Behaviors of the pelagic red crab Pleuroncodes planipes observed ...

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74 D. C. Tulipani & M. A. BoudriasFig. 5. Fifth pereopod of Pleuroncodes planipes. 1)Scanning electron micrograph of the distal end ofthe chelate 5th right pereopod. The limbs are usedmainly clean the exoskeleton and setae. Females uselimbs to stir the fertilized eggs after mating. 2) Rightlateral view of posterior body showing 5th pereopodtucked along the side of the carapace (black arrow).Scale: 1 block = 2 mm.body surface as well. A crab would sometimesscrub the tip of the cleaner pereopod againsta groove in its exoskeleton to loosen particlestrapped by the setae. Crabs folded their chelipedsmedially and posteriorly, enabling thecleaner pereopod to reach the entirety of theselimbs. Additionally, crabs used their maxillipedsto wipe the end of the cleaning pereopods,their antennae and their eyes. When groomingstopped, crabs typically folded and partiallytucked the distal ends of the fifth pereopodsunder the lateral posterior edge of the carapace(Fig. 5).Frequently, grooming behavior was immediatelyfollowed by feeding behavior wherePleuroncodes planipes would use their maxillipedsto wipe off the tip of the fifth pereopod,which was immediately followed by the anterior-posteriormovement of the mandibular palps.The movement of the maxillipeds towards themouth region appeared to transfer particles tothe mouth. The mandibular palps would waverapidly towards and away from the mouth tomove the particles into the mouth. In addition,captive red crabs were observed feeding,independent of grooming, while standing onthe bottom of the aquarium or hanging from aside wall. With the rostral spine tilted slightlyupwards, crabs would extend and retract theirmaxillipeds away from and then towards theirmouth region generating water movement(visible as the movement of food particles)towards their mouth while intermittently oscillatingtheir mandibular palps. Occasionally,red crabs quickly scraped the dactyl tip of oneof the pereopods along the bottom, kicking upparticles and moving them towards the maxillipeds,which in turn would capture any particlesthen move them towards the mandibular palpsand mouth. In the aquarium, red crabs quicklysensed any food dropped into the tank, moveddirectly towards it, and collected as much asthey could hold with their chelipeds and/ormouthparts. Some crabs even performed a coupleof tail-flips moving away from other crabspreventing them from potentially stealing thefood.Regarding interactions between individuals,crabs were often observed walking into,bumping, pushing, or landing on other crabsthat were standing on the bottom or hangingfrom a side wall (Fig. 2). When a crab approachedor bumped into another, typical agonisticbehaviors observed were either apparentaggressive behaviors or avoidance behaviors.Both crabs would usually move away whensuch an encounter occurred. Sometimes neithercrab moved to avoid the other despite antennaeflicks or tail fan flexions from the opponent.Sparring occurred when one crab approachedanother head-on without giving way (Fig. 2).The stationary one would open the dactyls onits chelipeds and gesture forward while raisingthe chelipeds up slightly towards the approach-
Behaviors of Pleuroncodes planipes 75ing crab. This type of gesturing usually resultedin the approaching crab changing directions.Sparring between males also occurred whenone male approached a female that was beingguarded by another male either prior to or justafter mating.DISCUSSIONPleuroncodes planipes have been describedas dividing their time between the benthos duringthe day and the epipelagic zone to feed atnight (Boyd, 1962; Blackburn, 1977). However,the crabs were also found in large surfaceswarms during the day (Boyd, 1967; Aurioles-Gamboa, 1992) and are considered the mostabundant nektonic organism off southern BajaCalifornia waters (Blackburn & Thorn, 1974).When the crabs were initially collected forthis study, they were small (standard carapacelength < 32 mm), which corresponded with thepelagic phase of their life cycle (Boyd, 1967);they floated frequently and could not walk wellwhen stranded on the beaches. However, afterbeing introduced into the aquarium with numeroussurfaces to rest on, P. planipes began toexhibit more benthic behaviors. Once in captivity,the red crabs seemed to grow quickly andmany of those collected in April 2002 moltedafter only a month in the aquarium. Over time,they adopted a mainly benthic lifestyle withintermittent periods of swimming, hovering andpassive sinking. This change in the red crabs’behavior from planktonic to mostly benthic wasalso observed of the captive crabs kept in anaquarium by Boyd (1962).Boyd (1967) described red crabs as voraciouseaters, with an omnivorous diet. This hasbeen supported by observations in the watercolumn off southern Baja California where redcrabs feed on phytoplankton and zooplankton(Longhurst, 1967) and even ingest particulateorganic matter when on the ocean bottom(Aurioles-Gamboa & Pérez-Flores, 1997). Redcrabs moved their maxillipeds and then theirmandibular palps every few seconds ingestingfood particles or moving water towardsthe mouth. Similar limb motions have beenobserved in palinurid lobsters, and many othermalacostracan crustaceans, where they flicktheir antennules and mouthpart appendages andcreate convective flows of water that facilitatethe tracking of the chemical signatures of food(Goldman & Patek, 2002).Additional limb motion, possibly related tofeeding behaviors, occurred frequently whenthe red crabs groomed themselves. In contrastto some species of brachyuran spider crabs(Macropodia spp.) that attach pieces of algaeto their carapace or those that have modifiedpereopods to hold sponges to their bodies forcamouflage (Dromidia spp.) (Warner, 1977;Ross, 1983), Pleuroncodes planipes spent timemeticulously cleaning their exoskeleton of particlesthat had adhered to it with their chelatefifth pereopods. Bauer (1989) listed numeroussources of fouling particles, such as sediment,unicellular algae, and parasites that can accumulateon the exoskeleton of aquatic crustaceansand negatively impact normal functionsof sensory, respiratory and locomotory behaviors.Accumulated debris on the exoskeleton ofthe red crab may be akin to fouling organismson the hull of a ship (Bauer, 1989). If the exoskeletonwas not frequently cleaned betweenmolts, the fouling particles would build up,eventually modifying the natural streamliningof the red crab’s carapace, pereopods, abdomenand tail-fan with their associated setae.Maintaining the exoskeleton appeared to be animportant function that may enhance the pelagicred crabs’ swimming efficiency by reducingthe drag along the body (Tulipani, 2005).Furthermore, these accumulated particles werea food source for the red crabs.When red crabs interacted with each otherin the aquarium, they preferred keeping somedistance between each other, a common behavioramong crabs which allows them to avoidone another (Warner, 1977; Salmon & Hyatt,1983). When distance was not maintained, agonisticbehaviors (gesturing with chelipeds withdactyls abducted or fighting) and avoidancebehaviors (brief tail-flips away) were observedof red crabs and have been similarly describedfor other decapod crab species (Pilumnus sp.,
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