Effect of Food Availability on Reproduction and Brood Size in a Freshwater Brooding Bivalve

Brood size is often constrained by the amount of energy available to produce offspring. Yet, energetic constraints may not be as important if the physical capacity to brood offspring is limited. Investigating the relative importance of energetic and physical constraints on brood size is necessary to understand how reproductive strategies are molded by natural selection. We investigated how food availability affects brood size in Sphaerium striatinum, a freshwater bivalve. We reared juveniles to adulthood under three food levels in a common garden experiment. The number of reproductive attempts, brood size, and stage of offspring development were measured. Clams reared with the most food reproduced more often, produced more offspring per reproductive attempt, and contained larger broods than clams reared with less food. These data support the notion that food availability is an important factor in the production of offspring and overall brood size. However, the number of offspring surviving to later stages of development was not different among treatments. In fact, clams reared with the most food lost proportionately more offspring than clams reared with less food. We conclude that physical constraints are more important in determining overall brood size than energetic constraints in S. striatinum

Parental Care in Sphaerium Striatinum Lamarck: Evidence for Retention of Competent Offspring

The timing of offspring release is a fundamental turning point in the life history of any organism. It represents the end to many of the most costly forms of parental care (e.g., provisioning of nutrients for developing eggs and zygotes) and the beginning of an independent life for the offspring. Generally temporal variation in this event is attributed to a variety of physiological and evolutionary trade-offs. Here we examine the retention of offspring in the freshwater clam Sphaerium striatinum Lamarck. Brooded offspring are typically not released into the environment until they are 4.0 mm shell length (SL). We provide evidence that offspring as small as 2.0 mm SL are competent to process particles and produce both fecal and pseudofecal material. Furthermore, such small clams are shown to experience higher mortality than larger individuals once they are removed from brood pouches. Hence, the retention of competent offspring within brood pouches represents a form of extended parental care

Scale-dependent variation in coral community similarity across sites, islands, and island groups

Community similarity is the proportion of species richness in a region that is shared on average among communities within that region. The slope of local richness (α diversity) regressed on regional richness (γ diversity) can serve as an index of community similarity across regions with different regional richness. We examined community similarity in corals at three spatial scales (among transects at a site, sites on an island, and islands within an island group) across a 10 000-km longitudinal diversity gradient in the west-central Pacific Ocean. When α diversity was regressed on γ diversity, the slopes, and thus community similarity, increased with scale (0.085, 0.261, and 0.407, respectively) because a greater proportion of γ diversity was subsumed within α diversity as scale increased. Using standard randomization methods, we also examined how community similarity differed between observed and randomized assemblages and how this difference was affected by spatial separation of species within habitat types and specialization of species to three habitat types (reef flats, crests, and slopes). If spatial separation within habitat types and/or habitat specialization (i.e., underdispersion) occurs, fewer species are shared among assemblages than the random expectation. When the locations of individual coral colonies were randomized within and among habitat types, community similarity was 46–47% higher than that for observed assemblages at all three scales. We predicted that spatial separation of coral species within habitat types should increase with scale due to dispersal/extinction dynamics in this insular system, but that specialization of species to different habitat types should not change because habitat differences do not change with scale. However, neither habitat specialization nor spatial separation within habitat types differed among scales. At the two larger scales, each accounted for 22–24% of the difference in community similarity between observed and randomized assemblages. At the smallest scale (transect–site), neither spatial separation within habitat types nor habitat specialization had significant effects on community similarity, probably due to the small size of transect samples. The results suggest that coral species can disperse among islands in an island group as easily as they can among sites on an island over time scales that are relevant to their establishment and persistence on reefs

Dynamics of coral communities

by Ronald H. Karlson.x, 250 p. : ill., map ; 25 cm

Apparent Sibling Rivalry in the Freshwater Clam Sphaerium Striatinum

In Sphaerium striatinum, a freshwater brooding bivalve, up to 97.5% of offspring that adults initially produce fail to reach independence. Marsupial sacs, specialized extensions of gill filaments that act as nurseries, initially contain multiple offspring in various sizes and stages of development. However, by the time offspring reach later stages of development, marsupial sacs typically contain only one offspring. Brood mortality is hypothesized to be the result of competition among embryos for nutrients and/or space. Sphaeriid eggs do not contain enough yolk for offspring to complete development. Adults supply additional nutrients required to reach independence. Brood capacity is limited by adult size. Adults cannot physically brood all offspring they produce. Here, we examine the validity of the competition hypothesis for brood mortality. We reared offspring, in vitro, through metamorphosis under varying nutrient levels and embryo densities. While hatching success and time to hatching were not influenced by nutrients or density, both factors had significant effects on the percentage of embryos completing metamorphosis and timing of metamorphosis. A higher percentage of offspring completed metamorphosis in higher nutrient levels and lower densities. Offspring reared with higher nutrient levels and lower densities also completed metamorphosis more rapidly. We discuss these results in relation to hypotheses for the overproduction of offspring, sibling rivalries, as well as factors that might explain brood mortality in this species

Density-dependent dynamics of soft coral aggregations: the significance of clonal growth and form

In clonal plants and animals, stolons and runners often promote rapid directional growth and escape from crowded microhabitats. Here we evaluate the effects of density on clonal growth and dispersal by stolons, on colony mortality, and on recruitment in the soft coral Efflatounaria sp. This colonial organism forms dense aggregations on mid-shelf and outer reefs of the Great Barrier Reef, Australia, where it is subjected to frequent physical and biological disturbances. Stolonal growth and asexual recruitment of new colonies (by budding) were enhanced by experimentally reducing local density. Within unmanipulated aggregations of Efflatounaria, per-capita rates of asexual recruitment were higher at low density, but colony survivorship was lower. Furthermore, the effect of density on stolonal growth and dispersal of daughter colonies varied as a function of a colony's history of disturbance. Disturbance was simulated by detaching from the substrate a newly budded colony that was still connected by a stolen to the parent colony. At low density, these pairs of partially detached colonies moved apart, while at high density, stolonally connected colonies moved closer together.\ud \ud Our results suggest that Efflatounaria employs a plastic life-history strategy that promotes recovery from injuries and the formation of dense aggregations. At low density, stolons facilitate rapid directional growth, asexual recruitment, and aggregation. At high density, clonal growth is inhibited, and mortality rates are greatly reduced. Enhanced survival within aggregations provides the adaptive context for interpreting the influence of density and disturbance history on the population dynamics of this clonal organism

Spatial variance in abundance and occupancy of corals across broad geographic scales

Species assemblages vary in structure due to a wide variety of processes operating at ecological and much broader biogeographical scales. Cross-scale studies of assemblage structure are necessary to fully understand this variability. Here, we evaluate the abundance and occupancy patterns of hierarchically sampled coral assemblages in three habitats (reef flat, crest, and slope) and five regions (Indonesia, Papua New Guinea, the Solomon Islands, American Samoa, and the Society Islands) across the west-central Pacific Ocean. Specifically, we compare two alternative models that unify spatial variance and occupancy via the negative binomial distribution. The first assumes a power-law scaling between the mean and variance of abundance; the second assumes a quadratic variance–mean relationship and a constant abundance-invariant aggregation parameter. Surprisingly, the well-established power-law model performs worse than the model assuming abundance-invariant aggregation, for both variance–mean and occupancy–abundance relationships. We also find strong evidence for regional and habitat variation in these relationships and in the levels of aggregation estimated by the abundance-invariant aggregation model. Among habitats, corals on reef flats exhibited lower occupancy and higher levels of aggregation compared to reef crests and slopes. Among regions, low occupancy and high aggregation were most pronounced across all habitats in American Samoa. These patterns may be related to habitat and regional differences in disturbance and recovery processes. Our results suggest that the spatial scaling of abundance and occupancy is sensitive to processes operating among these habitats and at regional scales. However, the consistency of these relationships across species within assemblages suggests that a theoretical unification of spatial variance and occupancy patterns is indeed possible

Appendix B. Mean local richness per transect for corals sampled across depth and habitat gradients.

Mean local richness per transect for corals sampled across depth and habitat gradients

Community structure of corals and reef fishes at multiple scales

Distributions of numerical abundance and resource use among species are fundamental aspects of community structure. Here, we characterise these patterns for tropical reef fishes and corals across a 10,000km biodiversity gradient. Numerical abundance and resource-use distributions have similar shapes, but they emerge at markedly different scales. These results are consistent with a controversial null hypothesis for community structure, according to which abundance distributions arise from the interplay of multiple stochastic environmental and demographic factors. Our findings underscore the importance of robust conservation strategies that are appropriately scaled to the broad suite of environmental processes that help sustain biodiversity