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  Predators & defenses
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  Righting response

Defenses of asteroids include righting response, considered here, and SPINES & OSSICLES, PEDICELLARIAE, ESCAPE BY CRAWLING, DEFENSIVE CHEMICALS, MUCUS, AUTOTOMY, and CAMOUFLAGE, considered in other sections.

Righting after being overturned is an ability possessed by all asteroids. Its survival value for life in turbulent waters and/or shifting substrats is obvious, but no research appears to have been done on righting ability in relation to predation in west-coast sea stars. The first question that should be addressed is "does a sea star right itself faster in the presence of a predator?". More general information on righting can be found at: LEARNABOUT SEA STARS: TUBE FEET & LOCOMOTION: RIGHTING RESPONSE.

Research study 1

photograph of sunflower star Pycnopodia helianthoides just commencing rightingphoto of ochre star Pisaster ochraceus with nerve locationg indicatedSome 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 2

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