column spacer Defenses

Fish Defenses


Defenses of fishes can be categorised into an heirarchical "cascade" beginning with:

1) avoid detection
2) take evasive action if spotted
3) prevent capture
4) prevent being eaten if captured
5) escape

Of these, the last is obviously the most important, but success in any of the others will aid in achieving it. Note that costs and risks to the prey escalate from first to fourth. The fifth and ultimate category, that of escape, can come at any time and won't be considered further here. The first task of any potential prey fish is to avoid detection. Helfman et al. 1997 The diversity of fishes. Blackwell Sci Publ.


hot buttons for defensive "tasks" for fishes

Fish defenses: prevent being eaten if captured: erectable spines

  If captured by a predatory fish, the "task" of the intended prey fish is to prevent being eaten. This may include having erectable spines, a topic considered here, or having POISONOUS FLESH, POISONOUS SECRETIONS, or being able to deliver an ELECTRICAL SHOCK, topics dealt with in another section.
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a whitespotted filefish taken from a video

"Some fishes, like this whitespotted filefish, can erect a sturdy dorsal spine to make it painful for the predator to close down its jaws. I don't think the spine is poisonous, but no matter, it would still be quite a mouthful!" - Turneffe Island 2002. Video courtesy Andy Stockbridge, Belize.

NOTE Cantherhines macroceros

  photographs of filefishes Aluterus scriptus and Cantherhines macrocerus
Several types of reef fishes have erectable spines that appear to have a double function: to lock the intended prey fish into tight crevices and, if the prey fish is caught, to prevent the predator from easily swallowing it. The spines do not appear to be toxic. Photograph of whitespotted filefish courtesy Anne Dupont, Florida.

photograph of a  squirrelfish Holocentrus adscensionisBecause they face backwards, spines in the fins of intended prey fishes require that the predator swallow the prey headfirst. Secondary manipulation of the prey for head-first gulping could provide a split-second opportunity for the prey to escape.




Sqirrelfish Holocentrus adscensionis
with dorsal spines fully erected 0.7X

  photographs of honeycomb cowfishes Lactophrys polygoniaIn addition to a box-like body shape that makes a cowfish difficult to capture, there are also sharp spines above the eyes that project forwards. Both features could increase the difficulty of a predator swallowing the prey headfirst.
  photograph of surgeonfishs Acanthurus coeruleus and A. bahianusSurgeonfishes have scalpel-sharp spines in the caudal region for protection against capture. In some or perhaps all species the spine can be compressed into the body and then erected and locked into place when needed. Rapid tail-beating as would come when the surgeonfish tries to escape from a tailward-directed strike, provides a lacerating defense.
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a queen triggerfish taken from a video

"If you look carefully at this queen triggerfish you can see a spine lying flat just in front of the dorsal fin. It's sturdy, and strongly armoured, can be erected to irritate a predator or to prevent being bitten, but I don't think it carries a toxin." - Turneffe Island 2002. Video courtesy Andy Stockbridge, Belize.

NOTE Balistes vetula


photograph of a queen triggerfish Balistes vetula
In the same manner as filefishes, triggerfishes have a spine on the dorsal surface located just in front of the dorsal fin. It normally lies flat, as in the accompanying photograph, but can be erected and locked into place when needed. The spine functions both for protection against predators and to lock the fish into crevices during rest-times.



Queen triggerfish Balistes vetula 0.15X


photograph of dorsal spine of a triggerfish Balistes vetulaThe spine of a triggerfish has evolved from 2 ancestral dorsal spines. The first is the larger one, while the second is smaller and acts to lock the first into place; hence, is the "trigger".



In this view of a spine found in the fish harbour of Grand
Cayman Island the smaller locking "trigger" has been lost. In the living
animal both spines and the supporting base are covered by skin 1.2X

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

"Well, who's this lurking in the seaweed?...a spotted scorpionfish! Not just camouflaged, but also well defended. Many of its spines bear venom glands. Get stuck by one of these babies, and it's off to the hospital!" - Turks & Caicos 2003

NOTE Scorpaena plumieri


photograph of spotted scorpionfish showing dorsal spines, courtesy Anne Dupont
Scorpionfishes have 17 poisonous spines for protection, 12 of which support the dorsal fin. The spines are grooved on either side and within each groove is a ductless venom gland. On penetration of the spine into flesh, as would happen if bitten by a predator, the protective skin covering the spine is torn away and the venom released. Photo courtesy Anne Dupont, Florida.

NOTE poisonous spines of scorpionfishes are considered in more detail at DANGEROUS MARINE ORGANISMS: SHARP SPINES/BRISTLES




Spotted scorpionfish Scorpaena plumieri, view from above.
The fish is oriented with its head facing to the left 0.4X


photograph of a splendid toadfish courtesy Anne Dupont, florida
Toadfishes bear 2 short doral spines and 2 opercular spines extending from either side of the head. A bite from a predatory fish causes venom to be released from each spine in a manner similar to that described for scorpionfishes. Photograph courtesy Anne Dupont, Florida.





Splendid toadfish Sanopus splendidus 4X

hot button for take evasive action if spotted part of fish defenses hot button for prevent capture part of fish defenses hot button for prevent being eaten if captured part of fish defenses