Locomotion
 

Topics relating to locomotion include RIGHTING RESPONSE, considered here, and FUNCTION OF THE MADREPORITE, TUBE FEET, LIGHT PERCEPTION, and ARM NUMBER, considered elsewhere.

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Righting response

  When a sea star finds itself on its aboral surface, that is, upside-down, it endeavours to right itself. Righting has obvious survival value in conditions of rough water and/or shifting substrata, and likely also in protection from predation. The mechanism and behaviour of righting is considered here, while its selective value in defense of predators is dealt with in another section LEARNABOUT SEA STARS: DEFENSES: RIGHTING RESPONSE.
 
Research study 1
 

photograph of sea star Stylasterias forreri completing a righting movement of the under-arm-creeping typeAn early study at the marine laboratory in La Jolla, California describes righting in the fish-eating sea star Stylasterias forreri.  The author describes several methods used in this behaviour, including somersaulting, twisting, flipping, under-arm creeping, and raising & toppling, with several variants of each depending upon how many arms are used and the direction of the righting movement.  Unfortunately, and perhaps understandably, both the photos and descriptions provided are hard to understand, and the author might have made a better contribution by providing sequential drawings for each of the various methods.  Jennings 1907 Univ Cal Publ Zool 4 (2): 53.

NOTE  the author also devotes several pages to a description of training and learning in Stylasterias but, again, lack of clarity in descriptions an an absence of graphed or tabulated data make the presentation hard to understand

End of an under-arm creeping sequence of righting involving, first, the
attachment of 4 arms, then the free one being swung over, followed by
2 of the arms creeping underneath the other 2. Not very clear, is it?

 
Research study 2
 

photo of ochre star Pisaster ochraceus with nerve locationg indicatedphotograph of sunflower star Pycnopodia helianthoides just commencing rightingSome west-coast asteroids move so slowly that escape by crawling is not an option.  When the tide is out, intertidal species protect themselves by nestling into crevices or anchoring firmly to the substratum.  Righting ability of sea stars after being dislodged by a predator or by waves has long been of interest to researchers. Turn-of-the-century studies on ochre and bat stars, Pisaster ochraceus and Patiria miniata, respectively, show that righting is initiated from no preferred arm location; rather, the arm whose tube feet first touch the substratum determines the course of the righting.  This arm begins at its distal end to twist the aboral side upwards, joined later by one of the adjacent arms, which also obtains a hold and twists.  The other 3 arms and body pass overhead in a somersaulting motion. 

The nerves controlling righting are the oral nerve ring and the radial nerves (see photo/schematic upper Right). The former is located around the mouth and the latter are in the ambulacral grooves. If the oral ring is cut at the location shown in the illustration, righting still occurs, but is less well coordinated and takes 2-3 times longer than in an undamaged individual.  Control cuts made from the aboral surface downwards without injuring the nerve do not disrupt the coordination of righting.  The author correctly concludes that there is no “nervous centre”, and proposes that any one of the 5 arms can give rise to impulses which affect only the 2 adjacent arms.  The author proposes that these impulses would diminish in strength as they travel from their point of origin, thus affecting arms other than adjacent ones only imperceptibly. Moore 1910 Biol Bull 19: 235; Moore 1910 Amer J Physiol 27: 207.

NOTE the radial nerves are located just under the epidermis in the ambulacral grooves. With care they can be exposed and stripped out. The cut as described here is from the oral surface towards the aboral surface, but just deep enough to sever the oral ring

NOTE  although not relevant to short-term studies as described here, such cuts heal quickly.  Thus, experiments lasting longer than a few hours generally require the insertion of some sort of block, as a piece of wax or plastic, to prevent the nerve ends rejoining

 
Research study 3
 

photograph of Leptasterias hexactis courtesy Dave Cowles, Walla Walla University, Washingtonphotograph of blood star Henricia leviuscula courtesy Dave Cowles, Walla Walla University, WashingtonAnother question relating to leading arms during locomotion, is whether there a preferred/leading pair of arms used during righting. This is tested in a study in Oregon using Henricia leviuscula and Leptasterias hexactis.   Most asteroids right themselves by somersaulting, or a combination of folding-over and somersaulting, and these 2 species are no exception.  Henricia only rights by somersaulting, while Leptasterias uses both somersaulting and folding-over, sometimes in combination.  The authors report a tendency in Henricia for the arm pair opposite the madreporite to lead during righting, but with a more general pattern being exhibited in Leptasterias.  As expected, size has an effect on righting times, with larger animals taking significantly longer to right.  However, the authors do not specifically investigate scaling effects of size and this might be an interesting project for further work. Neither righting method was significantly faster than the other, suggesting no advantage for survival in the field.  Polls & Gonor 1975 Biol Bull 148: 68. Somersaulting information from Polls 1970 MSc Thesis, University of Oregon. Photographs courtesy Dave Cowles, Walla Walla University, Washington diagram explaining the somersaulting method of righting in a sea star Henricia leviuscularosario.wallawalla.edu.

NOTE  in somersaulting all 5 arms curl aborally bringing their tips in contact with the substratum (see drawings on Right).  One adjacent pair of arms then twists so that they face one another.  These 2 arms begin to move to the sides causing the remaining arms to rise up and swing over the body.  The folding-over method is similar but without the curling of the arm-tips.  A third, rarely seen method, involves all arms rising up to form a tulip shape.  Several arms then collapse causing the animal to flop over.  Interest in the subject dates from 1862.

Another example of somersaulting is provided by a group of student researchers in China for Asterias amurensis. The righting (see photos below) proceeds as described for Henricia: 1) individual is upside-down, 2) arms uniformly rise and curl, 3) 2 adjacent arms attach, 4) these 2 arms walk distally, raising the other arms, 5) opposite arm rises up, and 6) body flips over. Somersaulting is complete. Wu et al. 2000s Undergrad Innov Prog, China Agricultural Univ, Beijing.
photographs of Asian sea star Asterias amurensis righting itself

Other west coast somersaulters are Pisaster ochraceus and Patiria miniata, while somersaulter/folding-overs include Pycnopodia helianthoides. The photo series below shows a juvenile P. helianthoides righting itself apparently by the folding-over method, although for a multi-armed species it is hard to see the difference between folding over and somersaulting. The sequence goes from Left to Right.

 
photograph of a juvenile sunflower star Pycnopodia helianthoides righting itself by the folding-over method stage1 photograph of a juvenile sunflower star Pycnopodia helianthoides righting itself by the folding-over method stage2 photograph of a juvenile sunflower star Pycnopodia helianthoides righting itself by the folding-over method stage3 photograph of a juvenile sunflower star Pycnopodia helianthoides righting itself by the folding-over method stage4
 
 
photograph of a sunflower star Pycnopodia helianthoides righting itself taken from a video

CLICK HERE to see a video of a sunflower star Pycnopodia helianthoides righting itself.

NOTE  the video replays automatically

 
Research study 4
 

graph showing mean righting times of seastars Pycnopodia helianthoides, Henricia leviuscula, Leptasterias hexactis, and Patiria miniataA comparison of righting behaviour and righting times in 4 species of sea stars done by undergraduate researchers in an “adaptations of marine animals” class at the University of Oregon is worth mentioning here, especially since it shoud lead to further follow-up research.  Results indicate faster absolute righting times in sunflower stars Pycnopodia helianthoides, which the authors correlate with the species’ faster locomotory ability and predatory life-style.  The authors address the more interesting aspect of relative righting speeds in the 4 species, but unfortunately are not clear in their definitions of “size of the body” and “body diameter”, so the results are not included here.  The study is valuable in the number of questions that come to mind, such as righting times relative to body size and arm length, noted above, and broader questions relating to righting times and habitat occupied, feeding habits, presence of predators, locomotory speeds, and so on.  Pearson & Pedemonte 2008 Res Rep, Adaptations of Marine Animals, Exploratory 1, University of Oregon.

NOTE  species are selected on the basis of “availability and size”, and include Pycnopodia helianthoides, Henricia leviuscula, Leptasterias hexactis, and Patiria miniata

NOTE  sample sizes used in the experiments are also small, ranging from single individuals for 2 of the species, to 3 and 6 individuals for the other species. Perhaps for this reason, the authors consider statistical tests to have been inappropriate, although the deficiency could have been easily remedied

 
Research study 5
 

photograph of Leptasterias aequalis courtesy Kirt Onthank and Dave Cowles, Walla Wall University, WashingtonResearchers at Louisiana State University and the National Marine Fisheries Servicve, Alaska investigate righting speeds of sea stars Leptasterias spp. collected at  San Juan Island, Washington and Lena Cove, Alaska, at different air and water temperatures, and find that activities of the Washington species are similar in water of 12-18oC, but significantly reduced at temperatures above 27oC.  Exposure to air for 3h, typical for mid-intertidal-inhabiting Leptasterias spp. has no significant effect on righting activity at 9oC or 15oC, but exposure at 19oC significantly reduces activity.  The study is really a physiological/biochemical one, using righting response as a measure of metabolic activity, but may be of interest to researchers interested in ecological aspects of righting in Leptasterias and other sea stars.  Stickle & Rice 2010 p. 403 In, Echinoderms: Durham (Harris et al., eds) Taylor & Francis Group, London. Photograph courtesy Kirt Onthank and Dave Cowles, Walla Walla University, Washington wallawalla.edu.

NOTE  the 3 Washington species are L. hexactis (48% representation based on results of other genetics studies), L. aequalis sp. B (28%), and L. aequalis sp. A (24%), while the Alaskan species are predominately L. alaskensis with a few L. hexactis

 

Photograph of Leptasterias aequalis 2X

 
Research study 6
  histogram comparing righting times in several species of west-coast sea starsA study by a student researcher at Friday Harbor Laboratories involves comparing righting times of 6 species of sea stars on a seawater table.  Results are as expected, with the multi-armed fast-moving sunflower star Pycnopodia helianthoides righting the fastest.  Ochre stars Pisaster ochraceus, as might be expected from their bulky shape and sedentary lifestyle, are slow but not much different from other sedentary species (see histogram).  No species shows a preference for a specific leading arm.  The author does not specifically comment on how each species rights itself, but it seems that somersaulting is a common mode.  Burdi 2012 Mar Invert Zool Rept, Summer 2012, 23pp.

NOTE  individuals are of average adult size with an of 6 for each species.  The author compares righting times on sand for each species but, since all of the species normally inhabit rocks, these data are not included here

 

Photographs are not in scale

 
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