Most knowledge of anglerfish behavior is associated with the specialized adaptations that have made them such effective and successful ambush predators. Most important among them are the angling apparatus and the functional morphological features of the feeding mechanism. These adaptations, along with certain reproductive adaptations, have made the an-glerfish among the most successful predators in the deep sea.

The angling apparatus (illicium) is derived from the ante-riormost spine in the dorsal fin. The illicium and second dorsal fin spine (if present) articulate with a single, enlarged pterygiophore. The illicium has a rather elaborate set of as sociated muscles, which allows the angler to move the spine rapidly, thus thrashing the bait (esca) vigorously above or in front of the mouth. In some anglerfishes the bait may be a simple bulb or clublike structure, but in others it can be quite

An angler (Lophius piscatorius) resting on the sandy bed of the Atlantic Ocean near Brittany, France. (Photo by Jeff Rotman/Photo Researchers, Inc. Reproduced by permission.)
An anglerfish "angling" with lure for catching prey. (Illustration by Joseph E. Trumpey)

elaborate. In many deep-sea species, the esca is bioluminescent, while in forms that live in sunlit regions, it may be an elaborate fleshy structure, occasionally resembling a small, shrimplike crustacean in some lophiid species or even a small fish in one species of antennariid.

The mouth in most anglerfishes is cavernous, and in many species it is lined with long, sharp, thin, depressible teeth. The functional morphological features associated with the angler-fish feeding apparatus are quite fascinating. Like many derived teleostean fishes, anglerfishes are "gape-and-suck" feeders. Many anglerfishes, however, have a greatly elongated and highly mobile suspensorium, or hyopalatine arch (the portion of the skull that supports the lower jaw). This feature serves to permit swallowing of very large prey and allows the volume of the buccal cavity to increase to a much greater extent than in other fishes, which results in much greater negative pressure or suction when the mouth is opened rapidly.

Not only can anglerfishes create much greater negative pressure when opening the mouth and expanding the buccal cavity, but they appear to be capable of doing so much more rapidly than other fishes. Pietsch and Grobecker used highspeed cinematography (800 and 1,000 frames/second) to study the functional morphological characteristics of the feeding apparatus in antennariid anglerfishes. They showed that maximum oral expansion and subsequent prey engulfment may occur in as little as four milliseconds, and the average speed for three species of Antenna-rim was seven milliseconds. For comparison, this takes 40 milliseconds in the European perch (Perca fluviatilis), 16 milliseconds in the freshwater butterfly-fish (Pantodon buchholzi), and 15 milliseconds in the stonefish (Synanceia verrucosa).

During typical ambush feeding, the anglerfish remains motionless (either on the bottom or in the water column, depending on the family) until suitable prey is detected. Many benthic anglerfishes (e.g., antennariids and lophiids) are capable of remarkable cryptic coloration, whereas the midwa-ter forms are usually very dark brown or black (cryptic in a pelagic environment with little or no light). Upon sensing prey, the angler brings the illicium into play, attempting to attract the prey to within reach (aggressive mimicry). When

A sargassumfish (Histrio histrio) floating in a sargassum mat at the ocean surface. (Illustration by Joseph E. Trumpey)

the prey reaches the strike zone, the mouth of the angler opens rapidly, and the buccal cavity expands greatly, creating a strong suction that draws in the prey.

Betta Fish

Betta Fish

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