The large number and sheer diversity of protostomes necessitates a restriction on the kinds of behaviors (and species) that can be discussed in this chapter. The behaviors highlighted here are based in part on their importance to the survival of an individual organism.
Protostomes are some of the most morphologically complex, ecologically diverse, and behaviorally versatile organisms in the animal kingdom. They consist of more than one million species divided into approximately 20 phyla. Major representative phyla include the Platyhelminthes (flukes, pla-narians, and tapeworms), Nematoda (roundworms), Mollusca (chitons, clams, mussels, nautiluses, octopods, oysters, snails, slugs, squids, and tusk shells), Annelida (bristleworms, earthworms, leeches, sandworms, and tubeworms), and Arthro-poda (ants, centipedes, cockroaches, crabs, crayfish, lobsters, millipedes, scorpions, spiders, and ticks).
When considering what a protostome is, it is important to note that the answer appears to be changing with the accumulation of new information. Evidence from studies of morphology and the fossil record generally support the view that animals in the phyla Annelida, Mollusca, and Arthropoda are indeed protostomes. However, new data based on rRNA analysis suggests that some animals in the Pseudocoelomate phyla (gastrotrichs, rotifers, and roundworms) and in the Acoelomate phyla (flukes, planarians, tapeworms, and ciliated worms) are also protostomes.
Despite the impressive diversity of organisms in this group, the vast majority of protostomes share certain basic characteristics of embryonic development. Indeed, the very name protostomes means “first mouth” and nicely illustrates the common characteristic that the initial opening to the digestive tract in the embryo develops into a mouth. Additional protostome characteristics include an embryonic stage known in the literature as mosaic development. Mosaic development produces a series of cell divisions (cleavage patterns) in which the fate of individual cells following the first cell division is fixed (determinate cleavage), while subsequent cell divisions are arranged spirally (spiral cleavage). Moreover, in proto-stomes the origin of the mesoderm (the germ layer producing such structures as the heart, muscles, and circulatory organs) is created from both the ectoderm (the germ layer producing the skin or integument, nervous system, mouth and anal canals) and endoderm (the germ layer producing the linings of the digestive tract and related glands) in a region known as the 4d cell. Protostomes also have an internal body cavity situated between the digestive tract and the body wall known as the coelom. Two cylindrical masses constructed from mesodermal cells split and the resulting cavities enlarge and combine to form an internal body cavity (coelom) that is surrounded on all sides by mesoderm cells (schizocely).
All protostomes must engage in activities that lead to survival and reproduction. The honey bee and ant, for example, must find and digest food and protect the colony. The pla-narian and crab must also meet nutritional requirements, reproduce, and defend themselves, but do so often in an aquatic environment. An earthworm is faced with similar problems of survival, but usually solves them underground. Protostomes that fly, swim, or burrow are all faced with the same set of problems. The solutions to these problems represent an interaction of environment and morphology, and here lies the differences in what is called behavior.
The word “behavior” is ambiguous. A physiologist, for example, may be comfortable describing the “behavior of a neuron,” while a behavioral scientist might find this objectionable. Moreover, among behavioral scientists there are often discrepancies in the definition of behavior. John B. Watson, who popularized an early form of “behaviorism” in the early twentieth century, once defined behavior as muscle contractions and glandular secretions. Other behavioral scientists such as B. F. Skinner have used several definitions of behavior, including “the movement of an organism in space in relation either to its point of origin or to some other object.” Many of these definitions give a novice the impression that, for behavioral scientists, the subject matter consists of bodily movements and mechanical responses. Many define behavior not as movement of an organism (which is the proper study of kinesiology), but as an act. Defining behavior in terms of actions captures the notion that behavior has consequences, in other words, scientists are primarily interested in what an organism “does.” By defining behavior in terms of actions and consequences, the focus of a behavioral analysis is not on the individual movements that constitute a behavior (as important as this is), but what the behavior “accomplishes.” For instance, how an organism acts in a social situation, responds to threats, or captures food is
A clam siphon at work.
A clam siphon at work.
intimately related to its body plan. Protostomes have a symmetrical body plan (e.g., planarians, earthworms, lobsters, or ants). One of the more interesting body plans is radial symmetry. Animals with radial symmetry have no front or back and take the general form of a cylinder (e.g., sea stars, and sea anemones) with various body parts connected to a main axis. Such animals have feeding structures and sensory systems that interact with their environment in all directions. Such a body plan is most common among animals that are permanently attached to a substrate (e.g., sea anemones) or drifting in the open seas (e.g., jellyfishes).
Another type of symmetrical body plan found in proto-stomes is bilateral symmetry. Invertebrates with bilateral symmetry (e.g., planarians, earthworms, crustaceans, insects, and spiders) have a definite front and back, left and right, and backside and underside orientation. Animals with such a body plan generally can control their locomotion, unlike sessile or drifting species (radial symmetry). The front end (anterior) contains an assortment of feeding and sensory structures, often encapsulated in a head (cephalization) that confronts the environment first. Moreover, the underside (ventral surface) typically contains structures necessary for locomotion, and the backside (dorsal) becomes specialized for protection.


Feeding behavior

Feeding behavior consists of several different types of acts associated with discovery, palatability, and ingestion. The expression of feeding behavior is a combination of evolutionary and environmental pressures. Depending on the species, pro-tostomes consume an infinite variety of food ranging from microscopic organisms, vegetable matter, and other protostomes; some even grow their own food. Despite the large and varied number of protostomes, some generalizations can be found. First, the strategies for finding food can be reduced to those organisms that find food by living on it, foraging for it, waiting for it to pass by, growing it, and having other organisms provide it. Second, the mechanisms associated with feeding can be reduced to those that singly and/or in combination feed by suspension, deposition, macroherbivory and predation.


Crustaceans such as daphnids, brine shrimp, copepods, and ostracods (i.e., those not considered in the class Malacostraca) are excellent examples of filter- or suspension-feeding proto-stomates. Suspension feeders obtain food by either moving through the water or by remaining stationary. In both cases, bacteria, plankton, and detritus flow through specially designed feeding structures.
Interestingly, because of the large amount of energy required to continuously filter water, there are relatively few protostomes that actually use continuous filtration. A less expensive and also the most commonly employed strategy are to develop specialized filter mechanisms that contain a “sticky” substance such as mucus. An example of this is found in some species of tube-dwelling polychetes that direct water through their burrows and trap food particles in mucus. The mucus is then rolled up into a “food pellet” and manipulated by ciliary action to the mouth where it is consumed.


Well-known examples of protostomes that obtain food from mud and terrestrial soils include most earthworms and some snails. Direct deposit feeders extract microscopic plant matter and other nourishment by swallowing sediment. Such feeders can be either burrowing or selective. In contrast to burrowing feeders such as earthworms, selective feeders obtain food from the upper layers of sediment.


Macroherbivory feeders obtain food by consuming macroscopic plants. One of the best protostome examples of plant feeders is the order Orthoptera (crickets, locust, and grasshoppers). Members of this order have developed specialized mouthparts and muscle structures to bite and chew. The African Copiphorinae, for example, uses its large jaws to open seeds. Biting and chewing mouthparts are also seen in beetles and many orders of insects. Two other types of mouth-parts common to macroherbivory feeders are sucking and piercing. Sucking mouthparts enable insects such as butterflies and honey bees to gather nectar, pollen, and other liquids. Protostomes such as cicadas feed by drawing blood or plant juices. The leaf cutter ants (Atta cephalotes) are interesting example of macroherbivory feeders. These ants cut leaves and flowers and transport them to their nests where they are used to grow a fungus that is their main food source. A related feeding behavior is also found in termites (Isoptera). Termites of the species Longipeditermes longipes forage for detritus such as rotting leaves that become a culture medium for the fungi on which they feed.


Arguably the most sophisticated protostome feeders are those that obtain food by hunting, which requires the animal to locate, pursue, and handle prey. Most invertebrates locate prey by chemoreception; others use vision, tactile, or vibration, or some combination thereof. Predators can be classified as stalkers, lurkers, sessile opportunists, or grazers.
Planarians (Platyhelminthes) are an excellent example of animals that obtain food through hunting. The vast majority of planarians are carnivorous. They are active and efficient hunters because of their mobility and sensory systems. They feed on many different invertebrates, including rotifers, nematodes, and other planarians, and have several different methods of capture. One of the most common methods is to wrap their body around a prey item and secure it with mucus. An interesting example of this behavior can be found in terrestrial planarians. The terrestrial planarian Microplana termitophaga feeds on termites by living near termite mound ventilation shafts. The planarian stretches itself into the shaft and waves its head until a termite comes in contact, at which time the termite becomes stuck on the mucus produced by the worm. An interesting note is that it is not generally agreed upon that Platyhelminthes are protostomes.
Another interesting method that protostomes use to stalk prey can be found in members of the phylum Onychophora. These are wormlike animals that some scholars believe bridge the gap between annelids and arthropods. The velvet worm Macroperipatus torquatus forages nocturnally on crickets and other selected invertebrates and approaches its prey undetected by utilizing slow movements. When the potential prey is recognized as an item to be consumed, the worm attacks it by enmeshing the organism in a glue-like substance squirted from the oral cavity.
Perhaps the most well-known examples of hunting proto-stomes are the spiders in the phylum Arachnida. Members of the family Lycosidae, colloquially known as wolf spiders, can hunt by day, although some species hunt at night. Some wolf spiders pounce on prey from their burrow, while others actively leave the burrow on hunting trips. The jumping spiders of the family Salticidae and some lynx spiders of the family Oxyopidae also hunt for prey. Once found, the spiders can leap upon it from distances as much as 40 times their body length. Other examples of hunting behavior can be found in the metallic hunting wasp Chlorion lobatum, which specializes in capturing crickets, and in the army ant Eciton burchelli, which forms large colonies and searches for prey on the forest floor.

Defensive behavior

Protostomes 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 protostomes in response to danger is adopting a threatening posture. When, for example, a specimen of Brachypelma smithi (mygalomorph spider) is threatened outside its burrow, it reacts by making itself appear larger by shifting weight onto the rear legs while simultaneously raising the front legs and exposing the fangs. Another physical defense mechanism of many species of my-galomorphs is that they use the fine sharp hairs that cover them to pierce their predators. This is not only painful, but may be toxic (these hairs can pierce human skin to a depth of 0.078 in [2 mm]). A spider can release these hairs by rubbing the hind legs against the abdomen. In addition to making themselves appear larger and covered with sharp and, in some cases, toxic hairs, they can also squirt a liquid from their anus.
A novel form of defensive behavior in spiders is found in females and immature males of the black widow (Latrodectus hesperus). When threatened, the black widow emits strands of silk and manipulates the silk to cover its vulnerable abdomen and, sometimes, the aggressor. Especially interesting is the defensive behavior of the cerambycid beetles (genus Ham-maticherus) that use spine-like appendages on their antennae to whip their aggressor. Equally fascinating is the behavior of arctiid moths that produce a series of clicks when detecting the sound made by hunting bats.
A well-known active defense system is found in social insects such as honey bees, termites, and ants. The latter two organisms actually maintain a caste of “soldiers” for colony defense, as do several species of aphids (Colophina clematis, C. monstrifica, C. arma). When threatened, these organisms attack by injecting venom into the aggressor and can use their powerful mandibles to incapacitate. In aquatic organisms such as those found in the order Decapoda, cuttlefish and squid defend themselves not only by an ability to escape, but also by discharging ink that temporarily disorientates the aggressor. Some decapods in the order Octopoda, which includes the octopus, have a similar ink defense system. At least one case has been observed in which Octopus vulgaris was recorded actually holding stones in its tentacles as a defensive shield against a moray eel.
In general, organisms during early ontogenetic development approach low-intensity stimulation and withdraw from high-intensity stimulation. Protostomes can always escape high-intensity stimulation offered by an aggressor by crawling, swimming, flying, or jumping. Such behavior is easily observed in grasshoppers and the decapod Onychoteuthis, popularly known as the “flying squid.” The flying squid can escape aggressors by emitting strong water bursts from its mantle to propel the animal into the air where finlike structures allow it to glide for a brief period of time. Fleeing is not always effective. The katydid, Ancistrocerus inflictus, does not confront aggressors by an active defense system such as that found in spiders, ants, honey bees, and termites. Rather, Ancistrocerus may be found living near the nests of several wasp species. It is these wasps that provide protection for the katydid.


There are various forms of mimicry. Some of the best-known protostomes that engage in mimicry are butterflies. The species Zeltus amasa maximianus (Lycaendae) increases its chances of surviving an attack by giving its enemy a choice of two heads—one of which is a decoy. By presenting a predator with a convincing false target, the probability of surviving an attack is increased. The decoy, or “false head,” is created by morphological adaptations present on the wing
A toxic nudibranch (Phyllidia coelestis) and juvenile sea cucumber (Bohadschia graeffei), both displaying aposematic or warning coloration.
A toxic nudibranch (Phyllidia coelestis) and juvenile sea cucumber (Bohadschia graeffei), both displaying aposematic or warning coloration.
tips. False-head mimicry requires not only morphological adaptations, but also that the animal be able to engage in behavioral patterns that will focus a predator’s attention on the decoy. One of the methods a butterfly might use to focus a predator’s attention on their false head is to make their morphological adaptations seems more “attractive”; this process is accomplished by certain butterflies using the ribbon-like structures located near their wing tips. When the butterfly moves its wings the ribbons begin to resemble antennae, diverting attention away from the true head located on the opposite side of the butterfly.
A similar strategy is also common in caterpillars. In species of Lirimiris (Notodontidae), the animal actually inflates a head-like sac at its rear. The resulting fictitious appendage draws the attention of the predator away from the actual head to the comparatively tough rear end. Another version of the false head is found in crab spiders (Phrynarachne sp.), and longhorn beetles (Aethomerus sp.), whose mimicry resembles bird feces, and the Anaea butterfly caterpillar (Nymphalidae) that resembles dried leaf tips.
In contrast to false mimicry, some protostomes actually mimic the behavior of other species. Active mimicry is common in a wide range of invertebrates. An interesting example is found in Acyphoderes sexualis (Ceramybiid). This beetle mimics the behavior of two different animals, depending on the threat. When the beetle is touched, it resembles some species of ponerine ants (Formicidae), and when threatened in flight, the behavior changes to resemble polybiine wasps. Another example is tephritid flies (Rhagoletis zephyria). At least two genera emit behaviors that resemble salticid spiders—their main predator. Many other fascinating examples can be found, including wasps (Ropalidia sp.) that create nests resembling fruit, and assassin bugs (Hiranetis braconiformis) that reduce the probability of serving as a parasitic host by imitating the walking pattern of its impregnator, complete with fake ovipositor.


When stimulated by an aggressor, some protostomes quickly modify their posture to make them appear larger and at the same time to quickly present a “flash” of color. The various postures and displays are characterized by their position, such as frontal displays and lateral displays. The colors associated with these displays are often effective forms of defense because aggressors learn to associate certain colors with results that may have occurred through prior interaction with the intended prey. For example, if the prey had exhibited a certain color to its attacker, and then the predator became sick after ingesting the prey, or the intended prey sprayed the aggressor with a disagreeable fluid its body produces, the predator learns to associate that outcome with the flash of color it had seen and will attempt to avoid repeating the situation. Rapid display of color is also effective because the display itself will often frighten aggressors away. An example of a flash display is found in the katydids (Neobarrettia vannifera). When disturbed, this animal quickly opens its wings to reveal a polka-dot pattern. A display resembling a large face awaits any aggressor who disturbs the peanut bug, Laternaria later-naria (Fulgoridae), and flag-legged insects (Coreidae) quickly wave a brightly colored leg that it can afford to lose.

Learned behavior

The reasons for studying learning in protostomes are varied. Some scientists hope to exploit the nervous system of invertebrates in an effort to reveal the biochemistry and physiology of learning. Other scientists are interested in comparing invertebrates with vertebrates in a hunt for the similarities and differences in behavior. Still other scientists use learning paradigms to explore applied and basic research questions such as how pesticides influence honey bee foraging behavior and if learning is used in defensive and social behaviors.
A prerequisite for the study of learning is that be clearly defined and that the phenomena investigated as examples of learning be clearly defined. When reviewing studies of learning, the scientist should be aware that definitions vary from researcher to researcher. For example, a researcher may consider behavior controlled by its consequences (i.e., behavior that is rewarded or punished) as an example of operant behavior, while others believe that it depends upon the type of behavior being modified (either operant or instrumental learning). Moreover, some believe that any association between stimuli represents examples of Pavlovian conditioning, while others believe that the “conditioned stimulus” must never elicit the response to be trained prior to any subsequent association.
Here, learning is defined as a relatively permanent change in behavior potential as a result of experience. Several important principles of this definition include the following:
• Learning is inferred from behavior.
• Learning is the result of experience; this excludes changes in behavior produced as the result of physical development, aging, fatigue, adaptation, or cir-cadian rhythms.
• Temporary fluctuations are not considered learning; rather, the change in behavior identified as learned must persist as such behavior is appropriate.
• More often than not, some experience with a situation is required for learning to occur.
To better understand the process of learning in proto-stomes, many behavioral scientists have divided the categories of learning into non-associative and associative.


This form of behavior modification involves the association of one event, as when the repeated presentation of a stimulus leads to an alteration of the frequency or speed of a response. Non-associative learning is considered to be the most basic of the learning processes and forms the building blocks of higher order types of learning in protostomes. The organism does not learn to do anything new or better; rather, the innate response to a situation or to a particular stimulus is modified. Many basic demonstrations of non-associative learning are available in the scientific literature, but there is little sustained work on the many parameters that influence such learning (i.e., time between stimulus presentations, intensity of stimulation, number of training trials).
There are basically two types of non-associative learning: habituation and sensitization. Habituation refers to the reduction in responding to a stimulus as it is repeated. For a decline in responsiveness to be considered a case of non-associative learning, it must be determined that any decline related to sensory and motor fatigue does not exert an influence. Studies of habituation show that it has several characteristics, including the following:
• The more rapid the rate of stimulation is, the faster the habituation is.
• The weaker the stimulus is, the faster the habituation is.
• Habituation to one stimulus will produce habituation to similar stimuli.
• Withholding the stimulus for a long period of time will lead to the recovery of the response.
Sensitization refers to the augmentation of a response to a stimulus. In essence, it is the opposite of habituation, and refers to an increase in the frequency or probability of a response. Studies of sensitization show that it has several characteristics, including the following:
• The stronger the stimulus is, the greater the probability that sensitization will be produced.
• Sensitization to one stimulus will produce sensitiza-tion to similar stimuli.
• Repeated presentations of the sensitizing stimulus tend to diminish its effect.


This is a form of behavior modification involving the association of two or more events such as between two stimuli or between a stimulus and a response. In associative learning, the participant does learn to do something new or better. Associative learning differs from non-associative learning by the number and kind of events that are learned and how the events are learned. Another difference between the two forms of learning is that non-associative learning is considered to be a more fundamental mechanism for behavior modification than those mechanisms in associative learning. This is easily seen in the animal kingdom. Habituation and sensitization are present in all animal groups, but classical and operant conditioning is not. In addition, the available evidence suggests that the behavioral and cellular mechanisms uncovered for non-associative learning may serve as the building blocks for the type of complex behavior characteristic of associative learning. The term associative learning is reserved for a wide variety of classical, instrumental, and operant procedures in which responses are associated with stimuli, consequences, and other responses.
Classical conditioning refers to the modification of behavior in which an originally neutral stimulus—known as a conditioned stimulus (CS)—is paired with a second stimulus that elicits a particular response—known as the unconditioned stimulus (US). The response that the US elicits is known as the unconditioned response (UR). A participant exposed to repeated pairings of the CS and the US will often respond to the originally neutral stimulus as it did to the US. Studies of classical conditioning show that it has several characteristics, including the following:
• In general, the more intense the CS is, the greater the effectiveness of the training.
• In general, the more intense the US is, the greater the effectiveness of the training.
• In general, the shorter the interval is between the CS and the US, the greater the effectiveness of the training.
• In general, the more pairings there are of the CS and the US, the greater the effectiveness of the training.
• When the US no longer follows the CS, the conditioned response gradually becomes weaker over time and eventually stops occurring.
• When a conditioned response has been established to a particular CS, stimuli similar to the CS may elicit the response.
Instrumental and operant conditioning refer to the modification of behavior involving an organism’s responses and the consequences of those responses. It may be helpful to conceptualize an operant and instrumental conditioning experiment as a classical conditioning experiment in which the sequence of stimuli and reward is controlled by the behavior of the participant.

Studies of instrumental and operant conditioning show that they have several characteristics, including the following:

• In general, the greater the amount and quality of the reward are, the faster the acquisition is.
• In general, the greater the interval of time is between response and reward, the slower the acquisition is.
• In general, the greater the motivation is, the more vigorous the response is.
• In general, when reward no longer follows the response, the response gradually becomes weaker over time and eventually stops occurring.
Non-associative and or associative learning has been demonstrated in all the protostomes in which it has been investigated, including planarians (for some scientists, turbellarians are not considered protostomes), polychaetes, earthworms, leeches, water fleas, acorn barnacles, crabs, crayfish, lobsters, cockroaches, fruit flies, ants, honey bees, pond snails, freshwater snails, land snails, slugs, sea hares, and octopuses. While there is no general agreement, most behavioral scientists familiar with the literature would suggest that the most sophisticated examples of learning occur in Crustacea, social insects, gastropod mollusks, and cephalopods. Many of the organisms in these groups can solve complex and simple discrimination tasks, learn to use an existing reflex in a new context, and learn to control their behavior by the consequences of their actions.

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