Defenses
 
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Invertebrate defenses

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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 spicules, considered in this section; SIZE REFUGE, STRONG/HEAVY SKELETON, and LEATHERY SKIN in a second section; and SPINES in a third section.

 
 

Invertebrate defenses: structure: spicules

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of fan gorgonian taken from a video "Nothing worse at a beach picnic than getting a mouthful of sand when you bite into your sandwich.  The gorgonians that we just passed and these tube sponges have spicules embedded in their flesh, but it’s not certain whether these are for structural support or defense." - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.
 
 

photograph of orange-icing sponge Mycale laevis
Most Caribbean sponges have spicules embedded in their tissues. Unlike in gorgonians, where the spicules are comprised of calcium carbonate, the spicules in sponges are made of oxidised silicon, or glass. Depending upon their size and shape, the spicules in sponges are thought to function for protection and/or structural support.photograph of 2 types of sponge spicules in close view

Monaxonic spicules form a protective barrier around the exhalent siphons of an orange-icing sponge Mycale laevis 0.5X

NOTE lit. "single axis". These spicules consist of a single rod, with one or both ends pointed



Cross-section of a sponge showing 2 types of spicules:
a type of aster for structural support, and a long, slender
monaxonic type (lower Right, blurred) for defense 40X



 

photomicrograph of an assortment of sponge spiculesThere are many different types of sponge spicules and a complex nomenclature has been developed to describe them. It's actually quite difficult to determine from a spicule's shape what its function is. For example, try to guess the function of each of the spicules highlighted in red on the Right, whether primarily for defense, or primarily for structural support.

 

 

No-one knows the answer, but a clue to defensive function might be in the degree of pointiness of a spicule, especially if only one end is pointed, while a clue to supportive function might be in a shape that holds or grips the tissues together. So, here is one "best guess" for the 8 highlighted spicules, but readers may have their own ideas.

array of sponge spicules with "best guesses" as to function

  graph relating spiculte content of a sponge to its palatability to potential predatorsThe truth is that our present knowledge does not allow us any more than a "best guess" as to function. However, if we generally accept that spicules likely act in some way to deter potential reef-fish predators, then the palatability of a sponge should decrease as spicule content rises, as shown in this graph.
 

graph showing palatability of 4 Caribbean sponge-spicules to bluehead wrassesTo test this idea on Caribbean sponges and their potential predators, researchers incorporate the spicules of 8 different sponge species into reconstituted food-pellets at natural concentrations and offer them to bluehead wrasses. Results are shown here for 4 commonly occurring sponges tested. The results show that for bluehead wrasses, spicules of any shape appear not to deter feeding, at least not when included in an otherwise highly edible food-pellet form. Chanas & Pawlik 1995 Mar Ecol Progr Ser 127: 195.

NOTE with the exception of the two long spicules shown for Geodia neptuni, all spicules shown are approximately 10X life size. The two long spicules are 2X life size

NOTE the pellets consist of powdered squid flesh in a natural gel, which the wrasses eat completely in control trials. The pellets contain spicules in concentrations mimicking those in the living sponges

 

photograph of a sponge-eating Caribbean nudibranch Discodoris mortensi courtesy Sandra Millen, University of British Columbia
Spicules are also no deterrent to all Caribbean sponge-eating nudibranchs, whose feces are filled with them. Photograph courtesy Sandra Millen, University of British Columbia.

 

 

 

Dorid nudibranch Discodoris
mortensi
, with feces 2X

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a hawksbill turtle eating sponge, courtesy Andy Stockbridge, Belize

"Well, here’s old greedy-guts…haven’t we seen you before?  Or someone just like you?…You sure do like your sponge, but how do you like the scratchiness and all those spicules?  Your gut must look like an inside-out pin cushion!" - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize

hawksbill turtle Eretmochelys imbricata

 
  Let's ask a sponge about its defenses:
 
photograph of azure vase-sponge Callyspongia plicifera
cartoon 1 of a series of 4 with diver talking to a sponge about defensive spicules cartoon 2 of a series of 4 with diver talking to a sponge about defensive spicules
Azure vase-sponge Callyspongia plicifera 0.25X
  cartoon 3 of a series of 4 with diver talking to a sponge about defensive spicules cartoon 4 of a series of 4 with diver talking to a sponge about defensive spicules
 
 

photograph of sponge spicules in the feces of a hawksill turtlephotograph of hawksbill turtle
Sponge spicules certainly don't deter feeding by hawksbill turtles. Studies show that intestinal contents of hawksbills may contain 75% dry mass of spicules. This means that for an average-sized turtle, there may be as much as 500g of spicules in the digestive tract at any given time. Meylan 1988 Science 239 (4838): 393.


Close view of spicules isolated from
the feces of a hawksbill turtle 10X

 

Hawksbill turtle Eretmochelys imbriocota eats
a leathery barrel-sponge Geodia neptuni 0.5X

  What special features of mouth and gut would we look for in a turtle (or any animal) that allow it to eat a diet of glass spicules? The illustration below shows what we might theoretically find in features of BEAK, THROAT, GUT, and ANUS of such an idealised spicule-eater, along with what we actually find in a hawksbill turtle Eretmochelys imbricata.
 
illustration showing the EXPECTED features that we might find in a sponge-eating marine turtle with what we ACTUALLY find in a hawksbill turtle
 
CONCLUSION: hawksbill turtles seem to have no special mechanism for dealing with the prickly nature of their prey.
 

photograph of hawksbill turtle and angelfish swimming away from a partially eaten sponge
Angelfishes Pomacanthus spp., one shown here with a hawksbill turtle, also eat sponges. A common behaviour for the fish is to let the turtle do the hard work biting through the tough outer layers of the sponge and then begin eating when the turtle moves off.

 





Leathery barrel sponge Geodia
neptuni
with a chunk missing from it

 

 

 
 

photograph of a sea-rod gorgonian Eunicea sp.photograph of spicules of a gorgonian
Gorgonians are defended by chemicals and perhaps also by calcareous spicules embedded in their flesh. Some genera have over 70% of their dry tissue-mass as spicules, and perhaps these play a dual role in structural support and defense.

 

Gorgonian spicules 100X

 


Sea-rod gorgonian,
possibly Eunicea sp. 0.25X

 

photograph of flamingo-tongue snail Cyphoma gibbosum eating a gorgoniangraph relating residence time of flamingo-tongue shells on their prey gorgonians in relation to spicule content of the gorgoniansThere is evidence from studies in St. Croix that residence time of predatory flamingo-tongue shells Cyphoma spp. on gorgonians Eunicea spp. and Muricea spp. may be inversely related to spicule density in their prey. Harvell & Suchanek 1987 Mar Ecol Progr Ser 38: 37.

The graph shows that flamingo-tongue shells spend less time
sitting on gorgonians that have high levels of spicules

 


Flamingo-tongue shell Cyphoma
gibbosum
eating a gorgonian 1.3X

 
 
Now let's hear what the snails have to say on the matter: cartoon 1 in a series of 7 dealing with the notion of spicules in gorgonians acting in defense from flamingo-tongue shells Cyphoma gibbosum
cartoon 2 in a series of 7 dealing with the notion of spicules in gorgonians acting in defense from flamingo-tongue shells Cyphoma gibbosum cartoon 3 in a series of 7 dealing with the notion of spicules in gorgonians acting in defense from flamingo-tongue shells Cyphoma gibbosum
cartoon 4 in a series of 7 dealing with the notion of spicules in gorgonians acting in defense from flamingo-tongue shells Cyphoma gibbosum cartoon 5 in a series of 7 dealing with the notion of spicules in gorgonians acting in defense from flamingo-tongue shells Cyphoma gibbosum
cartoon 6 in a series of 7 dealing with the notion of spicules in gorgonians acting in defense from flamingo-tongue shells Cyphoma gibbosum cartoon 7 in a series of 7 dealing with the notion of spicules in gorgonians acting in defense from flamingo-tongue shells Cyphoma gibbosum
 

photograph of spicules from the skin of a sea cucumber
Other coral-reef invertebrates that contain calcareous spicules in their skin include dorid nudibranchs and sea cucumbers. The spicules are thought to function in defense, either by increasing general toughness of the prey or by presenting a photograph of dorid nudibranch Discodoris mortensi courtesy Sandra Millen, University of British Columbiapotential predator with a prickly mouthful. However, as we've seen in the above accounts for sponges and gorgonians, there is a need for more research work to be done to clarify the functional relationship of spicule content and palatability in invertebrates.

Spicules isolated from the
skin of a sea cucumber 50X


The skin of this dorid nudibranch
Discodoris mortensi contains abundant spicules 2X

 
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