Predators & defenses
 

Among world species of sea cucumbers, defensive features of adults include a leathery body wall, swimming, withdrawal into crevices, evisceration/autotomy, skin ossicles, unpalatability/toxic chemicals, nocturnal and/or cryptic behaviour, and sticky, noxious tubules released from the cloaca.  All but the last defensive strategy are found in west-coast sea cucumbers.  When damaged by a predator or when deliberately crushed into a tidepool for fishing purposes, some Indo-Pacific species have sufficient toxins in the body to kill or incapacitate fishes. The main predators appear to be sea stars. 

NOTE  for a review of predators of world species of holothuroids see Francour 1997 Invert Biol 116 (1): 52

NOTE  chemicals of possible defensive function include saponins (soap-like substances) and various terpenoids, some with known feeding-deterrent capability.  Bryan et al. 1997 J Exper Mar Biol Ecol 210: 173.

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  Predatory sea stars
  Predatory sea stars will be considered in this section, while defenses such as THICK BODY WALL/SKIN OSSICLES, SWIMMING, WITHDRAWAL INTO CREVICES/"CATCH" CONNECTIVE TISSUES, EVISCERATION & REGENERATION, and TOXIC CHEMICALS/UNPALATABILITY will be considered in other sections.
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Research study 1
 

Studies in San Juan Islands, Washington suggest that sea stars may be important predators of sea cucumbers Psolus chitonoides, but not fishes, possibly owing to the presence of aversive chemicals.  Observations in the field of over 100 feeding sea stars show the proportions feeding on Psolus.

Dermasterias imbricata        39%
Pycnopodia helianthoides     19
Solaster stimpsoni                 8
Solaster dawsoni                   8              

Time to digest a Psolus varies from 3d for Dermasterias to 1-2d for the other sea-star species.  Bingham & Braithwaite 1986 J Exper Mar Biol Ecol 98: 311.

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Research study 2
 

photograph of sea star Solaster stimpsoniThe chief sea-star predator of holothuroids on the west coast is Solaster stimpsoni.  One of its prey is the small holothuroid Pseudocnus lubricus, a species often found growing in virtual monoculture in areas of the San Juan Islands, Washington and elsewhere (densities exceeding 400 individuals . m-2).  Although the sea star readily eats Pseudocnus in the field, in laboratory tests 2 other holothuroid species are preferred, Eupentacta quinquesemita and E. pseudoquinquesemita.  Both of these species show escape responses to Solaster, while Pseudocnus does not.  Also, both Eupentacta species are thought to compete for space with Pseudocnus.  The author speculates that S. stimpsoni may contribute to the maintenance of natural monocultures of P. lubricus by preferentially consuming or driving away the Eupentacta species that are competing for space with PseudocnusEngstrom 1988 p. 445 In, Echinoderm biology (Burke et al., eds) AA Balkema, Rotterdam.

NOTE  in Puget Sound, Washington this species is recorded as having a diet of 100% holothurians, including Pseudocnus lubricus (Cucumaria lubrica), and Eupentacta sp., Psolus chitonoides, and Cucumaria miniata. Mauzey et al. 1968 Ecology 49: 603

NOTE  in preference tests, even if the biomass of Pseudocnus is doubled relative to that of Eupentacta, there is still a clear selection for the 2 Eupentacta species

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Research study 3
 
Most attacks by sea stars on holothuroids observed by SCUBA divers involve Parastichopus californicus. This holothuroid species lives openly on the sea bottom where it feeds; it does not burrow or inhabit rocky areas with crevices. Although the sea cucumber's swimming ability to escape from attack by a fast-moving predator like the sunflower star Pycnopodia helianthoides is highly effective, not all contacts with the predator are certain to end well.
photograph of a sea cucumber Parastichopus preparing to swim from a sunflower star Pycnopodia helianthoides
Here, the intended prey is just beginning its series of rhythmic contractions preparatory to swimming 0.3X
photograph of a sea cucumber Parastichopus preparing to swim from a sunflower star Pycnopodia helianthoides
Despite close contact, Parastichopus' robust swimming contractions will likely propel it to safety 0.5X
photograph of a sea cucumber Parastichopus preparing to swim from a sunflower star Pycnopodia helianthoides
This outcome is less certain but, if you were to bet on the outcome, put your money on the sea cucumber 0.6X
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Research study 4
 

map showing distribution/abundance of sea cucumbers Pachythyone rubragraph comparing predation by different invertebrates on sea cucumbers Pachythyone rubraPatchy distribution is often associated with species that brood their young and release them as crawl-away juveniles.  Such patchiness is common in the sea cucumber Pachythyone rubra in the California Channel Islands.  In this region the species may cover up to 50% of the substratum in kelp-forest and urchin-barren habitats, and high-density patches (>1000 individuals . m-2) are known to persist and be stable over many years.  A researcher from UC Santa Barbara investigates possible causes of patchy distribution from the standpoints of  predation, sedimentation, water flow, and food supply, and uses transplants of individuals from areas of high density to areas of low density to monitor survival and growth. Laboratory tests with several potential predators show that only 2 species consume P. rubra: sunflower stars Pycnopodia helianthoides and spiny lobsters Panulirus interruptus (see graph). In the western part of the archipelago, cool phytoplankton-rich water provides good conditions for growth and reproduction, but pockets of Pycnopodia sea stars take their toll (see map).  In the eastern region of the Channel Islands, the water is warmer and has less phytoplankton.  This leads to less growth and reproduction in Pachythyone, but sunflower stars are absent, so the patches of sea cucumber are more stable over space and time.  Of the various experiments done, those relating to predation seem to be most convincing in explaining the patchy distribution.  A single P. helianthoides moving through a dense area of sea cucumbers will leave an empty swath as it moves along, and infrequent visits by individual sea stars could produce the patchy distributions seen over time.  Eckert 2007 J Exper Mar Biol Ecol 348: 121.

NOTE  some patches on Santa Rosa Island exceed 4000 individuals . m-2

NOTE  field tests with pelleted dry flesh of Pachythyone show that fishes find something distasteful in them and are deterred from eating

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