Defenses
 
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Invertebrate defenses: structure

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The topic considered here relating to defenses of invertebrates deals with structure, including size refuge. Other invertebrate defenses are accessible via the icons.

Structural defenses in coral-reef invertebrates include spines, considered in this section; SIZE REFUGE,STRONG/HEAVY SKELETON, and LEATHERY SKIN in a second section; and SPICULES in a third section.

 
 

Invertebrate defenses: structure: spines

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of black-spined sea urchin Diadema antillarum taken from a video

"Here’s somebody to avoid.  Look how the spines wave about when we approach.  Although sea urchins have no eyes, there are light-sensitive cells on the skin that are responsive to brightness and shadows. " -

NOTE the spines of this sea-urchin species, Diadema antillarum, are extremely sharp and will penetrate rubber wet-suits, swim flippers, and human flesh...and are very painful 

 
  Calceous spines are common defensive elements in reef invertebrates. A predator may be pierced outright and wounded, or the time and effort to bite at or digest bits of a spiny prey may outweigh the nutritional benefits. Protective spines are possessed...
 
photo collage of spiny defenses of various Caribbean coral-reef organisms
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a sponge-inhabiting brittlestar taken from a video

"Lots of species of brittle stars hide under rocks and in crevices, but there’s at least one type that likes to hang out on sponges.  It’s a vulnerable place to be so we can guess that they are protected in some way." 

NOTE could this be a mutualistic association?

 
 

photo collage of bluehead wrasse looking over a blunt-spined brittle star Ophiocoma as a source of food
While many species of brittle stars are highly motile and crawl quickly away from touch or other irritation, others seem to hang out in plain view, and appear to rely more on passive defenses of spines and calcareous plates. There is little fleshy component to a brittle star, and potential predators may get turned off by getting just a mouthful of spines and other calcareous parts of little nutritional value. Studies in St. Croix testing the palatability of several species of brittle stars with bluehead-wrasses and other predators show that the blunt-spined brittle star ranks second in order of susceptibility to being eaten out of 5 species tested. The author attributes this to its comparatively fragile construction and readily autotomisable arms. Aronson 1988 Bull Mar Sci 43: 93.

NOTE autotomy: lit. "self cut" G. More on this subject can be found in DEFENSES: INVERTEBRATES: BEHAVIOUR

 
 
But wait! Aren't there other possible explanations for the susceptibility of blunt-spined brittle stars to being eaten by bluehead wrasses? The author of the study provides several alternative hypotheses, as listed here. Check them over and then read explanations of their respective usefulness for further experimental testing below. Ideas from Aronson 1988 Bull Mar Sci 43: 93. table of alternative hypotheses explaining susceptibility of blunt-spined brittle stars to being eaten by bluehead wrasses
 
Explanations for the above alternative hypotheses:

1. YES, this is possible, and should be readily testable.

2. YES, this is also testable. While chemical defenses are not well studies in brittle stars, an acid mucus has been described in a few species that may be aversive to fishes.photograph of long-spined brittle star Ophiothrix suensonii courtesy Anne Dupont, Florida

3. well, this is certainly possible, but recall that the brittle stars being tested are tethered to weights on the sea bottom, so the idea is not really applicable here.

4. YES, this is plausible. The author remarks that one of the least-preferred species in the study Ophiothrix oerstedi is notable in its possession of "long, sharp, glassy, arm spines". The photograph on the Right shows such long arm-spines in a closely related species O. suensonii. Photograph courtesy Anne Dupont, Florida.

5. YES, this is a good hypothesis and experiments could be designed to test it.

6. YES, this is possible, and is related to #5 above.

7. well, yes, we know this already. The question is why are the blunt-spined ones more readily eaten by wrasses and other predators?

 
 
   

So, there is actually a number of quite good alternative hypotheses to investigate.

Let's turn to a professional, S.L.P. Dick, a well-known Brittle-Star Investigator, to help us sleuth out what would be a good design for an experiment on preferential selection of brittle-star prey by bluehead wrasses.

 

cartoon 1 showing start of investigation of palatability of brittle stars to bluehead wrasses
 
S.L.P. Dick: "Shown here are 4 ideas to test 3 species of brittle stars for 'tastiness'. Examine each experimental protocol and think about how well each would test the hypothesis. Afterwards, I will give you my opinion". number 1 in a series of 4 experiments to test the "tastiness" of brittle stars to bluehead wrasses number 2 in a series of 4 experiments to test the "tastiness" of brittle stars to bluehead wrasses
1. in this experiment you snip off equal-sized bits of arms of each species and offer them cafeteria-style to the wrasses 2. here you grind up each species, bag the grindings in mesh so that the flavour can be sensed, and present bags to the wrasses
  number 3 in a series of 4 experiments to test the "tastiness" of brittle stars to bluehead wrasses number 4 in a series of 4 experiments to test the "tastiness" of brittle stars to bluehead wrasses
  3. in this experiment you grind up each species, then create pellets out of each grinding in a pellet-press , and offer a selection of pellets cafeteria-style to the fishes 4. here you grind each species, extract grindings with solvent, filter bits from solvent, allow solvent to evaporate, incorporate dry extract into regular fish-food pellets, and present to fishes
   
 
S.L.P. Dick: "Well, there are some interesting and innovative submissions here, but few if any actually address the hypothesis to be tested, that is, to test the 3 species of brittle stars for 'tastiness'. Here are my comments:"

1. "this experiment is straight-forward enough, but it doesn't account for the fragility aspect brought up in the original study. In this design it may still be the most weakly constructed species that is preferred owing to its ease of ingestion"

2. "this design offers a good way to assess olfactory attractiveness, but not to test whether the fish finds one species more tasty than another. The problem is that the fishes are never given an opportunity to eat the grindings!"
  3. "this design is, as we say, "elegant in its simplicity". The homogenisation and pelleting eliminates variability in physical construction, and tests only for tastiness. But for a single sticky point, this is an excellent research idea...or, at least, a good start on one." 4. "here we give credit to ingenuity and careful lab techniques, but not to the design. This idea might be good for identifying predator-deterrent chemicals in the brittle stars, but not for testing our idea. What if the "tastiness" has been left behind in the original un-extracted grindings?"
   
  NOTE the "sticky point" referred to relates to the colour of the pellets. For all we know, bluehead wrasses may have evolved preferences for certain colours of food. One possible way to get around this would be to...WAIT!...think about this, then check the bottom of this page for a suggestion...
 
 

photograph of fighting conch Strombus pugilis
Many gastropods have adornments of spines along with thick shells for protection.

 

 

 

 

 

Spines on this fighting conch Strombus pugilis
are not sharp enough to pierce a predator, but
they might make it more difficult for a predator
to catch hold or take a bite 1.2X

 
  drawing showing bluehead wrasse being conditioned to feed in the dark


A possible solution to the "sticky-point" problem is simply to habituate the wrasses to eat their pelleted foods in the dark. Good thinking if you thought of this possibility, and there may be others...
 
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