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

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76 D. C. Tulipani & M. A. BoudriasPagurus sp., and Callinectes sapidus) (Warner,1977).Crustaceans move from one location toanother for purposes of feeding, mating, or dispersalof juveniles (Dunham, 1983; Wilber &Herrnkind, 1986; Chatterton & Williams, 1994;Tankersley et al., 1998) with the pelagic red crabbeing no different. For example, in the wild,they migrate to surface waters to feed (Boyd,1967; Longhurst, 1967; Blackburn, 1977).The red crabs’ benthic inshore-offshore movementswere associated with coastal upwellingof cold water and the crab’s reproductive cycle(Aurioles-Gamboa, 1992; Aurioles-Gamboa& Pérez-Flores, 1997). Red crabs occupy boththe water column and the benthos in the wild,and the movements observed in the aquariumclearly fell into two subcategories reflective ofwhere they occurred. Of all locomotory behaviors,those occurring in the water column wereobserved more often than the ones occurring onthe bottom of the aquarium (58.1% vs. 41.9%,respectively) (Table 1). Differences in benthicbehaviors in captivity vs. in situ can probablybe attributed to influences of external environmentaleffects of each habitat (Chatterton &Williams, 1994).Swimming in the water column or at thesurface were the most frequently performed activebehaviors of Pleuroncodes planipes (Table1). Boyd (1962) and Kato (1974) anecdotallydescribed the same intermittent swimming patternas a way for the crabs to maintain their relativeposition in the water column. In the wild,pelagic red crabs feed on plankton in the surfacewaters (Longhurst, 1967; Longhurst et al.,1967), therefore, it would be advantageous tothe red crab to stay within the epipelagic zoneas long as possible where phytoplankton andzooplankton are abundant. Swimming rapidlyupwards, followed by slow sinking, in the captivered crabs mirrors the classic behavior patternsseen in their coastal habitat. This uniquelyefficient swimming ability (Tulipani, 2005) hadnever previously been described for galatheids.The red crab’s ability to quickly transition to itsstreamlined swimming posture from its highdragsinking position is probably an adaptationto life in the water column.Red crabs also swim to escape predation,though not like typical swimming brachyurancrabs. Red crabs are not like Callinectes sapidus(Portunidae), which use modified fifthpereopods as swimming paddles (Warner,1977; Welch et al., 1999), nor are they likeTritodynamia horvathi (Pinnotheridae), whichuses its heavily setose pereopods to swim(Takahashi et al., 1999). In physical appearanceand swimming mode, pelagic red crabs moreclosely resemble lobsters and crayfish, whichlead to their common name of squat lobsters(Newland et al., 1992). Red crabs tuck in alltheir thoracic limbs; adopt a streamline bodyshape and swim backwards using tail-flip escapeswimming (Boyd, 1962; Tulipani, 2005).Spanier et al. (1991) described a similar typeof escape response and a similar shape of thetail-fan for the Mediterranean slipper lobster,Scyllarides latus (Scyllaridae). The marineshrimp Crangon crangon, also completely flexesits abdomen, squeezing the tail-fan againstthe thorax giving the appearance of folding itsbody completely in half (Arnott et al., 1998).The mantis shrimp, Squilla mantis, exhibits amaximal tail-flip response to rostral stimuli bycompletely inverting itself then righting itselfwith a half-roll before swimming away (Heitleret al., 2000). Other crustaceans similarly performtail-flip escapes but with species-specificmaneuvers, such as half-twists, lateral rolls, orinversion of the body, during the execution ofthe escape movement (Newland et al., 1988,1992; Arnott et al., 1998; Heitler et al., 2000).Thus P. planipes swims much more like a lobsteror a shrimp than like a brachyuran crab.Further evidence of unusual swimming behaviorsfor a crab can be seen in the hoveringbehavior which could permit red crabs to easilytransition to either swimming or sinking postures.It may have developed in the aquariumbecause there were solid surfaces against whichthe crabs could “lean” while in the water column.Just as treading water can help conserveenergy, hovering has the potential for energeticsavings for captive red crabs, enabling themto swim to the surface with less effort than
Behaviors of Pleuroncodes planipes 77launching from the bottom of the aquarium.Comparisons of field observations of crustaceanactivity to observations of laboratory activitypatterns have shown that the two are notnecessarily the same, and that many organismsmodify their behavior in captivity (Chatterton& Williams, 1994). This seems to be the casewith this hovering behavior of P. planipes in theaquarium. We have not seen any evidence ofhovering in any in situ observations (Tulipani,2005).Yet Kato (1974) suggested that red crabsmaintain their position in the water column,akin to hovering, by alternating between activelyswimming and slowly sinking. Analysisof in situ video of Pleuroncodes planipes supportsthese conclusions (Tulipani, 2005) wherethey swim rapidly upward with several tail-flipsand then slowly sink with limbs extended. Theslow sinking was the only passive locomotorybehavior since the red crabs did not movetheir pereopods or chelipeds while they sank.Movement of their mouthparts indicated typicalfeeding behavior while sinking, observedby Kato (1974) and seen on in situ videotapes(Tulipani, 2005). The repeated surface swimmingand sinking pattern probably enables thelarge swarm of crabs to feed without expendingmuch energy once they have reached the surface.Other smaller crustaceans, such as copepods(Buskey et al., 1996) and mysids (Buskey,1998), also form large aggregations in surfaceswaters yet are usually continually swimmingto maintain their position and expending muchenergy to do so. Feeding while sinking is probablybeneficial energetically for the red crabsbecause once they reach the surface, they stopswimming and simply float before beginningtheir slow, passive descent. After each sinkingsegment, they only need to swim a short time toreach the surface again. This may be analogousto blue crabs (Callinectes sapidus) using tidalcycles to assist long distance travel, migratingup to 1000m day –1 to reach spawning areas (Full& Weinstein, 1992; Tankersley et al., 1998) apotential energetic benefit. It is particularly evidentin gravid female Callinectes sapidus whoapparently use ebb-tides to travel seaward to releaseembryos and then use flood-tides to returnup-estuary afterwards (Tankersley et al.1998).Thus, it appears that Pleuroncodes planipesutilize their complex suite of locomotory behaviors,especially the unique alternation ofefficient jet-propelled swimming and low costslow sinking patterns, to feed more efficiently.Analysis of their body design and fluid dynamicparameters support the contention thatthey are very well adapted to active swimmingand passive sinking (Tulipani, 2005). Thoughlocomotion accounts for only 15% of their totalrepertoire, pelagic red crabs show a clear synergybetween their functional morphology andtheir behaviors. They may not appear that differentfrom other decapods in their stationarybehaviors but their unique swimming modes setthem apart from other crabs.Acknowledgements.—Special thanks go toLisa M. Baird, Mary Sue Lowery, and Anne A.Sturz for their editorial assistance and criticalreview of this manuscript, as well as to MissVanessa Ruiz for the wonderful illustrationsincluded in this paper as well as my Master’sthesis. We would also like to thank the Schoolfor Field Studies staff in Puerto San Carlos,BCS, Mexico for their logistic support. Fundingfor this research was provided by the MarineScience Dept. at the University of San Diego,the TransBorder Institute at the University ofSan Diego, a Hannon Foundation fellowship(DCT) and the Coca-Cola Foundation.LITERATURE CITEDArnott, S. A., Neil, D. M., & Ansell, A. D., 1998.Tail-flip mechanism and size-dependentkinematics of escape swimming in thebrown shrimp Crangon crangon. Journal ofExperimental Biology, 201:1771–1784.Aurioles-Gamboa, D., 1992. Inshore-offshoremovements of pelagic red crabs Pleuroncodesplanipes (Decapoda, Anomura, Galatheidae) offthe Pacific Coast of Baja California Sur, Mexico.Crustaceana, 62:71–84.Aurioles-Gamboa, D., Castro-González, M. I., &Pérez-Flores, R., 1994. Annual mass strandingsof pelagic red crabs, Pleuroncodes planipes(Crustacea: Anomura: Galatheidae), in BahíaMagdalena, Baja California Sur, Mexico. Fishery
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