The locomotion of the comatulid Florometra serratissima (Echinodermata: Crinoidea) and its adaptive significance

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Abstract
Swimming and crawling activities are described from observations made in situ during SCUBA diving and in aquaria. Initial vertical swimming lifts the crinoid to 29 cm mean height. Subsequently, horizontal swimming displaces the crinoid, oral surface foremost, at a mean speed of 6.8 cm/s. Horizontal swimming is achieved by differential arm extension. Behavioral adaptations make use of ambient currents to augment displacement. Swimming mechanics are based on sequential repetition of arm strokes made approximately simultaneously by three coordinated groups of arms, verified experimentally by selective arm removal. The Reynolds number while swimming is calculated to be 8112, with a drag coefficient of 0.0148. Swimming occurs in short, repeatable bursts of 10–30 s. Continuous swimming beyond 4 min provokes a refractory period lasting 5–17 min during which individuals are incapable of swimming. Spontaneous swimming was never observed in situ nor in aquaria, but was elicited by mechanical stimulation or contact with potential predators. Trials with 17 cooccurring invertebrate species of diverse feeding modes show that swimming is provoked by opportunistic carnivores, notably asteroids. Swimming is an escape response against potential predators. Crawling is the primary means of moving from place to place. Crawling is induced by directional currents and by intraspecific contact. Spacing out of individuals within an aggregation may be accomplished by crawling following agonistic contact behaviour. Functional regionalization of the arms is discussed in relation to autotomy and predation.