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

72 D. C. Tulipani & M. A. BoudriasTable 1. Activity budget of Pleuroncodes planipes.Detail of locomotory behaviors. See text fordefinition of each behavior.Locomotory behaviors (15%)Active PassiveWater Column:Surface Swimming 23.5%Swimming 16.2%Sinking 9.6%Hovering 8.8%Subtotal 48.5% 9.6% 58.1%Benthic:Walking 35.7%Climbing 6.2%Subtotal 41.9% 0.0% 41.9%Total 90.4% 9.6% 100.0%of all locomotory behaviors, while benthicbehaviors (climbing and walking) comprisedthe other 41.9% (Table 1).Therefore, the crabsspent the majority of the locomotory time activelymoving around in the water column.Locomotory behaviorA swimming bout was a finite event thatbegan once a crab launched itself from a surfacein the aquarium and ended when the crablanded on some other surface in the aquarium(Fig. 1). For example, a complete swimmingbout could start with a crab hanging head downfrom one of the side walls of the aquarium, thenlaunching itself away from the wall, performingseveral tail-flips, alternatively sinking andtail-flip swimming, until landing on the bottom.A ‘power swim’ segment was defined as a portionof a swimming bout when a crab rapidlymoved in a curved or straight path using manyconsecutive tail-flips without stopping until itreached the surface of the water, a side wall, orperformed another maneuver (Fig. 1B).Swimming consisted of propulsive tail-flipsexecuted with the abdomen and tail fan whilethe crab adopted a streamlined swimming position,moving around in the water column. Atail-flip consisted of five sequentially occurringmovements that, when combined, generatedthe propulsive force that moved a crab rapidlyin a posterior and dorsal direction (Fig. 4A–D). First, a tail-flip started from the positionwhere the abdomen was contracted and the tailfan was flat against the thoracic plate (Fig. 4A).Second, there was an extension of the abdomenand tail fan posteriorly (Fig. 4B). Third,when the abdomen was at its maximum extension,the tail fan was extended perpendicular tothe abdomen at the fourth abdominal segmentwith a simultaneous abduction of the uropods(Fig. 4C). Fourth, this was followed by a rapidcontraction of the abdomen with the telson anduropods still expanded (Fig. 4D). Fifth, the tailflipended with a contraction of the abdominalmuscle to press the retracted tail fan againstthe thoracic plate of the crab producing a jet ofwater that propelled the crab posteriorly anddorsally. This final position was the same as thefirst and completed the tail-flip propulsion (Fig.4A).During some swimming bouts, crabs couldhover in mid-water by using several tail-flickswhere the abducted tail fan beat against theabdomen without the full abduction-adductioncycle of the uropods (Fig. 2B). In this case,the crab positioned its body with its rostrumpointing to the bottom of the aquarium, thecurved dorsal edge of its abdomen pointingto the water’s surface, and all of thoracic legswere extended away from the body; once inthis posture, it executed numerous tail-flicks.This particular motion did not create a full jetof water like that in tail-flip swimming. Thusthe crab maintained its relative vertical positionin the water column with this sequence ofmovements. While hovering, crabs sometimesdrifted horizontally due to the water flow in theaquarium.Passive sinking segments were identifiedeasily when a crab splayed out all of its legs,extending them so that none were touching, andslowly sank without moving any appendages.The general body shape in the passive sinkingposture was similar to the shape of an openedparachute, and the setae that line the pereopodsand chelipeds passively extended away fromboth the medial and lateral edges of all limbs tobe perpendicular to the limb (Tulipani, 2005).