Predators & defenses
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  Considered in this section is swimming, while other topics on predators & defenses including, including SEA-STAR & NUDIBRANCH PREDATORS, BURROWING, BIOLUMINESCENCE, and TOXIC CHEMICALS/UNPALATABILITY, can be found in other sections.
Research study 1

photograph of a leather star Dermasterias imbricata
It is not uncommon for sea pens Ptilosarcus gurneyi to swim/float from different types of predators, including leather stars Dermasterias imbricata.  After uprooting from the substratum the behaviour combines full body inflation with gentle undulations.  While not “swimming” in the conventional sense, the sea pen becomes almost neutrally buoyant and water currents may carry it several tens of meters away from danger.  Why burrow or withdraw if swimming is effective? The answer may be that sea pens have a graded response to danger.  Why uproot and be displaced from the bed if partial or complete burial is sufficient?




Leather star Dermasterias imbricata exhibiting an unusual behaviour.
It is tempting to think the sea star is testing the current for traces
of prey, but this would be more convincing if more than just a few tube
feet were being exposed to the current to sense the supposed prey 0.4X

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Research study 2
  drawing series showing swimming response of a sea pansy colony to the approach of a predatory sea starSea pansies Renilla amethystina (koellikeri) can live on either solid or sandy substrata.  They prefer the latter, however, and anchor in it by means of an expandable, fleshy peduncle (stalk).  When predators come near, such as sea stars Astropecten armatus or the nudibranch Armina californica, Renilla escapes by withdrawing its anchor, flattening the rachis, catching a ride on the current, and then re-attaching. Kastendiek 1982 Oecologia 52: 340.
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