Defenses & predators
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  Toxic chemicals/unpalatability
  Topics of toxic chemicals/unpalatability will be considered in this section, while THICK BODY WALL/SKIN OSSICLES, SWIMMING, WITHDRAWAL INTO CREVICES/"CATCH" CONNECTIVE TISSUES, EVISCERATION & REGENERATION, and SEA-STAR PREDATORS will be considered in other sections.
 
Research study 1
 

Early research on toxicity of extracts of tissue of west-coast holothurians to various fishes collected in Santa Catalina, California and San Juan Islands, Washington yield positive results for Cucumaria salma, C. piperata, Pseudocnus lubricus (Cucumaria lubrica), and Eupentacta quinquesemita out of a total of 14 species tested.  Only body walls are found to be toxic, and not viscera or gonads.  In comparison, similar tests on holothurians in Mexico and Cocos Islands show toxicity in all but one of 12 species.  The author credits the almost universal toxicity in these forms to greater numbers and higher species diversity of potentially predatory fishes. Bakus 1974 Biotropica 6: 229.

NOTE  potentially toxic compounds are isolated as follows: a standard mass of holothurian (skin, gonad, gut, and so on) is extracted in a solvent (ethanol, ether, seawater), the solution evaporated by boiling, and the extracted residue dissolved in 100ml seawater or freshwater (depending upon what type of fish is to be tested, e.g., goldfish, cottid, rockfish) in a fingerbowl.  A test fish is added and time to death recorded.  A "strongly toxic" holothurian is defined as causing death within 15min, whereas a "weakly toxic" one causes death in 20-45min.  Control experiments consist of placing fishes in the same volume of clean seawater or freshwater as that used in the experiments

 
Research study 2
 

photograph of the tentacles of the sea cucumber Psolus chitonoidesStudies in San Juan Islands, Washington suggest that sea stars may be important predators of sea cucumbers Psolus chitinoides, but not fishes possibly owing to the presence of aversive chemicals.  Sculpins Oligocottus maculatus avoid eating tissues of Psolus and, if force-fed, will die.  Similarly, kelp greenlings Hexagrammos decagrammus that commonly prey upon other sea cucumbers, such as Cucumaria miniata and Eupentacta quinquesemita, will avoid eating Psolus Bingham & Braithwaite 1986 J Exper Mar Biol Ecol 98: 311.

 

 

 

Fishes have good colour vision and may be warned off
from attacking a potentially toxic meal by the brightly
coloured tentacles of Psolus chitinoides 1.5X

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On first reading, the figure legend in Research Study 2 above seems to make sense, but what may wrong with it? Here are some possibilities. CLICK HERE for explanations.

Psolus is protected by armoured plates and, other than the tentacles, is inedible for reasons other than possible toxicity; hence, colour is irrelevant.

Red colours may seem bright to us but, at depths greater than 2-3m, they will appear black.

Fishes hunt more by smell than by sight, especially in the dim waters of Puget Sound.

The tissues of Psolus, as other holothuroids, contain a myriad of ossicles and these, not toxins, are what make the tissues unpalatable.

 
Research study 3
 

histogram showing amounts of sea cucumber eggs and larvae eaten by fish,tunicate, and sea-anemone predatorsphotograph of the tentacles of a sea cucumber Cucumaria miniataEggs in lecithotrophic holothuroids are often brightly coloured, suggesting that they may be warning of the presence of unpalatable chemicals. Studies in San Juan Islands, Washington show that eggs and larvae of Cucumaria miniata are rejected as food by sculpins Oligocottus maculosus, and by sea anemones Anthopleura elegantissima and Metridium senile, but not by tunicates Cheylosoma productum.  The histogram shows the proportion of eggs/larvae eaten (yellow bars) by each of the predators in comparison with proportion of control food eaten (blue bars). For example, the sculpins eat no eggs or larvae of Cucumaria but show that they are, indeed, hungry by eating 89-100% of the control food. In contrast, tunicates find the eggs and larvae of Cucumaria as palatable as brine-shrimp eggs. The authors conclude that because these stages lack obvious structural or behavioural defenses, the most likely source of their deterrence is chemical (likely saponins).  Iyengar & Harvell 2001 J Exper Mar Biol Ecol 264: 17.

NOTE only larvae, not eggs, are tested with the sea anemones

NOTE control foods are small pieces of ripe ovaries of sea urchins for the sculpins and sea anemones, and brine-shrimp eggs for the tunicates

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Research study 4
 

photograph of an eviscerating sea cucumber Eupentacta quinquesemita
Predation by fishes appears not to be an important source of mortality for Eupentacta quinquesemita.  A study on the process of evisceration in this species shows that the expelled viscera are not eaten by fishes and the author suggests that the adult itself may be unpalatable. Byrne 1985 Ophelia 24: 75; Byrne 2001 J Exper Biol 204: 849.

 


A mid-way stage in evisceration in E. quinquesemita. The tentacles are visible,
attached to the introvert (the round ball). In a living animal the introvert is
pulled in for protection by 10 muscles bands known as tentacle retractor
muscles
. In this photo these muscles have been broken and the contracted
stubs of 4 or 5 are visible around the base of the introvert. At a later stage
the introvert, along with various visceral elements, is completely expelled 2X

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Research study 5
 

Analysis of several Puget-Sound holothuroids shows high concentrations of saponins in tentacles and viscera and, to a lesser extent, in the body wall.  Of 4 species analysed, Psolus chitinoides has 2-4 times higher concentration of saponins than Eupentacta quinquesemita, Cucumaria miniata, and C. piperata.  Interestingly, homogenised tentacles of Psolus incorporated into gelatin cubes are rejected by sculpins Oligocottus maculosus, but identical preparations of purified saponins from tentacles or body wall are readily eaten by the fishes; however, in most or all cases these ingestion lead to death. This suggests to the authors that the saponins themselves are not distasteful and that some other factor(s) may be involved in Psolus’ unpalatability. Bingham & Braithwaite 1986 J Exper Mar Biol Ecol 98: 311.

NOTE  soap-like chemicals widespread in plants, especially in outer layers where they form waxy protective coating.  Many types of saponins are toxic.  For example, foxgloves (Digitalis spp.) contain a suite of cardiac and steroidal glycosides (saponins) that contribute deadly toxicity to the entire plant.  Toxic saponins are known generally as sapotoxins and, in holothurians, as holothurins or holotoxins.  Hothothurin is a steroid glycoside first isolated from the Cuvierian tubules of tropical Holothuria spp. and Actinopyga agassizi (see Nigrelli et al. 1955 Zoologia 40: 47)

NOTE in control tests, gelatin alone is readily eaten in cube-form by the fishes.  The authors test similar preparations of other species of sea cucumbers (data not shown here) and use red food-colouring in the cubes to standardise the visual appearance of the cubes to the fishes

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