Behavioral characteristics of invertebrates

This chapter will familiarize you with issues and examples related to the behavior of invertebrates. The large number and sheer diversity of invertebrates requires a restriction in the types of behaviors (and species) that can be discussed. The behaviors selected were based in part on their importance to the survival of an individual organism. Since there is little known about the behavior of many of the lower invertebrate and deuterstome phyla, examples of insects and other protostomes were used to illustrate the various kinds of behavior mentioned.

All animals are metazoans and are characterized by being multicellular. The principles of behavior discovered in unicellular organisms are fundamentally the same in multicellular organisms. The similarities that exist between both forms of organisms are fascinating because the evolution of multicellu-larity has led organisms to develop a fantastic array of complex activities and modifiable behavior. Consider, for instance, the behavior of the marine sponge Sycon gelatinosum (phylum Porifera). During the larval stage it is a free-swimming animal that moves toward light at the water's surface. As it develops, it lives near the substrate and eventually becomes a sessile adult. The adult sponge is no longer a free-swimming animal and is entirely incapable of active movement. Reactive behavior is elicited from the adult sponge when individual cells are stimulated directly. The resulting responses are localized, slow, and uncoordinated. The inability to hunt food items such as bacteria, plankton, and detritus requires the sponge to develop specially designed feeding structures that bring food to it.

Contrast the poorly coordinated behavior of sponges with the more active behavior of animals in the phylum Cnidaria, which includes multicellular marine animals such as jellyfish, corals, sea anemones, and the freshwater Hydras. In cnidari-ans, cells performing the same function are grouped into tissue. The creation of tissue allows cnidarians to behave in more complex ways than sponges. Hydras, for example, coordinate their tentacles to grasp prey, contract their entire bodies in response to strong mechanical stimulation such as predatory attacks, and move a single tentacle in response to a non-threatening organism or passing shadow.

A great advance in behavior is seen in worms of the phylum Platyhelminthes. This phylum contains animals such as planarians, flukes, and tapeworms. In these animals we find the first evidence of characteristics critical for the development of complex behavior. These characteristics include bilateral symmetry, the appearance of a brain, polarized neurons, and definitive anterior and posterior ends—with the anterior end containing a head, and eyes. The advances present in this phylum make complex orientation possible. The first examples of complex learning are also present in flat-worms. An example of this development is the flatworm's ability to to discriminate between two signals—one of which leads to a biologically relevant stimulus. This organism's ability to modify its behavior based on the possible consequences of an encounter in order to avoid dangerous situations moves an existing reflex into a new context. Although primitive types of behavior modification are possible in members of the phyla Porifera and Cnidaria, they are not as complex as those found in flatworms, nor are they retained for as as long as they are in worms.

The advances first seen in members of the phylum Platy-helminthes and elaborated by worms in the phylum Annelida (e.g., polychetes, earthworms, leeches) and reach their apex in members of the phyla Mollusca (e.g., snails, clams, squid, octopus) and Arthropoda (e.g., spiders, crabs, crayfish, lobsters, honey bees, wasps, ants). For example, the cephalopods' neural development, problem solving capability, sensory apparatus, and ability to modify behavior is unsurpassed among the invertebrates. Among the arthropods, social insects such as the honey bee and ant have astonishing examples of defensive, social, and learned behavior patterns. What is behavior, and who studies it? Behavior is not easy to define and various definitions exist. For example, physiologists might describe the "behavior of a neuron," but some comparative psychologists would find this objectionable. Generally speaking, behavior is defined as "what organisms do." Behavior is the action an animal takes in order to adjust, manipulate, and interact with its environment. Actions such as moving, grooming, and feeding can be referred to as maintenance behavior. Action that influences members of the same and/or different species can be called communication behavior. Behavior that is modifiable is known as learned behavior. In general, each of these three types of behavior defines or "orientates" the animal in space.

Various disciplines have contributed to the study of invertebrate behavior. These disciplines include comparative psychology, ethology, physiology, ecology, and entomology.

A stalked jellyfish (Haliclystus auricula) attaches to kelp and eelgrass near British Columbia. (Photo by ©Neil G. McDaniel/Photo Researchers, Inc. Reproduced by permission.)

Scientists engaged in the study of behavior often do so from an interdisciplinary approach in which psychologists, etholo-gists, physiologists, and entomologists all work alongside each other. Comparative psychologists have a special interest in searching for similarities and differences in behavior.

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