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Invertebrate defenses: structure

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The topic considered here reating to defenses of invertebrates deals with structure. Other invertebrate defenses are accessible via the icons.

Structural defenses in coral-reef invertebrates include size refuge, strong heavy skeleton, and leathery skin, considered in this section, and SPINES and SPICULES considered in their own sections.

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a sponge taken from a video "Sessile reef invertebrates may seem vulnerable, but most are protected…these sponges, for example, are doubly protected by spicules and deterrent chemicals." - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.

Invertebrate defenses: structure: size refuge

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a milk conch on the sand, taken from a video "Well, here's a tasty meal for a predator, but it would take a big predator to handle its large size. Conchs like to sit out in the open, where they must attract attention, so they get looked a lot, and consequently are quite shy."- Little Cayman 2001
  Many invertebrates reach a size too large for predators to eat or, if they do eat them, only non-lethal bits are removed. The intended prey have reached a refuge in size. Some common examples of such "biteable" invertebrates are sponges, gorgonians, and corals.
photo collage of coral-reef invertebrates that have reached a refuge in size from predation

Invertebrate defenses: structure: strong/heavy skeleton

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of cathedral coral taken from a video

"Some of these corals sure are massive. Corals rely on their heavy calcareous skeletons for defense, especially during the day when their polyps and contained stinging cells are mostly withdrawn." - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.

NOTE pillar coral Dendrogyra cylindrus


Corals have a heavy structural skeleton of calcium carbonate. During daytime, when polyp-foraging predators such as butterflyfishes are active, the delicate expandable part of each polyp is withdrawn into the skeleton for protection. Later at dusk the polyps expand.

The series shows polyps of a boulder coral
Montastrea sp. in variable states of withdrawal 6X

  photograph of expanded polyp of boulder coral Montastrea sp. photograph of expanded polyp of boulder coral Montastrea sp. photograph of expanded polyp of boulder coral Montastrea sp.

photograph of queen parrotfish Scarus vetula biting photograph of parrotfish bite-marks on boulder coral MontastreaThe skeleton of a coral offers little defense against parrotfishes whose heavy jaws scrape off the outer layer of living polyps. The fishes do this to obtain nutrients in the form of the coral's plant-like symbionts or, alternatively and in the case of terminal males, to mark out their territories.


Smoothly excavated areas of boulder
coral Montastrea sp. are indicative of
biting activities of parrotfishes 0.5X

Queen parrotfish Scarus
bites at a coral 0.7X


Heavy exoskeletons of crustaceans and shells of molluscs provide protection from predators.

NOTE lit. "outside skeleton"

photograph of channel clinging-crab Mithrax spinosissimus
photograph of milk conch Strombus costatus with growth of fire coral on shell
Despite their large size and heavy exoskeleton, channel clinging-crabs Mithrax spinosissimus usually hide away during the day and forage for food at night 0.15X
The massive shell of this milk conch Strombus costatus bears a further, and incidental, defensive armament of fire coral Millepora sp. 0.67X

photo/drawing illustrating the cracking-resistance of a molluscan shell
The shell of a milk conch and other snails consists of large crystals of calcium carbonate deposited in layers on an internal framework of protein. The shell's microstructure resembles a ceramic plywood made up of layers of crystals running in different directions. Just like a layered plywood, when stressed it is resistant to catastrophic failure by its tendency to form micro-cracks and to undergo crack-bridging across the different structural layers. Kamat et al. 2000 Nature 405: 1036.


In this simulation, a crab bites at the shell edge
(made difficult to start with by its smooth roundedness),
and initiates cracking across the internal structure.
Note, however, how the crack becomes discontinous, and
thus weaker, owing to the structural layering of the crystals


photograph of hermit crab Dardanus venosus courtesy Anne Dupont, Florida
Semiterrestrial and marine hermit crabs make secondary use of snail shells for defense. As they grow larger, the crabs replace their shell-domiciles with progressively photograph of semiterrestrial hermit crab Coenobita sp. larger ones as opportunity presents. Photograph of Dardanus venosus courtesy Anne Dupont, Florida.


Star-eye hermit crab
Dardanus venosus 0.33X

Soldier hermit-crabs Coenobita sp. scavenge for food in the back-reef area. These and other
semiterrestrial crabs must live close to the sea for reproduction. Each year the breeding females
make their way to the ocean and release their larvae directly into it. After several weeks developing in
the plankton, the juveniles regain the shore, find an empty shell, and adopt the land habit of the adults 1X


photograph of a crab specially adapted to use abandoned clamshells for protectionIf nothing else, just find an old bivalve shell and hold it onto your back with your legs, as done by this small back-reef crab.

NOTE an unidentified species, possibly Indo-Pacific


Invertebrate defenses: structure: leathery skin

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of sea cucumber taken from a video "Sea cucumbers are quite leathery-looking.  You never see them being attacked, so the skin must be too tough, or perhaps has deterrent toxins." - Little Cayman 2001

photo collage of Caribbean sea cucumbers Holothuria mexicana and Holothuria thomasi
The leathery skin of sea cucumbers must provide protection from at least some types of predators, perhaps fishes and rays, but the exact predators are not known. Most sea cucumbers have the same shape most of the time, but tiger-tail sea cucumbers can stretch their bodies to extreme distances of a meter or more.





In Jamaica, tiger-tail sea cucumbers hide in coral crevices at least partially during daytime, feed at night, and move only about once every 4d to a new location. When feeding they may stretch out a meter or more, and their extended shape may prove quite spooky to SCUBA-divers at night. Hammond 1982 Bull Mar Sci 32 (2): 549.


photograph of intertidal Caribbean chiton Acanthopleura granulata
Other common reef-invertebrates with tough, leathery outer coverings are chitons and sea squirts. In addition to a tough outer covering, the former have 8 embedded shell plates and often a coating of bristles or tubercles. The shell plates articulate, allowing the body to deform as the chiton slowly crawls over surface irregularities. These structural features are thought to protect chitons from drying, pounding surf, UV irradiance, and bird and sea-star predators.


Fuzzy chitons Acanthopleura granulata scrape algal
foods from the rocks in the intertidal region 1.5X


photograph of siphons of the giant tunicate Polycarpa spongiabilisThe protective outer covering of sea squirts is made up of a non-living leathery cellulose-like substance called tunicin.




Close view of the inhalent and exhalent siphons of the giant tunicate Polycarpa
The siphons are distinguishable by the presence of a ring of
yellow-coloured tentacles protecting the opening of the inhalent one 0.5X