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206 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES Series 4, Volume 60, No. 10 It would be fallacious to argue against the protective role of a metabolite on the grounds that the delivery system might be improved. The most effective arrangements must have been evolved over a long series of generations, and we would only expect to find intermediate stages in their elaboration. Likewise, one must not be misled by the fact that defensive metabolites are not completely effective against predators. We should expect them to reduce the amount of predation, and to occur together with other means of defense. The opisthobranch Oxynoe feeds upon the alga Caulerpa, and uses metabolites from it defensively. However, it is well camouflaged on the alga. In Photo 27, a specimen of Oxynoe olivacea on Caulerpa prolifera can be seen exuding a cloud of toxic material. Oxynoe can also deal with predators by autotomy. Like lizards that shed their tails when caught, these slugs can break off the end of their foot and crawl away. Such autotomy is quite common in opisthobranchs, and is often accompanied by behavioral and physiological adaptations that enhance the effect. We discuss a few examples in later sections of this monograph. Many opisthobranchs have escape reactions that are elicited by the attack of a predator. These often involve detachment from the substrate and writhing from side to side allowing the slug to be carried away from the predators by currents. In some cases opisthobranchs have well-coordinated swimming movements due to undulations of the body or the flapping of parapodia. The two orders of pteropods, Thecosomata and Gymnosomata are characterized by the presence of “wings” that allow them to swim very effectively and spend their lives in the open water. This kind of locomotion could easily have been derived from swimming used to escape predators. Considering the various forms of defense that occur in opisthobranchs from a bioeconomic point of view, there is a complex pattern of costs and benefits that deserves further study. We have noted that there may be several lines of defense, and some of these are more costly than others. Avoiding the attacks of visual predators by some kind of crypsis, such as camouflage or nocturnal habits, is very common among opisthobranchs. A slug that is overlooked expends very little in the way of resources for the very reason that it does not get attacked. Crypsis, however, restricts the animal to those times and places where it is effective and may conflict with a need to forage for food or find a mate. Chemical defense may allow an opisthobranch to survive in more exposed positions, and if very effective the animals may be virtually immune from predators. The metabolites must cost something—how much nobody has yet attempted to estimate. Obtaining them from food must be cheaper than biosynthesizing them de novo, but the cost of foraging must be balanced against the savings thus realized. Chemically defended opisthobranchs are often attacked, as is shown by the frequency with which damaged specimens are found in natural environments. Although they often survive, they would be better off not having been attacked at all. Autotomy may allow an animal to crawl away while the predator feeds upon the autotomized part. The autotomized parts are often dorsal processes that writhe and squirm when detached. Having the dorsal processes contain diverticula of the gut, rather than more nutritious parts of the body such as gonads, is one way of reducing the costs of autotomy. Swimming may allow a slug to escape from benthic predators, and if faced with a high probability of being eaten, exposure to other predators may be a risk well worth taking. Once the predator has been evaded, however, there are a whole new set of problems. The slug may have a hard time locating an appropriate environment. Defensive anatomy, for instance, that of shells, displays trends that can be followed in the fossil record as well as on a geographical basis (Vermeij, 1978, 1987). Animals are particularly well defended in places where species diversity is high, as in the seas around New Guinea and the Philippines. Although chemical defense had not left a fossil record per se, nonetheless the same kinds of trends that characterize shells have been documented in metabolites. Just as tropical shells tend to be more robust, tropical marine animals tend to more toxic (Bakus, 1974, 1981; Coll,
CIMINO & GHISELIN: CHEMICAL DEFENSE AND EVOLUTION OF GASTROPODS 207 LaBarre, Sammarco, Paul, Williams & Bakus, 1982). This has been well corroborated, though the Antarctic fauna does not fit the trend very well, being a rich source of defensive metabolites (McClintock & Baker, 1997). There are similar trends that go along with the seasons. The problem of what causes such global patterns, including those of species diversity, body size, the amount of genetical recombination, and all sorts of other things may have no simple answer (<strong>Ghiselin</strong>, 1974). But they do indicate adaptation, however obscure the causes may be. One interesting study on the seasonal changes in the amount of toxic materials in a sponge shows a low level in the late winter and a higher one in the summer and fall (Turon, Becerro & Uriz, 1996). The kind of naïve reasoning that would relate the difference to temperature as such does not work, for the curve lags far behind. The pattern is rather like that of the number of eggs per clutch in certain birds, which declines during the reproductive season as food becomes more limited and it is more important to protect the young from predators. We say more about such topics at the end of this monograph. Many of the metabolites that occur in marine animals are pigments, a heterogeneous class of colorful compounds (Bandaranayake, 2006). Some of these are photosynthetic pigments contained in symbiotic algae and cyanobacteria. In other cases they are thought to be sun-screens that prevent damage from ultraviolet light. Others, including alkaloids produced by symbionts, are toxic or distasteful. Still others would appear to have mainly a visual effect, providing for color patterns that may conceal the animals on the one hand (camouflage), or render them conspicuous on the other (warning coloration). The fact that animals rich in secondary metabolites are often brightly colored is one more piece of evidence in favor of their having a defensive role. They supposedly have a kind of warning coloration that advertises the fact that they are distasteful to the predators. Some of the most conspicuous opisthobranchs in tropical waters are ones that have high levels of metabolites. For instance, dorid nudibranchs of the family Phyllidiidae, such as Phyllidia ocellata shown in Photo 64, which are well protected by isocyanides and have few natural predators, are generally active during the day and are quite conspicuous (Brunckhorst, 1991). On the other hand it has often been pointed out that many of the colorful opisthobranchs are actually quite inconspicuous when on their food. The mimics of brightly-colored animals may not be as distasteful as their models. Furthermore, the mere fact that an animal is drab and inconspicuous does not necessarily mean that it lacks chemical defense. Opisthobranchs that live among algae and feed on them are often well camouflaged, yet have well-developed chemical defense. Warning coloration is most strongly developed in those opisthobranchs that for one reason or another benefit from being able to venture out into the open with relative impunity. For example, dorid nudibranchs of the genus Roboastra pursue other nudibranchs and eat them, thereby deriving both nutriment and defensive metabolites (see Chapter X). Warning coloration and mimicry are controversial topics that have received a great deal of attention from biologists (for opisthobranchs see the works of Edmunds, 1974, 1987, 1987, 1991; Gosliner & Behrens, 1990; Gosliner, 2001). We will not discuss the issues in detail, but there are some peculiarities of opisthobranchs that make them particularly interesting in this connection. It has been something of a puzzle how natural selection could favor the evolution of conspicuous color patterns in the first place. One might think that a predator interacting with a varied population of prey would tend to notice and to eat the more conspicuous items first. The more conspicuous organisms would be the ones most often to die and would fail to pass their genes on to the next generation. The simplest answer is that often enough the prey escape either uninjured or not mortally so. There is experimental evidence for this (Penney, 2004; Gosliner, 2001). Furthermore, many animals tend to avoid conspicuous prey. Various other ways of getting around that problem have been suggested, but we will only consider one of them (but see Marples, Kelly & Thomas,
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