Academic journal article The American Midland Naturalist

The Effect of Size-Limited Brood Capacity on Brood Size in a Freshwater Bivalve

Academic journal article The American Midland Naturalist

The Effect of Size-Limited Brood Capacity on Brood Size in a Freshwater Bivalve

Article excerpt

ABSTRACT.-Size limited brood capacity is common among species with hard exoskeletons or shells. In these species, brood size is limited by the physical capacity to hold offspring. Here we present evidence that brood size is limited by physical constraints in Sphaerium striatinum, a small brooding bivalve. Sphaerium striatinum is a sequential brooder and produces offspring throughout the year. Offspring are brooded in marsupial sacs located on the inner demibranch. In an unconstrained brooder one would predict that brood size would increase as a function of adult length cubed, a volumetric relationship. In S. striatinum, brood size increases as less than a function of adult length squared. We demonstrate that brood size is limited by two general constraints: marsupial sacs and the retention of extra-marsupial offspring. The number of marsupial sacs increases as less than a function of adult length squared. This relationship may be a result of physiological process such as feeding and respiration. Offspring size at independence is a crucial factor in determining offspring survivorship. The retention of extra-marsupial offspring promotes growth inside a safe environment and increases survivorship upon independence. However, the exponent relating brood size to adult length is significantly less for adults that contain extra-marsupial offspring than compared to adults that do not contain extra-marsupial offspring. Although the evolution of brooding in S. striatinum has resulted in severe constraints on brood size, the benefits of brooding outweigh the cost of limited brood capacity. We discuss our results in relation to brooding strategies and size limited brood capacity in other brooding bivalves.

INTRODUCTION

Brooding is associated with small body size in many invertebrates (Strathmann and Strathmann, 1982). The explanation for this association is that larger adults produce more offspring than they can physically brood (Strathmann et al, 1984). Following allometric principles of morphological design, the ability to produce and brood offspring is directly related to adult size (Huxley, 1932; Thompson, 1942). For example, fecundity is directly proportional to gonad volume and scales to the cube of adult length (Peters, 1983). The surface area of a body increases as an exponential function of adult length squared. For organisms that brood offspring along the surface of a body wall, the capacity to brood offspring will increase at a slower rate (proportional to length) than the ability to produce offspring. Therefore, the number of offspring larger adults can produce increases more quickly with increasing body size than the capacity to brood offspring.

Size-limited brood capacity is regulated by the arrangement of offspring inside adults, duration of incubation and size of offspring at independence (Strathmann, 1995). Fixing adult size, organisms that arrange offspring in a three-dimensional mass can retain proportionately more offspring than adults that arrange offspring along a two dimensional plane (Strathmann and Strathmann, 1982). If offspring size increases during incubation, then the amount of space available per offspring becomes proportionately smaller as offspring increase in size (Glutton-Brock, 1991). While physical constraints need not limit the initial production of offspring, they may become increasingly severe over the course of offspring development.

Size-limited brood capacity is common among species with hard exoskeletons such as crustaceans (Perrin, 1987; Glazier, 2000) and molluscs (Bayne et al, 1983; Calow, 1983; Sehens, 1987). Numerous species of bivalves brood offspring in both marine and freshwater environments. While all brooding bivalves retain offspring within the mantle cavity beneath or within the demibranchs, the mode of brooding varies widely across species. Variation in brooding is apparent in the physical arrangement of offspring and is influenced by adult size and size and number of offspring. …

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