Intraspecific predation, the process of both killing and eating an individual of the same species, is a significant and widespread process that until recently has not received the attention it merits. It is a major factor in the biology of many species and may influence population structure, life history, competition for mates and resources, and behavior. It is commonly observed among many animals: For this review I read over 900 papers describing such predation in about 1300 species. Laurel Fox's 1975 review of cannibalism (54) should be read for additional information. Here I attempt to present new or different data, references, and ideas. One of my purposes is to establish a theoretical framework for the organization and interpretation of the numerous observations of intraspecific predation. Certain topics included here are speculation intended to spark interest and indicate important problems that need further research. Because of page limitations many appropriate and important citations are not cited here but may be found elsewhere as indicated by "& incl. refs."

The Evolution and Dynamics of Intraspecific Predation

Many benthic marine invertebrates develop by means of free-living, dispersive larval stages. The presumed advantages of such larvae include the avoidance of competition for resources with adults, temporary reduction of benthic mortality while in the plankton, decreased likelihood of inbreeding in the next generation, and increased ability to withstand local extinction. However, the direction of evolutionary change appears generally biased toward the loss of larvae in many clades, implying that larvae are somehow disadvantageous. Possible disadvantages include dispersal away from favorable habitat, mismatches between larval and juvenile physiological tolerances, greater susceptibility to environmental stresses, greater susceptibility to predation, and various costs that may be associated with metamorphosing in response to specific chemical cues and postponing metamorphosis in the absence of those cues. Understanding the forces responsible for the present distribution of larval and non-larval (aplanktonic) development among benthic marine invertebrates, and the potential influence of human activities on the direction of future evolutionary change in reproductive patterns, will require a better understanding of the following issues: the role of macro-evolutionary forces in selecting for or against dispersive larvae; the relative tolerances of encapsulated embryos and free-living larvae to salinity, pollutant, and other environmental stresses; the degree to which egg masses, egg capsules, and brood chambers protect developing embryos from environmental stresses; the relative magnitude of predation by planktonic and benthic predators on both larvae and early juveniles; the way in which larval and juvenile size affect vulnerability to predators; the extent to which encapsulation and brooding protect against predators; the amount of genetic change associated with loss of larvae from invertebrate life cycles and the time required to accomplish that change; and the degree to which continued input of larvae from other populations deters selection against dispersive larvae. The prominence of larval development in modern life cycles may reflect difficulties in losing larvae from life cycles more than selection for their retention.

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On the advantages and disadvantages of larval stages in benthic marine invertebrate life cycles

Summary

1. Modes of larval development play important roles in the ecology, biogeography, and evolution of marine benthic organisms. Studies of the larval ecology of fossil organisms can contribute greatly to our understanding of such roles by allowing us to race effects on evolutionary time scales.



2. Modes of development can be inferred for well preserved molluscan fossils because the size of the initial larval shell (Protoconch I in gastropods, Prodissoconch I in bivalves) reflects egg size. Other morphological criteria are also available, and a comparative approach based on related taxa with known development may be the most reliable method. By combining larval and adult traits, it is possible to recognize modes of larval development in at least some fossil bryozoans, brachiopods, and echinoderms as well.



(a) Planktotrophic larvae arise from small eggs, are released in enormous numbers with little parental investment per offspring, and suffer tremendous mortality during and shortly after a planktic existence. These larvae feed on the plankton during development, and are commonly capable of a prolonged free-swimming existence, and thus wide dispersal.



(b) Nonplanktotrophic larvae (which include both planktic lecithotrophic forms and ‘direct developers’) generally arise from large eggs, with relatively few young produced per parent. Relative to planktotrophic larvae, nonplanktotrophic larvae generally receive greater parental investment per larva, and larval mortality is generally lower. These larvae rely on yolk for nutrition during development, and planktic durations are generally much briefer than for species with planktotrophic larvae, so that dispersal capability is considerably less. Energetic investment per egg is generally higher than in planktotrophs, but as there are lower fecundities as well it is difficult to generalize about the total energetic cost of one mode of reproduction against the other.



3. Owing to the high dispersal capability of planktotrophic larvae, it has been suggested that species with such larvae will be geographically widespread, geologically long-ranging, and exhibit low speciation and extinction rates. Species with nonplanktotrophic larvae will tend to be geographically more restricted, geologically short-ranging, and exhibit high speciation and extinction rates (again, as a consequence of their characteristically low larval dispersal capabilities).



4. Recognition of differential dispersal capabilities can play a role in paleobiogeo-graphic analyses. Concurrent study of the distribution of groups with contrasting modes of development will permit testing of hypotheses concerning timing, magnitudes and frequencies of migration and vicariance events.



5. Larval types are not randomly distributed in the oceans, but relationships with other aspects of the organisms' biology and habitats are very complex. Mode of development varies with:



(a) Ecology. A simple r–––K model of adaptive strategies is clearly insufficient to explain the observed relationships: while many ‘equilibrium’ species have nonplanktotrophic larvae, and organisms living in less prdictable environments often have planktotrophic larvae, some of the most opportunistic marine species have nonplanktotrophic larvae. Nonetheless, planktotrophic development seems most suited for exploitation of patchy but widespread habitats.



(b) Latitude. At shelf depths, planktotrophy is predominant in the tropics, and decreases sharply at high latitudes.



(c) Depth. Incidence of planktotrophy decreases with depth across the continental shelf, at least in some taxa. Beyond the shelf, many deep-sea organisms are nonplanktotrophic (e.g. most bivalves, peracarid crustaceans), but planktotrophic development appears to be present in other groups (prosobranch gastropods, ophiuroids, and bivalves inhabiting transient habitats such as sunken wood and hydrothermal vents).



These trends in developmental types will be accompanied by trends in evolutionary rates and patterns as outlined above. The study of larval ecology by paleobiologists will yield insights into the processes that gave rise to ancient evolutionary and biogeographic patterns, and will permit the development and testing of hypotheses on the origins of the patterns observed in modern seas.

Larval ecology of marine benthic invertebrates: paleobiological implications

Hermit crabs are anomuran decapod crustaceans that have adapted to occupation of empty snail shells (and a few other types of cavities). Lack of calcification of the abdominal exoskeleton both allows and dictates the occupation of shelter not produced by the crabs themselves. Many animals utilize exogenous shelters, but almost all 800 species of hermit crabs are mobile while sheltered. The combination of mobility and protection afforded by this life style must contribute to the large numbers of these crustaceans found in virtually all marine environments, as well as on tropi­ cal terrestrial shores. The strong association between these crabs and their adopted shelters has greatly influenced almost all aspects of their biology. In most species, meta­ morphosis from larva to adult involves the assumption of morphological asymmetries associated with inhabitation of gastropod shells as well as behavioral changes as the free-swimming glaucothoe becomes a benthic, shell-seeking crab (3). Not surprisingly, most biological studies on hermit crabs have focused on the crab-shell association. Behavioral investigations on shell selection and shell exchange are obviously shelter-related, while the visual display systems utilized by these crabs (32, 46) are indirectly asso­ ciated with distribution of the shell resource. Most ecological studies have focused completely on the crab-shell interaction, and relatively little work has been done on the crabs as an element of marine ecosystems. Accord­ ingly, this paper only briefly reviews aspects of the crabs' behavioral ecology other than the crab-shell association and its influence on both individual behavior and population-related processes. While individual hermit crabs have been found in a variety of objects and some specialize in the use of scaphopod shells (some Py/opagurus spp.) (33) or even bamboo tubes

The Behavioral Ecology of Hermit Crabs

Water economy and thermal relations of plethodontid salamanders were studied in the laboratory and in the field. Laboratory measurements included behavioral responses in temperature and relative humidity gradients, rates of dehydration and dehydration at various relative humidities and soil—moisture levels, and determination of critical thermal maxima by rapid controlled heating to a definite endpoint. Salamanders were acclimated to a combination of three temperatures and two photoperiods: 5°C, 16 hr of light alternating with 8 hr of darkness (LD 16:8); 5°C, (LD 8:16); 15°C (LD 16:8); 15°C (LD 8:16); 25°C (LD 16:8); and 25°C (LD 8:16). Three series of experiments were conducted on 20 populations representing 14 species. Critical thermal maximum increased with an increase in acclimation temperature indicating that the salamander's heat resistance was readily altered by its previous thermal history. Salamanders selected a definite range of temperatures and did not merely avoid extremes in the thermal gradient. Thermal preferenda were relatively stable for each species and were not significantly affected by either acclimation temperature or photoperiod. Rate of dehydration was dependent upon body size, drying power of the air, and ambient temperature. Interspecific differences in dehydration rates appeared to be related in part to differences in size. Species composed of small individuals lost weight faster than species made up of large individuals. As the vapor pressure deficit increased the dehydration rate increased. Dehydration was more rapid at higher temperatures. Rehydration rate increased as the percentage of weight loss due to dehydration increased and was more rapid at higher temperatures. Salamanders absorbed water from soil when soil—moisture tension was as high as 2.8 atm at 25°C. All species absorbed water from unsaturated soil at similar rates that were dependent upon soil—moisture content. Salamanders in the humidity gradient responded positively to differences in the moisture content of the air, and all but one species were more than 70% successful in selecting the highest relative humidity available in the gradient. Interspecific differences were apparent, but were not always correlated with habitat preferences. Plethodon glutinosus did not show any specific adaptation in its thermal responses or water relations that would account for its widespread distribution in the eastern United States. Plethodon ouachitae and P. caddoensis apparently have survived in the Ouachita Mountains due to favorable microhabitats and their ability to burrow deep beneath talus—covered slopes during hot and dry summers. Surface activity of P. caddoensis is limited in the summer by hot and dry conditions in its microhabitat.

Role of Temperature and Water in the Ecology of Lungless Salamanders

During a six-year period, several thousand intertidal snails, Thais lamellosa and T. emarginata, were given individually numbered tags and times of death were noted. These snails provided most of the shells used by intertidal hermit crabs (primarily Pagurus granosimanus). Therefore changes in the hermit crab population can be related to shell availability. The crab population increased greatly during 1969, and then declined steadily until the end of the study (1973). Changes in the crab population were highly correlated with shell availability. This hermit crab population will remain constant in size if 0.8 shells become available for each crab each month.

Availability and use of shells by intertidal hermit crabs

Some embryos of Thais emarginata attain twice the shell length of others by the time of hatching (0.9 to 1.8 mm, +44% to -31%, about the mean). Larger hatchlings must have acquired more nurse eggs than smaller ones. Embryos tend to reach the same size when nurse eggs are scarce (in crowded capsules), and size differences are most apparent when nurse eggs are present in excess. Therefore, competition among embryos contributes little to hatching size differences. Embryos are distributed haphazardly among capsules; some capsules contain only one embryo which becomes large, while others contain as many as 33 embryos, each of which remains small. Nurse eggs are distributed more regularly. Embryos are also distributed haphazardly among capsules of Acanthina spirata. Each embryo of A. spirata has on average only 1.67 nurse eggs, and hatching size is correspondingly less variable (0.55-0.75 mm, +15% to -16%, about the mean). These nurse-egg feeders reach variable hatching sizes because some embryos share their yo...

Hatching size and the distribution of nurse eggs among prosobranch embryos

Specimens of six salamander species were exposed to different soil moistures and rates of water exchange were calculated from the changes in weight observed. Soil water was available to these salamanders in soils with moisture tensions between 0 atm. and 2 atm. At these tensions, salamanders could rehydrate fully and can therefore be expected to remain in water balance in any soil with a tension in this range. The measurements should be useful in determining the suitability of habitats for salamanders. The rate of water exchange between the salamander and the soil was a function of the soil moisture tension. The rate of uptake from saturated soils was correlated with the body weight and the dehydration deficit of the salamanders. There were no differences in rate of exchange or in absorption threshold between the different species. These characteristics of water exchange between salamanders and soil are related to the properties of the soil.

Tom M. Spight

Papers

Availability and use of shells by intertidal hermit crabs

Hatching size and the distribution of nurse eggs among prosobranch embryos

The water economy of salamanders: exchange of water with the soil