Defensive behavior represents a class of behavior referred to as communication behavior. Metazoans must defend themselves against an impressive array of predators. To survive against an attack, various strategies have evolved. These
strategies include active mimicry, flash and startle displays, and chemical/physical defense.
A common behavior exhibited by invertebrates in response to danger is the adoption of a threatening posture. For example, when the long-spined sea urchin (Diadema sp.) is threatened it will point its spines toward the predator or threat. Aquatic organisms found in the order Decapoda, such as the cuttlefish and squid, defend themselves by discharging an ink that temporarily disorientates the predator, allowing the organisms time to escape. Some decapods (such as the octopus) in the order Octopoda have a similar ink defense system. At least one case has been observed in which Octopus vulgaris was recorded holding stones in its tentacles as a defensive shield against a moray eel. When sea cucumbers are threatened they expel their intestines to confuse a predator and allow them to escape.
In general, organisms during early ontogenetic development approach low-intensity stimulation and withdraw from high-intensity stimulation (e.g., light intensity). Sessile invertebrates like anemones, corals, and tunicates will contract or withdraw to protect their most vulnerable body parts. Mobile invertebrates can usually escape an aggressor's high intensity stimulation by engaging in kinesis and/or taxis such as crawling, swimming, flying, or jumping. Such behavior is easily observed in the cephalopod Onychoteuthis (protostome) popularly known as the "flying squid." The flying squid can escape aggressors by emitting strong water bursts from its mantle, which propels the animal into the air where finlike structures allow it to glide for a brief period of time. Most other non-
sessile invertebrates—like flatworms, echinoderms, and arrow worms—crawl away to hide under a rock, or change direction and swim away to escape predatory stimuli.
There are various forms of mimicry and only a few can be mentioned here. Some of the more well-known invertebrates that engage in mimicry are butterflies. The species Zeltus amasa maximianus (Lycaenidae) presents a "false head" to aggressors that is made more attractive to the predator by the motion of its wings. False-head mimicry requires not only morphological adaptations but also an ability to engage in behavior patterns that force the predator to focus its attention on the false structure. By presenting a predator with a convincing false target the probability of surviving an attack is increased. A similar strategy is also common in caterpillars. Species of Lirimiris (Notodontidae) actually inflate a headlike sac that is found posteriorly. The resulting fictitious appendage draws the attention of the predator away from the actual head and toward the comparatively tough posterior end. An interesting version of false-head mimicry exists in crab spiders (Phrynarachne spp.), and longhorn beetles (Aethomerus spp.), which both mimic the appearance of bird feces, and the Anaea butterfly caterpillar (Nymphalidae), which mimics the appearance of dried leaf tips. Mimicry and false mimicry—where animals mimic another animal—are not well developed among the invertebrate taxa.
When some invertebrates are stimulated by an aggressor they quickly modify their posture in an attempt to make it appear larger, and at the same time their body will quickly present a "flash" of color. This type of behavior has evolved mostly in protostomes. However, many combjellies and jellyfish can produce flashes of bioluminescent light that deters or confuses predators.
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