The theme of this chapter is well summarized by the title of a collected volume edited by Toft and her colleagues (1991): ‘Parasite–host associations: coexistence or conflict?’. The answer to that question, over the entirety of biology, has proven quite complex. Here I examine a small, more easily tractible aspect of the problem, taking the perspective of freshwater molluscs.
Freshwater molluscs are known to host a wide variety of parasites. The first report of a haplosporidian from North America was Barrow's (1961) discovery in Michigan Lymnaea, Physa, and Helisoma. Haplosporidian disease subsequently decimated the oyster population on the east coast of North America, ending a way of life for thousands of fisherman. The oligochaete Chaetogaster lives on or in a wide variety of freshwater snails and may be parasitic (Buse 1971), commensal (Young 1974), or possibly even of some benefit to its host (Khalil 1961). This chapter will emphasize the Digenea, however, by virtue of the wealth of literature available. I conclude with brief discussions of two groups of parasites most associated with unionacean mussels, the aspidogastrid trematodes and unionicolid mites. Readers with interests in any other class of parasite to which molluscan flesh may be heir are referred to the primary literature directly.
The Digenea is by far the largest order of trematodes, that entirely parasitic class of flatworms generally termed ‘flukes’.
We begin this chapter with a review of the nominally abiotic factors that may contribute to the distribution of freshwater bivalves and gastropods on a regional scale. At least a dozen studies over the last 30 years have returned strong positive correlations between the abundance and/or diversity of freshwater molluscs and calcium concentration or related water quality variables. Laboratory experiments show both that calcium concentration in normal ranges can affect growth, survivorship, and fecundity, and that the various species offen differ strikingly in their calcium optima. Some workers have suggested that calcium concentration (and correlated variables) may not act on abundance in the field, however, but rather serve as a ‘filter for colonists’ only. Others have suggested that the effects of calcium on natural abundances of molluscs are real but indirect, calcium concentration influencing the abundance of food.
We next review a relationship with analogues throughout biology, the number of mollusc species as a power function of habitat area. About a dozen studies worldwide have returned species/area regression coefficients somewhat less than or equal to the ‘canonical’ value of 0.25. Species vary in the mean areas of the habitats they occupy, just as they vary in calcium range. And an interaction is sometimes noticeable between area and calcium in their effects on the freshwater mollusc community, such that sites with larger areas tend to have more species than might be predicted from their calcium concentrations.
In this chapter we will examine the relationships among populations within freshwater mollusc communities. We define the term ‘community’ broadly, noting as we do that its component mollusc populations may be of such diverse ecological character that the likelihood of interactions among them may be less than the likelihood of interactions between them and other elements of the benthos. The artificiality of the concept of the freshwater mollusc community does not, however, erase its utility.
Developing a theme first opened in Chapters 2 and 3, here we review a large body of literature approaching diet and habitat from a comparative standpoint. Differences in the gut content of co-occurring gastropod species can be substantial, but this seems primarily due to differing habitat choice, rather than selective grazing. Variation in substrate preference seems central in gastropod communities, with other aspects of the local environment (especially depth) important as they impact substrate. The overlap in diet and habitat observed within bivalve communities seems great.
We next review the several studies that have applied ordination techniques to variation in species distribution within freshwater mollusc communities, and offer several original analyses. The elements of most communities seem to associate into subsets according to features of the habitat. We also find one situation where species seem to aggregate apart, and cases where little structure of any sort is apparent. Sampling scale seems to be a key to detecting interspecific pattern in distribution.
Molluscs are edible, and easily subdued by predators of even the most modest ferocity. Thus most aquatic vertebrates seem to eat molluscs, at least under some circumstances. I include under this sweeping generalization not only fish, but semi-aquatic vertebrates such as amphibians, certain reptiles, and a few mammals. In a charming review entitled ‘Enemies of the land and freshwater Mollusca of the British Isles’, Wild and Lawson (1937) listed 20 vertebrate predators of Planorbis alone, excluding fish but including such eye-catchers as the natterjack toad, pheasants, and bats. In addition, a large fraction of the macroinvertebrate predators of fresh waters recognize molluscs among their prey. Among the fauna of Oneida Lake, New York, F. C. Baker (1918) recognized six species of insect potentially dining on molluscs, one crayfish, eight leeches, and two of the molluscs themselves (large Lymnaea). To the Oneida rogue's gallery Baker added 46 species of fish, 8 amphibians, 7 reptiles, 6 birds, and 3 mammals. Michelson's (1957) review of possible biological control agents for pulmonates included single paragraphs on predatory flatworms, leeches, crustaceans, predatory molluscs, and mammals, plus two paragraphs on birds, three on reptiles and amphibians, and four on insects. Molloy and colleagues (1997) catalogued 176 predators of zebra mussels.
The present review will proceed from mammals through the vertebrates and into the invertebrates, as predator body sizes decrease and densities rise.
If natural selection works to maximize the total offspring an individual leaves behind, how can it happen that one perfectly successful population of freshwater bivalves produces, on the average, 10 offspring per parent lifetime, while another produces 106? Life history studies address variation in fundamental demographic parameters such as birth rate (including age at reproduction, clutch size, and developmental time) and survivorship (lifespan, semelparity/iteroparity), as well as the relationship between individual age and size, from propagule to adult. Because the energy available to an organism is finite, life history studies are ultimately concerned with what have come to be called ‘trade-offs’ between parental growth, maintenance, and reproduction, semelparity and iteroparity, propagule size and number, and many other factors. Trade-offs are not inevitable, however, as we shall see.
The efficiency of natural selection on a trait is dependent on its heritability, that portion of the total phenotypic variance that is additively genetic. But the heritability of life history traits generally seems to be less than that observed for morphological, physiological, or even behavioural traits (Price and Schluter 1991). Because selection is expected to act most efficiently on traits as directly tied to fitness as survivorship and reproduction, the numerator of the heritability may be small. And because the expression of such traits is unusually sensitive to the environment, the denominator of the heritability will be large.
In this chapter we will review a few of the basic attributes of the biology of freshwater bivalves. Although filter feeding might at first seem a relatively simple process, closer examination shows a wide discrepancy between the particles in the medium (often largely inorganic) and the food actually assimilated (diatoms, green and blue-green algal cells, bacteria, and organics both dissolved and suspended). Our discussion of bivalve feeding will be divided into sections on particle retention, ingestion, and assimilation. There is some large-scale diet and habitat specialization in bivalves; Pisidium seems to have become adapted to filter waters from within the sediments, sometimes deep in the profundal zone, and Dreissena has colonized the hard bottoms. But in general, we will see that all bivalve populations seem to live in about the same habitat and eat about the same food at the same time. In light of the evidence that large populations of bivalves may substantially depress the concentration of suspended particles in even the largest lakes and rivers, the potential for food limitation and both intra- and interspecific competition must be acknowledged.
The freshwater bivalves are quite diverse in their modes of reproduction. We will see that unionoids are gonochoristic (although their mechanisms of sex determination are unclear) with widespread hermaphroditism. Their adaptation to hold developing larvae (‘glochidia’) and impose them parasitically upon fish hosts constitutes one of the more interesting natural history sagas of which I am aware.
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