Indirect costs of reproduction and the tradeoff between offspring size and number: a framework illustrated by fitness costs and benefits of ovarian fluid.

The theory describing the evolution of offspring size often assumes that the production cost per unit volume is the same for small and large offspring. However, this may not be true if indirect costs of reproduction (e.g., material and energetic costs of supporting offspring development) scale disproportionately with offspring size. Here we show how direct and indirect costs of reproduction can be explicitly modeled within the Smith-Fretwell framework and how observations of size-number relationships can thus be used to evaluate indirect costs. We applied this analysis to measures of egg volume and fecundity for over 300 individuals of a coastal fish species and found that the tradeoff was much stronger than the expected inverse (fecundity scaled with volume-1.843). Larger offspring were thus more expensive to produce. For our study species, an important indirect cost was that larger eggs were accompanied by disproportionately more ovarian fluid. Calorimetry and removal experiments were used to further measure both the energetic costs and fitness benefits of ovarian fluid. In addition, we show that indirect costs of reproduction can intensify size-number tradeoffs in a variety of fishes. Indirect costs of reproduction can be large and may therefore play an important role in the evolution of offspring size.

Effects of microplastics on the feeding rates of larvae of a coastal fish: direct consumption, trophic transfer, and effects on growth and survival.

Microplastics are now found throughout the world's oceans, and although many organisms ingest microplastics, less is known about how plastics in seawater may affect key processes such as feeding rate, growth, and survival. We used a series of laboratory experiments to test whether microplastics in seawater affected the feeding rates of larvae of the California Grunion, Leuresthes tenuis. In addition, we tested whether trophic transfer of microplastics from zooplankton to larval fish can occur and affect growth and survival of fish. We measured feeding rates of grunion larvae at various concentrations of 75-90 µm and 125-250 µm polyethylene microplastics and under both still water and turbulent conditions. In these experiments, exposure to microplastics had modest effects on feeding rates, though responses may be somewhat complex. Low concentrations of microplastics increased feeding rates compared to the control, but at higher concentrations, feeding rates were indistinguishable from those in the control group, though effects were small compared to natural variation in feeding rates among individual fish. Experiments to test for trophic transfer of microplastics revealed that grunion larvae that were fed brine shrimp exposed to high concentrations of microplastics had lower growth rates and elevated mortality rates. Overall, our results suggest that the direct effects of microplastics on feeding rates of California Grunion during the early larval phase are minor, while the trophic transfer of microplastics from zooplankton to larval fish may have significant effects on their growth and survival. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00227-021-04010-x.

The beach-spawning California grunion Leuresthes tenuis eats and digests conspecific eggs.

Apparent egg cannibalism was investigated in the beach-spawning California grunion Leuresthes tenuis. Three hypotheses were tested to determine whether L. tenuis regularly consumes and efficiently digests conspecific eggs. First, examination of the gut contents of adults collected at four spawning sites over two seasons showed that the intestines of most fish from all the sites (57-87%, n ≥ 30, each site) contained L. tenuis eggs. The two other hypotheses focused on digestion of the eggs. First, the force required to crush cannibalized eggs was significantly less than that for uncannibalized eggs (fertilized or unfertilized), indicating that ingestion weakens the egg chorions. Second, conspecific eggs fed to fish held in the laboratory visibly degraded as they passed through the gut. The eggs lost c. half of their protein content and about two-thirds of their lipid content as they passed from proximal to distal regions of the gut, indicating that digestion occurred. Digestive enzyme activities of the gut further confirmed that L. tenuis can break down the contents of ingested eggs. Trypsin activity decreased and aminopeptidase activity increased posteriorly along the gut, whereas amylase and lipase activities exhibited less clear patterns by gut region. As far as is known, this study is the first to show that L. tenuis is an egg cannibal.