Defenses
 
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Invertebrates
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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

 

 hot buttons for chemical-defenses part of BCCR
Chemical defenses of invertebrates fall into a category of nematocysts, considered here, and other categories accessible via the icons.
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a coral reef taken from a video "This array of gorgonians and sponges, and other reef invertebrates sure is pretty. Just so long as you don't try to eat them. Many have chemical defenses that would make you quite sick...stinging cells and noxious, pukey-tasting flesh. How else could they survive amongst all these hungry fishes?" -
 
 

Invertebrate defenses: toxic chemicals: nematocysts
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of gorgonians taken from a video "A common feature to all cnidarians, including these beautiful gorgonians is the possession of stinging cells or nemaatocysts. Nematocysts are arguably the best defensive and food-getting system in the entire animal kingdom, and they go a long way to explaining the success of the group." -
 
 

Nematocysts are produced only by cnidarians, and function in defense, food capture and sometimes locomotion. They are microscopic in size and are located in the epithelium of the polyps and, in corals and sea anemones, onphotograph of fire coral Millepora sp.digestive or mesenterial filaments (acontia) in the gut cavity.

NOTE lit. "thread bag" G., referring to the shape of the undischarged nematocyst. There are 3 main types: those that entangle prey, those that attach to the substratum, and those that pierceand kill. Only the last wil be considered herephotograph of knobby anemone Bartholomea annulata

 

Close view of nematocyst-
bearing defensive polyps of
a fire coral Millepora sp. 3X







Nematocysts are often clustered in distinctively coloured
or otherwise visually obvious "batteries" on the tentacles,
as in this corkscrew sanemone Bartholomea annulata 1.5X

 

Nematocysts on the mesenterial filaments in the gut cavity normally function to quell the movement of prey taken in as food, but in many species they can be extruded from the mouth or through pores in the body wall to act in defense. The photograph shows an attack of a bearded fireworm on a mound coral Montastrea sp. and the defensive response of the polyps.

NOTE these are also called digestive filaments or, more formally, acontia

 photograph of fireworm Hermodice carunculata attacking a coral Montastrea sp.

 
Mesenterial filaments are being extruded through the body walls of at least 2 polyps in defensive response to a bearded fireworm Hermodice carunculata. You can
see that the worm is itself irritated
by the encounter, as it has raised
its bristles in defense 3X
   
   
 

Nematocysts, as in a hydroid Halocordyle disticha, are produced in special skin cells near the polyp base and migrate from there to the tentacles. When formed, the nematocyst is fully "loaded" and is contained within a capsule. Within the capsule is an internally coiled thread suspended in a flocculent protein gel. On stimulation, usually requiring both touch and chemical, the capsule wall becomes permable and water is forceably drawn into the flocculent protein. The capsule volume increases and the pressure forces the thread to explode from the capsule. The rapid eversion is much like a finger of a rubber glove being blown outward. Spines and bristles formerly facing inwards, are now on the outside, and these help the thread to penetrate. In nematocysts that sting, the flocculent protein is toxic and is driven down the discharging tube and eventually injected into the prey. After discharge, the nematocyst is useless and must be replaced. Nematocysts are used by the hydroid in both defense and food-getting.

NOTE other types of nematocysts have sticky threads that wrap around or cling to the prey or predator
 
  The following sequence shows a hydroid capturing a copepod that has swum past and touched one of the polyps of the colony:
  composite drawing showing nematocyst discharge in the tentacle of a hydroid
 
 

photograph of sea star Solaster stimpsoni avoiding being stung by cup corals Balanophyllia sp.
Our knowledge of the pharmacological effects of nematocyst toxin invertebrates is scanty. However, it is clear from the reaction of various soft-skinned invertebrates to cnidarians that, while death from casual contact may be rare, avoidance is still the best strategy.

 

 

A temperate-inhabiting sea star Solaster stimpsoni
carefully avoids touching its sensitive tube feet
to the tentacles of cup corals Ballanophyllia elegans. 0.3X

 
  If human sensitivity to nematocysts bears any similarity to how invertebrates might be affected, then some "sure bets" for invertebrates to avoid would be...
 
...hydroid colonies, most species of which stand no more than a few centimeters in height... photograph of Christmas-tree hydroid Halocordyle disticha ...trailing tentacles of the Portuguese man-of-war... photograph of Physalia physalis courtesy Cindy Young, Vancouver
 
Christmas-tree hydroids
Halocordyle disticha will sting
sensitive human skin 2X
Under-view of Physalia physalis showing the fishing tentacles
that bear extremely potent nematocysts 1.5X. Photo
courtesy Cindy Young, UBC
 
...certain sea anemones... photograph of sea anemone Lebrunia sp.
Close view of the nematocyst batteries
on tentacles of the
corkscrew sea anemone
Bartholomea annulata 2X
 photograph of knobby anemone Bartholomea annulata
Sea anemone Lebrunia coralligens 2X
 
  photograph of fire coral Millepora sp. photograph of fire coral Millepora sp.
...and all forms of fire coral...

Fire corals Millepora spp. are generally free of invertebrates walking or crawling over its
surface, doubtless owing to the potency of its nematocysts.

However, one species that
commonly frequents fire coral
is the brittle star Ophiothrix suensonii. Whether it is tolerant
or resistant to the stings, sits
on its spine tips, or whether the
fire coral does not discharge its nematocysts, is not known 0.8X

  
 
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