Experimental studies on the feeding ecology of Munida subrugosa (White, 1847) (Decapoda: Anomura: Galatheidae) from the Magellan region, southern Chile

Crabs of the galatheid genus Munida have a worldwide distribution, and currently over 150 different species have been recorded (Macpherson, 1994). Studies on the composition of the megaepifauna in the Magellan Region have shown that Munida subrugosa, with a circumpolar distribution between 35°S and 58°S (Zeldis, 1985), is one of the most abundant decapods, and can constitute up to 50% of the benthic biomass south of the Beagle Channel (Arntz et al., 1999). For many predators at a higher trophic level, M. subrugosa represents a large part of the diet composition (Moreno and Jara, 1984). However, the state of knowledge about the autecology of M. subrugosa is low. Previous work SCIENTIA MARINA 71(1) March 2007, 187-190, Barcelona (Spain) ISSN: 0214-8358


INTRODUCTION
Crabs of the galatheid genus Munida have a worldwide distribution, and currently over 150 different species have been recorded (Macpherson, 1994).Studies on the composition of the megaepifauna in the Magellan Region have shown that Munida subrugosa, with a circumpolar distribution between 35°S and 58°S (Zeldis, 1985), is one of the most abundant decapods, and can constitute up to 50% of the benthic biomass south of the Beagle Channel (Arntz et al., 1999).For many predators at a higher trophic level, M. subrugosa represents a large part of the diet composition (Moreno and Jara, 1984).However, the state of knowledge about the autecology of M. subrugosa is low.Previous work -Se estudiaron los hábitos alimentarios y tasas de ingesta de alimento del crustáceo anomuro Munida subrugosa en la Región de Magallanes (Chile) bajo condiciones de laboratorio.Los cangrejos mostraron una marcada preferencia por la carne versus las macroalgas, con tasas de ingesta 14 veces más altas.Diversas especies de algas fueron clasificadas en función de la estructura y la capa detrítica.Las observaciones señalan que la alimentación sobre carroña y el canibalismo (restringido al consumo de individuos dañados o en estado de muda) constituyen una estrategia aplicada frecuentemente en el hábitat natural.Sin embargo, se sugiere que la principal fuente de alimento es de origen microscópico, dada la conducta alimenticia observada en ejemplares vivos, que se alimentan de material particulado y en suspensión.Los resultados de este estudio, apoyado por los resultados de análisis de contenidos estomacales realizados simultáneamente, demuestran que Munida subrugosa es un predador oportunista, que puede hacer uso de diferentes formas de obtención de alimento.
The present study is part of the FONDEF project D001-1181: "Development of the Fishery of the Langostino de los Canales (Munida subrugosa) in the 12th Region, Magellan Region and Chilean Antarctica", initiated in 2001 to prove the suitability of M. subrugosa as a commercial alternative for the traditionally captured lithodids in the sample area.To ensure a sustainable exploitation detailed studies about the biology of the species are necessary.Feeding habits in particular are of considerable interest because they help to determine the role of M. subrugosa in the natural habitat and its position in the local food web.In this work, studies on feeding preferences and observation of live specimens kept in the laboratory were used to provide an initial understanding of the feeding ecology of the species.

MATERIAL AND METHODS
Samples of M. subrugosa were obtained from Bahía Santa María, a shallow bight in a semienclosed basin in the Magellan Strait (53°24'S; 70°21'W) from February to March 2002 at sampling depths of 10-20 m.Immediately after capture by the means of specially designed, bottom-lying fishbaited traps, live crabs were transferred to the laboratory.Feeding behaviour was recorded with 20-30 specimens each held in Plexiglas aquaria (30 x 40 x 50 cm) with seawater in a temperature-controlled room at 8°C (±1°C) under approximately natural light conditions.To provide more natural substratum conditions and examine specific feeding habits, the bottom of one observation aquarium was covered with a 3 cm thick sand layer.Food items comprised live prey including various species of polychaetes and crustaceans as well as lumps of meat and algae.Each item was placed onto the water surface.
Food uptake rates and preferences were established in two feeding experiments over a period of five days.Meat tissue of M. subrugosa was known to be appreciated, but special interest was placed on consumption rates of macroalgae, since these were regularly found during stomach analysis.Single adult crabs of similar carapace length were placed in 21 x 23 x 30 cm sized experimental tanks with fil-tered seawater (<0.2 μm) and kept without food for 3 days.A pairwise choice test between meat tissue and the red algae Polysiphonia sp. was carried out.To detect any possible selectivity between algal species, a multiple choice test with 3 species of red algae, Ceramium sp., Polysiphonia sp. and Ballia sp., each with high abundances in the natural habitat, was run.The amount of ingested food was determined gravimetrically as wet weight.All resources were daily renewed, and leftover material was taken out thoroughly before new food was offered.Faecal pellets were analysed randomly.

RESULTS
M. subrugosa showed clear preferences in the comparison of meat versus macroalgae (Fig. 1).The mean daily ratio of meat consumption (560 mg) was 14 times higher than that of plant consumption (40 mg).Meat consumption showed a tendency to increase or at least continue on a high level throughout the experimental period, whereas algal uptake rates remained low.
The results of the multiple choice experiment with algae revealed a preference ranking by M. subrugosa: Polysiphonia sp.> Ceramium sp.> Ballia sp.(Kruskal-Wallis, p = 0.018).Nevertheless, all values lay far beneath meat consumption, ranging from 9 to 33 mg/Ind./day (Fig. 2).Algal consump- tion, regardless of species, showed a tendency to decline over the experimental period, especially after the first day.
Feeding behaviour in response to the investigated food items was highly specific.Algae were fed on rather hesitantly and were regularly rejected after a short period of time.Reaction intensity was very high in response to meat tissues as well as to live polychaetes.Immediately after supply a characteristic searching behaviour was exhibited, in which chelipeds were lifted and the endopods of the 3 rd maxillipeds were used to comb through the water column.Shortly after localisation, the crabs performed quick movements towards the desired food and severe fights between several or even all individuals in the aquarium began.Large food objects, such as lumps of meat or polychaetes, were grasped with the chelipeds, cut or ripped into smaller pieces and handed over to the endopods of the third maxillipeds for consumption, while smaller pieces were taken directly with the third maxillipeds.The feeding process could last up to a few hours.Cannibalism was frequently observed on injured or dead specimens, whereas agonistic behaviour towards live, healthy crabs of the same and other species was never observed.In this context, we made an interesting observation: after removal from the aquaria for approximately 10 min crabs showed a behaviour referred to as "playing dead" on being put back into the water.The investigated crab sank to the bottom motionless, predominantly landing in a supine position.Severe attacks of conspecifics forced it to recover its natural habitus after 2-3 minutes, whereupon the aggressiveness of the attackers abruptly stopped.
In the aquarium equipped with sand, the behaviour of "gathering sediment" was observed regularly and persistent in all animals in the case of lack of macroscopic food.Again, the endopods of the 3 rd maxillipeds were used to collect sediment and onlying particles.Additionally, a gathering of minute meat particles ranging from 0.2 to 2 mm out of the water column was observed, in which the endopods of the 3 rd maxillipeds combed through the water acting as filtering devices.DISCUSSION Observations as well as feeding experiments suggest that M. subrugosa has a rather broad niche width and tends to be a generalist feeder, promoted primarily by the ability to make use of different feeding habits.The literature reports Galatheids to be largely feeders on small particles, while typical decapod predatory habits have been given up or reduced (Nicol, 1932;Kaestner, 1993).Results of this laboratory study demonstrate the significance of scavenging and active preying for the nutrition of M. subrugosa.Scavenging apparently represents the dominant form of meat uptake in the natural habitat, as shown by behaviour towards (seemingly) weak conspecifics.In addition, active preying, recently demonstrated for Munida sarsi (Hudson and Wigham, 2003), seems to be an opportunistic strategy occasionally applied on suitable prey.The high and lasting meat consumption rates in the laboratory suggest that the species has a gorging food behaviour, ignoring saturation in the event of meat encounter, which is favourable to fully utilising food fall.Rapid response of crabs after meat supply to the aquaria is indicative of some chemoreception ability at a distance of at least 30 cm, a capability previously not documented for Galatheids.
While scavenging and preying are valuable supplements for the diet of M. subrugosa, deposit feeding represents the basic nutrition, as is also shown in stomach analysis (Romero et al., 2004).Another method of feeding on small particles seems to be filter feeding, a strategy which has not yet been documented for Munida, but is well known for the relat-SCI.MAR., 71(1), March 2007, 187-190 ed family Porcellanidae (Nicol, 1932), and effectively used for nutrition by Galathea strigosa (Havsfiskelaboratoriet, 1996).These two methods enable crabs to collect a wide range of particulate organic matter, thus apparently being specially adapted to the conditions in the sample area, where the lack of obligate filter organisms (Cattaneo-Vietti et al., 1999) promotes a high amount of detritus on the seafloor as well as suspended organic matter in the water column.There is evidence that algae are also primarily selected because of the epiphytic, microbial or detrital cover, which correlates with algal surface, a behaviour known for some invertebrates (Kornijów et al., 1995).The preferred Polysiphonia sp. with delicate ramifications is easy to handle and has the greatest surface.Selection by nutritional value is unlikely considering the state of the algae in faecal pellets, which suggests a low digestibility regardless of species.
In an environment like the Magellan region characterised by frequent events of disturbance (Antezana, 1999) leading to unpredictible food availability, M. subrugosa's ability to adapt feeding strategies to external circumstances is advantageous for surviving periods of unfavourable conditions and allows it to build up great biomasses and coexist with numerous other benthic decapods occurring in the sample area (Arntz et al., 1999;Gorny, 1999).M. subrugosa is assumed to play a crucial role in the pelagic-benthic coupling in this sub-Antarctic environment, but further estimates of filtering and gathering rates are needed to assess the size of this impact.
FIG.1.-Daily average consumption rates of Munida subrugosa (n = 5) in a choice experiment between crab meat and the red alga Polysiphonia sp.during the experimental period of 5 days.Vertical lines are standard deviations.