BCCR: Defenses

Defenses

seahorse dive leader for Biology of Caribbean Coral Reefs website

photograph of reef with fishes taken from a video

"When we swim over a reef like this, the fishes are watchful of us, and normal behaviour more or less stops until we go by. Every so often, though, you catch a sudden flurry of movement, and when you look, it's gone. These rapid-burst movements could be just territorial defense or a quick spawning, or they could be a strike by a predator. A good defense is the key to survival." - Cayman Brac 2003

Fish defenses

hot buttons for defensive "tasks" for fishes

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, and
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.

photograph of barrel sponge with fishes taken from a video

"If you are a fish and don't want to be eaten, then it's best not to be noticed. So, what are all these little fishes doing out and about? Well, they have to, to do their own thing, finding food and so on, and associated with this is exposure to predators." - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.

Fish defenses: avoid detection: hide away
There are many ways for a potential prey fish to avoid detection, but some of the main ones are to hide away as in crevices and under overhangs, considered in this section, and to use CAMOUFLAGE or otherwise CONFUSE THE PREDATOR, considered in their own sections.

photograph of a schoolmaster fish hiding in the shade taken from a video

"If a fish wants to avoid being detected by a predator, then it makes sense to hang out in shady areas. This schoolmaster seems content to let us swim up quite close." - Little Cayman 2001

NOTE Lutjanus apodus

photograph of a schoolmaster
Fishes that hide in shady areas can detect an approaching predatory fish that is bathed in sunlight much further away than the approaching fish can detect the one in hiding. But it works both ways, as a predator lurking in the shade has a better chance of catching prey fishes passing by in the sunlight.

Two predatory fishes, a schoolmaster Lutjanus
apodus and a Nassau grouper Epinephelus
striatus, hang out in a shaded area 0.1X

photograph of a green moray taken from a video

"Lots of morays in this area. A green one here, and over there a spotted one. Morays generally rest in protective crevices during the day, then come out to hunt after dark." - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.

NOTE Gymnothorax funebris

NOTE G. moringa

photograph of blackbar soldierfishes Myripristis jacobus photograph of longspine squirrelfish Holocentrus rufus
Many species of carnivorous fishes, including squirrelfishes, soldierfishes, and some groupers, rest in crevices during the day and come out to hunt at night.

Blackbar soldierfish
Myripristis jacobus 0,5X

Longspine squirrelfish
Holocentrus rufus 0.5X

photograph of a graysby

A graysby Epinephelus cruentatus rests in a
daytime crevice, perhaps waiting for prey
to happen by and/or for nightfall 1X

photograph of fish-filled crevice taken from a video

"This looks like a good spot for a fish to hide. I'll just poke my head in for a quick look. Yes, it's quite crowded...hard to make out who is who. I see some grunts, a yellow goatfish, and maybe some mahogany snappers." - Turks & Caicos 2003

NOTE Lutjanus mahogoni

In general, then, most daytime-feeding fishes, including all herbivorous ones, hide in protective crevices during the night and emerge at dawn to feed. Nighttime-feeding fishes do the reverse. Dusk and dawn, then, are periods of transition in activity of different trophic categories of fishes. Photograph of blue parrotfish courtesy Linda Ianiello, Florida.

NOTE lit. "feed", referring here to the major energy-transfer levels of an ecosystem (e.g., plants, herbivores, carnivores). Reef fishes mainly occupy trophic categories of herbivory and carnivory

photograph of initial-phase redband parrotfish Sparisoma aurofrenatum resting on bottom at night

Initial-phase redband parrotfish Sparisoma aurofrenatum rests on the sea-bottom during nighttime 0.7X

photograph of a blue parrotfish Scarus coeruleus courtesy Linda Ianiello, Florida

Blue parrot-
fish Scarus
coeruleus
rests in a crevice at night 0.2X

Reef fishes thus show a distinct daily pattern of activity.

Just before dawn, as shown in the accompanying images, squirrelfishes, grunts, and other nighttime predators descend from open water and seek out daytime resting spots. This is known as the dawn changeover.
After a 15 to 30-minute quiet period...
...zooplanktivorous fishes, such as chromises and other pomacentrids, and herbivorous fishes such as parrotfishes and damselfishes, emerge from their nighttime resting spots and commence feeding (see blue line).
At dusk the pattern reverses and all the daytime-feeding fishes sink down to the reef face. The reef is quiet for another 15-30min period, after which the nighttime-feeding fishes once again emerge. This is known as the dusk changeover.

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