Defenses
 
column spacer Defenses
 
 
Invertebrates hot buttons for invertebrate defenses part of BCCR hot button for behavioral defenses part of invertebrate defenses in BCCR hot button for toxic chemicals part of invertebrate defenses in BCCR hot button for structure part of invertebrate defenses in BCCR
The topic considered here reating to defenses of invertebrates deals with toxic chemicals. Other invertebrate defenses are accessible via the icons.
 
 

Invertebrate defenses: toxic chemicals

Chemical defenses of invertebrates fall into a category of toxic/nocious tissues or secretions, considered here, and other categories accessible via the icons.

hot buttons for chemical-defenses part of BCCR
 
 

Invertebrate defenses: toxic chemicals: toxic/noxious tissues or secretions

 
  Toxic/noxious tissues or secretions are employed as defenses by many reef invertebrates. Snails, crabs, cephalopods, & sea cucumbers are dealt with in this section, while SPONGES, ZOANTHIDS, & GORGONIANS are considered elsewhere.
 
 

Snails, crabs, cephalopods, & sea cucumbers

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs photograph of flamingo-tongue shells Cyphoma gibbosum feeding on a gorgonian taken from a video

"Flamingo-tongue shells are really visually obvious. Any time you see brightly coloured animals openly advertising themselves, they're likely to have a noxious surprise for a predator." - Turks & Caicos

NOTE Cyphoma gibbosum

 
 

The mantle of a flamingo-tongue shell is divided into right and left flaps that can be extended upwards on either side to enclose the shell. The result is a colorful display of orange spots rimmed in black. The flaps are not camouflaging; rather, they contain a toxin thought to be derived from photograph of flamingo-tongue shells Cyphoma gibbosumthe snail's gorgonian prey and are thought to be a warning to predators. This warning coloration, also known as aposemetism, provides an easy-to-remember message to visual predators, such as fishes. Although predators of flamingo-tongue shells have yet to be identified, tests in Curacao show that bluehead wrasses will reject or ignore bits of mantle tissue cut up and presented as food. Gerhart 1986 Mar Ecol Progr Ser 31: 255.

NOTE lit. "away message" G. or warning coloration, informing a potential predator to stay away. Orange, yellow, and black are common warning patterns in the animal kingdom and are used by bees, ants, wasps and many butterflies to give a clear, unambiguous message: "eat me if you dare"

 

Flamingo-tongue shell Cyphoma gibbosum showing
its colorful mantle. Note that the large foot on
which the animal crawls is also coloured 2X

 

photograph of flamingo-tongue shells with mantles partially withdrawnWhen undisturbed, the mantle flaps are usually up but, if touched, the snail retracts them to expose an otherwise nondescript-looking shell. This raises the question as to why the snail retracts its mantle. If the colours function to warn off potential predators, then why would the snail retract them? Note the tight fit of the thin mantle flaps to the shell. It is not known how the close fit is attained, nor how the flaps are raised or retracted.

NOTE divers often touch the flaps to make the snail retract them and, in a well-dived area, many snails have bared their shells possibly from this cause

 

Given that flamingo-tongue shells Cyphoma gibbosum generally retract
their mantles when touched by a SCUBA-diver, could the proportion
of retracted mantles in an area represent a kind of bioassay of the level of
"environmental conscientiousness" of the divers swimming ahead of your group? 4X

 

 
 
Let's see what the juvenile and adult bluehead wrasses Thalassoma bifasciatum think about all of this: cartoon 1 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey
cartoon 2 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey cartoon 3 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey
cartoon 4 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey cartoon 5 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey
cartoon 6 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey cartoon 7 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey
cartoon 8 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey cartoon 9 in a series of 9 showing wrasses talking about flamingo-tongue shells as tasty prey
 
 

drawing of sea hare Aplysia dactylomela releasing ink
When disturbed, back-reef inhabiting herbivorous sea hares produce a purple ink and, when more intensely bothered, a white opaline secretion. photograph of an inking sea hare Aplysia brasilianaBoth secretions have known irritating or toxic qualities. Additionally, a sea hare's skin and digestive gland contain more than 25 different chemicals of suspected noxious or toxic quality. All of these chemicals are derived from the animal's red algal foods. Photograph courtesy Anne Dupont, Florida.

NOTE a molluscan digestive gland functions similarly to the liver and pancreas of humans, involving chemical transformations and enzyme production, respectively

 

This sea hare Aplysia brasiliana has released purple ink
upon being handled. For this reason, sea-hare species
in some Caribbean areas are known as "ink-fish"

 

photo collage showing effects of diluted sea-hare ink on the behaviour of various reef organismsThere are many ideas as to the defensive function of the ink of sea hares. Included are smoke-screening, as in octopuses and squids, and increased unpalatability and irritability to potential fish and invertebrate predators. Recent studies in Jamaica on the effect of realistic concentrations of sea-hare ink on various carnivorous reef animals show a range of apparently irritation responses. Carefoot & Pennings 1999 J Exp Mar Biol Ecol 234: 185.

NOTE fresh ink diluted in seawater to mimic as closely as possible the concentrations produced by naturally inking sea hares

 

 
 

photograph of nudibranch Berthella stellata
Nudibranchs and their relatives are soft, mostly shell-less, and seem to be defenseless. However, as a group they actually have a repertoire of defenses unmatched by any other invertebrates. Such defenses include secondarily acquired stinging cells (nematocysts), defensive chemicals, skin-spicules and, as in the species featured here, acid secretions from the skin. Photo courtesy Sandra Millen, University of British Columbia.

 

 

 

Berthella stellata is a notaspid opisthobranch, related to
nudibranchs but with a thin, flattened shell embedded
within its mantle tissue. Its skin has a pH of about 3.5,
somewhat less acidic than vinegar, which is about 2.5-3.0.

2X


  photo/diagram of pathway of undischarged nematocysts from gut to cerata in a nudibranchIn a kind of second-hand defensive-chemical borrowing, aeolid nudibranchs that feed on hydroids and sea anemones can incorporate and use the prey's nematocysts in their own defense. The nematocysts are transferred through the gut undischarged, and housed in cells in special tiny sacs at the tips of the dorsal projections known as cerata. The location of the nematocyst-containing sacs is usually demarcated by white pigment on the cerata tips, and the whole visually obvious array is thought to represent warning coloration or aposemetism. The nematocysts sit within the cells of the sac with their discharge ends facing inwards towards a space known as a lumen. When the animal is irritated, special muscles at the base of the sac contract and squeeze the nematocysts out a hole at the top, at which time they promptly discharge. Photo courtesy Anne Dupont, Florida.
 

photograph of Caribbean aeolid nudibranch Learchis evelinae courtesy Anne Dupont, FloridaFrom the stomach of an aeolid nudibranch arises the multi-branched digestive gland, each branch contained within a ceras (pl. cerata). This the organ where final processing of the food takes place, and where nutrients and energy materials are absorbed into the body. The undischarged nematocysts pass through these branches and enter the sacs at the cerata tips. Photograph courtesy Anne Dupont, Florida.

 

 

The digestive-gland branches and
cerata tips are easily seen in this
aeolid species Learchis evelinae 5X

 

Although the function of the system seems obviously for defense, proof of this has been a long time coming. An early suggestion, based on observation that cerata are sometimes voluntarily cast off by the snail, was that it was a system to rid the body safely of undischarged nematocysts. However, the feces of the snail are packed with undischarged nematocysts, making a complex cerata-based system of removal redundant, and this idea has little present support. Much more likely is that the system has evolved to protect the snail. There have been several anecdotal reports in the scientific literature of fishes shaking their heads and rapidly expelling water from their mouths on contact with sea anemones and hydroids, and more than one researcher has felt the effects first-hand after nibbling on the cerata. photograph of Glaucus atlanticus, courtesy Carol Lalli, Brentwood Bay, B.C.

Strong correlative evidence has come from sea-bathers in Australia stung on contact with Glaucus spp., a type of pelagic snail known to sequester highly toxic Portuguese man-of-war nematocysts in its cerata. Thompson & Bennett 1969 Science 166 (3912): 1532. Photograph courtesy Carol Lalli, Brentwood Bay, British Columbia.

NOTE this is not recommended, as some people, especially ones previously sensitised, may develop strong and potentially life-threatening allergic reactions to the nematocysts' toxins

The pelagic snail Glaucus atlanticus eats Portuguese
man-of-wars Physalia spp.and incorporates
undischarged nematocysts into its cerata tips 0.8X
 

photograph of semiterrestrial crab Gecarcinus lateralis
Although the first line of defense for crabs and related crustaceans is their biting claws, some crabs when annoyed will regurgitate gut juices that may be noxious to potential predators.

 

 

 

 

 

Semiterrestrial Caribbean crab Gecarcinus
lateralis
regurgitating its gut fluids

 

photograph of beak of a Humboldt squid Dosidicus gigasCephalopods have formidable beaks, but these are almost never used in defense. All octopuses and squids, however, produce toxic secretions in their salivary glands that have blocking effects on sodium channels similar to the neurological effects of tetrodotoxin of pufferfishes. In octopuses and possibly also in some squids, the toxin can be released into the enclosed web-area of the arms to quell the activity of crustacean and other prey before they are eaten.

photograph of octopus beak showing jawsNOTE notable exceptions are the feisty European octopus Octopus vulgaris, which bites humans often but usually not fatally, and 3-4 species of the famed Indo-Pacific blue-ringed octopus Hapalochlaena, which are highly venomous and are known to have killed people. There is no antidote to the venom

Beak and radula of a dead Humboldt
squid Dosidicus gigas being squeezed
open between fingers 0.6X

 

Beak of an octopus Octopus sp. 0.6X

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a 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. - Bonaire 2003

NOTE donkey-dung sea cucumber Holothuria mexicana

NOTE in Fiji and other South-Pacific islands a practise still in use today is to anaesthetise tidepool fishes by pouring in the juices of a crushed sea cucumber

 
 

When certain species of tropical sea cucumbers are disturbed they release a mass of white tubules from the anus, known as Cuvierian tubules. These are not only sticky and entangling, but contain a saponin-like chemical known as holothurin. Holothurin is water-soluble and, in many or all sea cucumbers, not just the ones with tubules, is also present in the general epithelium covering the body. After their release, new tubules photograph of Indo-Pacific sea cucumber Bohadschia argus releasing Cuvierian tubulesregenerate within a few weeks.

NOTE named after the the French physiologist Georges Cuvier, who presumably first described the tubules

NOTE saponins are soluble, soap-like chemicals known as colloidal glycosides. Solutions of saponins froth on agitation, and many types such as holothurin are toxic or at least aversive to vertebrates. One of their actions is to break down red blood cells and, in sufficient concentration, they are effective fish poisons


Cuvierian tubules being relased from the anus of an Indo-Pacific species
Bohadschia argus
1X. Although several Indo-Pacific species of Holothuria
release tubules, related Caribbean Holothuria species appear not to do so

 
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