chilean channels, is one of the most abundant decapod crustaceans in the Atlantic Patagonian waters and the Tierra del Fuego Archipelago. Studies carried out in the coastal waters of the Strait of Magellan showed densities of 3 to 27 ind-m"² (Retamal & Gorny, 2001). In the Beagle Channel, densities of this species differ according to depth, with 1.03 ind-m"² at depths of less than 40 m and 0.27 ind-m^-2 at greater depths (Tapella, 2002).

The squat lobster is preyed on by various marine mammals such as whales (Matthews, 1932; Nemoto & Harrison, 1981), dolphins (Longhurst et al, 1967; Schiavini et al, 1997), sea lions (Rayner, 1935; Rodriguez & Bahamonde, 1986; Koen Alonso et al., 1998), and the sea otter Lutra felina (Sielfeld, 1990). Other predators include a variety of marine birds including cormorants (Rodriguez & Bahamonde, 1986) and penguins (Thompson, 1993). However, most of the squat lobster's predators are fish that feed on the lobster during its different life stages (Moreno & Jara, 1984; Rodriguez & Bahamonde, 1986; Sánchez & Prenski, 1996). Mollusks such as Octopus sp. (Bahamonde & Rodriguez, 1985) and Eledone massy a (Re, 1998), the squid Illex argentinus (Brunetti et al, 1998), and crustaceans such as the crabs Lithodes santolla and Paralomis granulosa (Campodónico & Hernández, 1983; Comoglio et al., 1989; Balzi, 1999) are also mentioned as squat lobster predators in the literature.

The diet of the studied galatheids covers a wide spectrum of feeding habits. Some species (e.g., Pleuroncodes planipes) feed almost exclusively on particulate organic matter (POM) (Aureoles-Gamboa & Pérez Flores, 1997) whereas others are detritivores (e.g., Munida tenuimana) (Cartes, 1993), opportunistic omnivores (e.g., P. monodon) (Madrid et al, 1997), and even occasionally cannibals, as observed in Munida gregaria off New Zealand (Zeldis, 1985). Galatheids studied in the Beagle Channel showed two different, simultaneous feeding behaviors: pre-dation on algae, polychaetes, and crustaceans; and bottom-feeding, consuming POM and organisms associated with the surface sediment layer (Romero et al, 2004).

The main objective of this article is to analyze squat lobster feeding and the diversity of the predators acting on this abundant species, which is an essential component of the demersal-benthic community in San Jorge Gulf.

MATERIALS AND METHODS

The present work was carried out along the central coast of San Jorge Gulf. Samples were taken between 1998 and 2000 in Caleta Cordova, Comodoro Rivadavia, and Caleta Paula using coastal vessels. Samples for the study of feeding behavior were obtained from freezer and high sea vessels in different sectors of San Jorge Gulf in 2005 and 2006 (Fig. 1). All samples were taken with bottom trawling nets with otters and/or beams; mesh sizes were 40 and 80 mm.

Stomach analyses were performed on ten samples taken in different sectors of the gulf and at different depths (Table 1). Sub-samples of 30-36 specimens of both sexes with carapace lengths (CL) over 10 mm were selected for the analysis. The CL was measured as in previous studies (Tapella et al, 2002; Romero, 2003; Romero et al, 2004). The repletion index was recorded for each stomach (RI: 0 = no content; 1 = < 25%; 2 = 25- 50%; 3 = 50-75%; 4 = > 75%).
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Stomach contents were first observed with a ste-reomicroscope (20X) to identify the larger items and then under optical microscopes (40x and lOOx). The frequency of occurrence of the different food items was calculated, as was the percentage of stomachs containing said item for the total stomachs analyzed in each sample.

Particulate organic matter (POM) has been defined as being composed of mucus from organisms, amorphous aggregates of previously dissolved organic matter, and complexes of associated microorganisms (Alldredge & Silver, 1988). Here, non-identified organic remains (ROM) were taken to be disintegrated and semi-digested tissular and cuticular animal remains that had been incorporated during feeding but that could not be identified.

In order to recognize M. gregaria predators, regardless of the squat lobster life stage, catches of diverse organisms from five coastal cruises were analyzed. A total of 124 specimens of 32 fish species, 26 specimens of 4mollusk species, and 25 specimens of 6 crustacean species were collected. Possible predators were identified on board the fishing vessels. The animals were selected for study when haul catches also included squat lobster; only animals with squat lobster in their stomachs were considered. Large fish were dissected, their stomachs extracted, and the contents analyzed macroscopically in situ. For small fish, the stomachs were removed or entire specimens were fixed in 7-8% formalin solution in sea water for later analysis. Individuals caught in sport fishing and manually along the coastal shoals and beaches near Comodoro Rivadavia were also considered, including an important number of live crabs caught off the beaches of Rada Tilly (2001-2002).

RESULTS

Altogether, 306 specimens of both sexes were analyzed; 57.2% were male. The average CL was 16.7 ±1.39 mm for females and 21.3 ± 2.36 mm for males. The RI varied from 0 to 4 in the studied specimens. Empty stomachs (RI = 0) were found in 9.2% of specimens (n = 28) and complete stomachs (RI = 4) in 5.5 % of them.

Figure 2 indicates the frequency of occurrence of food items found in the stomachs with content. In 84% of the squat lobster, amorphous POM was found; 92% had sediments. In all cases, POM represented the highest relative proportion of stomach content in the studied animals.

Decapod crustaceans were the second most important food item (58%). Patagonian prawn (Pleo-ticus muelleri) was the most frequent item and bits of squat lobster were also observed, although less frequently. Non-determined peracarid crustacean remains, ostracods, and copepods were also found.

Polychaetes represented an important food item.

Non-identified organic remains were also important (31%); they are believed to come mainly from highly digested crustaceans and polychaetes.

Cyanobacteria of the genera Chlorococcus sp. and Microcoleus sp. and dinofiagellates such as Ceratium sp. and probably Alexandrium sp. were also observed in the stomach contents of the squat lobsters.

Remains of red algae were scarce and consisted largely of Plocamium sp. and other non-determined species not normally found on the bottoms frequented by the species (soft bottoms at depths greater than 30-40 m). Diatom frustules corresponded mainly to the centric groups; the genera found were Coscinodiscus sp., Triceratium sp., Pseudo-nitzchia sp. and Chaeto-ceros sp., although their presence was limited.

Foraminifera were represented by various species, mainly Elphidium sp., Cassidulina sp., Quinqueloculina sp., Oolina sp., Eponides sp., Pyrgo sp., and Lagena sp. Remains of other loricate protists, probably thecamoebae and/or tintinnids, were also found.

Scarce remains of sponges were found, as were spicules having 2 to 4 radia and, possibly, spongin fibers. Mollusks were determined to be prey due to the presence of gastropod radulae. Other mollusk remains consisted exclusively of very disintegrated and small shell debris of gastropods and bivalves, probably ingested along with the sediments. The same seems to have happened with echinoderms, as highly disintegrated plates typical of Holothuroidea, spines, and remains of sea urchin and ophiuroid carapaces, as well as fish scales were recorded.

In the predator stomachs, the bright orange color of the squat lobsters facilitated their identification in most cases. However, determining the amount of squat lobsters consumed was not so simple because the remains were usually divided and partially disintegrated or in an advanced state of digestion.

Table 2 shows the predator species and the number of animals analyzed. No squat lobsters were observed in the stomach contents of the following fish: Galeorhinus galeus (tope shark), Myliobatis goodei (southern eagle ray), Parona signata (Parana leatherjacket), Stromateus brasiliensis (butterfish), Seriolella punctata (silver warehou), Percophis brasiliensis (Brazilian flathead), Prionotus sp. (sea-robin), and Squatina guggenheim (angular angel shark). Two fish of each species were studied, except for the tope shark, for which four specimens were analyzed.

Several of the analyzed species are very rare in the study area because they are distributed in the temperate-warm waters of the Argentina Province. This is the case of the angular angel shark, the southern eagle ray, and the searobin. Their presence in the gulf indicates an important penetration of a temperate-warm water mass.

No remains were found in the crabs Peltarion spinulosum, Libidoclaea granaría, and Platyxanthus patagonicus.

Previous studies indicate that galatheids characteristically feed on surface organic detritus. This strategy is alternated with other feeding methods such as predation and necrophagia (Nicol, 1932; Garni & Hoeg, 2000; Madrid et al, 1997; Romero, 2002; Romero et al., 2003).

In San Jorge Gulf, the squat lobster feeds on a reduced variety of benthic organisms. The stomach contents of nearly all the studied animals were mostly POM and sediments (fine sand, silt) with detritus (mollusk shells, skeletal remains of crustaceans and echinoderms) that were at times nearly reduced to the size of coarse sand grains. The squat lobster stomach contents also included sponge spicules, fish scales, and other remains that are were probably part of the surface sediments on the bottoms where these animals live.

In general, few findings indicate that this species acts as an active predator. The presence of algae, for example, indicates the ingestión of algal remains from shallow waters, since there are no algae on the bottoms where the samples were taken. Mollusks are also underrepresented in the diet and it is possible that their frequency is underestimated due to the consistence of their meat. The observed radulae indicate predation on some unidentified gastropod. Although there are various abundant bivalves in the area (e.g., Nuculana sulculata, Nucula puelcha, Kennerleya patagónica; Roux & Fernández, 1997), no remains were found that indicated predation on these species. Selectivity studies carried out on decapod crustaceans have demonstrated a negative selection on the mollusks, probably due to the species' inability to handle the specimens (Juanes, 1992). In the Beagle Channel, mollusks represented less than 1% of the relative abundance of the prey (Romero et al., 2004).

Necrophagia is a typical feeding behavior of decapod crustaceans. It is likely that crustaceans probably avoid feeding on some species when they are alive, but do feed on them when they are dead or decomposing. This can be seen in the decapod crustacean fisheries that bait their traps with dead animal remains. Sampling in Nuevo Gulf is being done by baiting the squat lobster traps with hake spines; this method has been very successful (P. Barón, pers. comm.) The presence of Pleoticus muelleri and squat lobster in the stomachs may result from feeding on dead animals that are quite common in the by-catch of coastal and beam-fishing vessels. This could also be the case with fish, although only scales were found and these at very low frequencies.

Only polychaetes and some gastropods seem to be preyed on by the squat lobster, although it is possible that this extends to peracarid crustaceans in some isolated cases. The animals in which these prey were found were all males. This could be due to the large size and development of the chelipods, which would give them an advantage over the female squat lobsters. This reveals an important correlation, as females have a higher rate of POM assimilation than males, whereas males assimilate prey organisms better than females (Romero et al., 2006).

The number of species involved in feeding in San Jorge Gulf is lower than that recorded in the Beagle Channel. Here, the species presents the two aforementioned habits: preying on small macroalgae and crustaceans and bottom-feeding, with POM constituting the main energy source for the species (Romero, 2002). This has also been observed in other galatheids, such as Munida tenuimana (Cartes, 1993) and Pleuroncodes planipes (Aureoles-Gamboa & Pérez Flores, 1997).

In San Jorge Gulf, the squat lobster is basically a bottom-feeder that sweeps the sea floor, scavenging particles with its third pair of maxillipeds. When doing this, they find non-digestible material such as sand, silt, and the hard remains of dead organisms, but also living organisms such as bacteria and pro-tists, the remains of decomposing organisms, and POM that can be assimilated. The squat lobster tears up the dead organisms and eats the meat and organs. Thus, as found in the Beagle Channel, the species represents an important link between POM and many benthic and demersal top predators.

Due to its particular bathymetry, San Jorge Gulf is a very favorable environment for the accumulation of organic substances, principally contributed by organisms that develop their life cycle in this area. In the coastal areas, the bottoms are made up of gravel and sand, and mud predominates in deeper areas, without algae and colonial fauna, which are common in shallow waters with course granulometry (Roux et al., 1995; Fernández et al., 2003). Here, there is an important flux of organic matter to the sea bottom, mainly in the central area, that represents a deposi-tional environment with fine-textured sediments and an accumulation of organic detritus from the water column (Fernández et al., 2005). These elements are taken advantage of by the fauna, which is dominated by filtering bivalves, sea urchins, polychaetes (Roux & Fernández, 1997) and, temporarily, adult squat lobster. The area in the western sector of the gulf with depths of less than 80 m is covered with sand and mud with deposits of mixed organic detritus; this area is the most frequented by the species.

The squat lobster also plays a preponderant role in the gulf's ecosystem due to its abundance. Coastal hauls between 40 and 60 m depth have been recorded in which squat lobster was the main catch, reaching over 300 kg per haul (G. Giannotta, pers. comm.). Some trawls have resulted in densities of 0.1 to 2.1 lobstersm² (Vinuesa, unpublished). Considering that the mesh size of the nets is 4 to 8 cm in diameter, these figures can be seen to be underestimates of the real amounts.

Our findings show 24 fish species, four crustacean species, and four mollusk species that prey on the squat lobster. In San Jorge Gulf, nine species were found to feed on the squat lobster: Dyscopyge tschudi, Atlantoraja cychlophora, Cheilodactylus bergi, Patagonotothen ramsayi, Odonthestes smitti, Eurypodius latreillei, Ovalipes trimaculatus, Loligo gahi, and Enteroctopus megalocyathus. These species have not been mentioned in previous studies.

Squat lobster is the main prey year-round of many benfhic-demersal fishes also recorded in this study:

Genypterus blacodes and G. brasiliensis, Dipturus flavirostris, Psammobatis scobina, Sympterigia bo-napartei, Acanthistius brasilianus, Salilota australis, Sebastodes oculatus, Pseudopercis semifasciatus, Xistreuris rasile, and Cottoperca gobio (Sánchez & Prenski, 1996).

In Chile, nine species mentioned in Table 2 also belong to those preying on the squat lobster: Schroederichthys bivius, Salilota australis, Macruro-nus magellanicus, Merluccius hubbsi, Cottoperca gobio, Callorhynchus callorhynchus, Dipturus flavirostris, Lithodes santolla, and rays of the genera Psammobatis (Rodriguez & Bahamonde, 1986). Three cormorant species are also mentioned, as are three species of sea lions Arctocephalus australis, A. gazella, Otaria byronia, and several flounder and other fish in the area, including snoek (Thyrsites atun). The Notothenidae from the Magellanic area deserve a special mention since six of the eight species frequent in the Tierra del Fuego channels have been recorded as predators of the squat lobster (Moreno & Jara, 1984; Isla & San Román, 1995). Lithodes santolla has also been mentioned as a predator of the species, both in the Beagle Channel (Comoglio et al., 1989; Tapella et al., 2002) and in San Jorge Gulf (Balzi, 1999), as has Paralomis granulosa in the Beagle Channel (Comoglio et al., 1989).

It is clear that the list of predators indicated in this article is partial, although it includes the extensive study on demersal fishes carried out by Sánchez & Prenski (1996). Nonetheless, it is very possible that other fish species and sea animals feed on the squat lobster in the waters of San Jorge Gulf. Some fish in which the squat lobster was not found probably feed on it occasionally, as is the case of the southern eagle ray and the angular angel shark, both benthic species. No squat lobsters were found in the pelagic parona leatherjackets, which feed mainly on juvenile fish and zooplankton (Cousseau & Perrotta, 2000). Although benthic, the Brazilian flathead feeds only on fish (Cousseau & Perrotta, 2000). Oddly, the squat lobster was not found in the tope shark (it was also not mentioned by Sánchez & Prenski, 1996), nor was it found in the searobin, which feeds on benthic crustaceans (Cousseau & Perrotta, 2000).

Birds and mammals were not included in this work; however, many species prey on the squat lobster in other places. In some seasons of the year, it is very common to see seagull guano with disintegrated remains of squat lobster carapaces; the same has been observed in places frequented by cormorants, penguins, and other sea birds. The ever-increasing frequency of the whale Eubalaena australis in the gulf's waters may be due to a search for food, as this species has also been mentioned as a predator of squat lobsters in Patagonian waters (Mathews, 1932; Rayner, 1935).

The species is also found in the Nuevo, San José, and San Matías gulfs, where other benfhic-demersal fish, also possible predators, are abundant (e.g., porgies, croakers, sandperchs, flounders, silver sides and other rays). These species are not present in San Jorge Gulf and the more southern temperate-warm waters.

Squat lobster may be easy for the fishing industry to extract if a decision was made to exploit them, as has occurred with similar species in the Pacific Ocean. However, mid-scale and especially small-scale fishing can reduce the stability of the gulf's trophic web. The squat lobster approaches the coast during the breeding season (Vinuesa, 2007) where it forms considerable concentrations, making it easy to catch. It is believed that, if exploitation of this resource begins, it should be done through the by-catch of the current fisheries, which has been estimated to be approximately 2,000 tons (CEDEPESCA, 2005).

ACKNOWLEDGMENTS

We are grateful for the collaboration of Mr. Gabriel Giannotta and Mr. Luis Badia, skippers of fishing vessels from Caleta Cordova and Comodoro Riva-davia. We also thank Ezequiel Murphy and Ricardo Alvarez of the Secretaría de Pesca, Chubut Province, for samples obtained from large fishing vessels. This paper was funded through grants received from the Argentinean National Committee of Scientific and Technical Research (CONICET) (PIP 02566/99).

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Received: 26 February 2007; Accepted: 24 July 20007

Corresponding author: Julio H. Vinuesa (jvinuesa@speedy.com.ar, julio.vinuesa@gmail.com)