Learn About Feather Stars: Attachment & locomotion

Research study 1

photograph of feather star Florometra serratissima ready to swim

Florometra serratissima is the only swimming species of crinoid on the west coast of North America. It swims by graceful undulation of its arms in 3 sets, each set moving successively but overlapping. Thus, while about one-third of the arms are in power stroke, another third are in recovery, and the last third somewhere inbetween. During the power stroke the arms extend out maximally for greatest frictional resistance, while during the recovery stroke they bend inwards to minimise resistance.

Florometra ready to swim 0.3X

Florometra just after take-off 1X

Research study 2

photograph of feather star Florometra serratissima close view of cirridrawing of ossicle construction of cirrus of feather star Florometra serratissima courtesy Holland & Grimmer 1981 Cell Tissue Res 214: 207Stalkless crinoids such as Florometra serratissima anchor to the substratum using flexible cirri. The cirri are jointed and can slowly bend and straighten.An individual can slowly move about its habitat by crawling on its cirri. There are several theories on how the cirri contract, including use of muscles, hydrostatics, ligament elasticity, and ligament contractility. No muscle cells are visible with a light microscope and there is no strong evidence to support the other ideas. However, electron-microscopical studies in southern California on Florometra and other stalkless species (from Japan and Europe) suggest that the bending may owe to contraction of nano-filaments in the cytoplasm of epidermal cells surrounding each cirrus.The contraction is thought to be antagonised by ligaments joining adjacent ossicles, and it is the elastic energy in these that is thought to straighten the cirrus. The authors conjecture that a cirrus is able to be locked in a rigid state by neurosecretory changes in the protein rubber of the ligaments. Holland & Grimmer 1981 Cell Tissue Res 214: 207.

NOTE of roughly 650 extant crinoids, about 570 (88%) are stalkless

Research study 3

representation of arm movements during swimming of feather star Florometra serratissimaA study by researchers at the University of Victoria, British Columbia outlines locomotory behaviour in feather stars Florometra serratissima. There are 2 modes, crawling and swimming. Florometra crawls in response to environmental cues such as water currents and intraspecific contact. Crawling first requires that the distal parts of all 10 arms attach to the substratum by means of hooks on their aboral surfaces. Extension of all arms then raises the calyx so that the cirri are lifted above the substratum. The arms then contract and extend on opposite sides of the body, which moves it in one direction or the other. Repetition of this behaviour will gradually move the individual to a new location.

Swimming is more complex and involves 3 sets of arms acting sequentially. The sets comprise two triplets and one quadruplet, are their composition with respect to specific arms is invariable (see sequence below). In the scenario shown, swimming is initiated by the blue triplet making a downstroke, followed 1sec later by the green quadruplet, and 2sec later by the orange triplet. An entire sequence is completed, then, in about 3sec, and the pattern may be repeated for up to 30sec. Although not discussed by the authors, lift appears to be generated by simple downstroke sculling (see drawing on Right). The pinnules extend laterally on the downstroke and fold in on the upstroke, thus increasing sculling efficiency. Swimming generally begins for a few strokes in the vertical plane, then an individual usually tilts 90o and swim horizontally. The authors note that 33 iindividuals stimulated by touching them with the arm of a sunflower star Pycnopodia helianthoides swim vertically to an average height of 30cm at an average speed of 5cm . sec-1. After about 4min of continual stimulation, an individual tires and lays on the bottom for up to about 20min. All individuals that swim horizontally do so with the same pair of opposing arms oriented vertically. A swim terminates with cessation of beating and a “parachuting” (rigid-arm descent) to the sea bottom. Shaw & Fontaine 1990 Can J Zool 68: 942.

NOTE some of the research is done in situ using SCUBA

NOTE the 3 sets of arms are colour-coded in the schematic. Their designation with respect to the anus is constant among individuals, but it is not clear whether the same sequence of movement of arm-sets is also constant

The beat sequence for the 3 sets of arms during swimming is as follows:
schematic showing arrangement of 10 arms in a feather star Florometra serratissimaFood grooves on the arms lead to a centrally located mouth
schematic showing the first sequence in arm-beat during swimmimg in a feather star Florometra serratissima

A single swim cycle is initiated by the first arm triplet

schematic showing the second sequence in arm-beat in a feather star Florometra serratissimaAbout 1sec later the arm quadruplet begins to beat
schematic showing the third sequence in arm-beat in a feather star Florometra serratissimaAbout 1sec later the second arm triplet begins its beat
Florometra serratissima swimming