Selection on life-history traits across a wave exposure gradient in the tidepool copepod Tigriopus californicus (Baker)

Abstract

Marine intertidal organisms inhabiting different wave exposure regimes can face different abiotic conditions and selection regimes. In this paper, salinity and temperature conditions for the tidepool habitat of the marine copepod Tigriopus californicus (Baker) are described across a wave exposure gradient, and the selective impact of salinity on life-history traits is inferred from laboratory and field studies. I found that desiccation and osmotic stress in the tidepool habitat of T. californicus were higher at an outer coast compared to a protected site in California. In the outer coast area with high wave action, tidepool salinity varied spatially, fluctuated temporally, and was at least two times greater than seawater salinity on average; tidepools frequently dried completely. Conversely, tidepool salinity in the protected area with low wave action was moderate and did not vary spatially or temporally. Culturing females at different salinities and temperatures suggested that there is a trade-off between osmoregulatory maintenance and growth or reproduction. At high salinity age of first reproduction and interclutch interval increased, and reproductive rate decreased. Extreme salinity produced age-specific mortality in the laboratory, where juveniles suffered significantly greater mortality than adults in salinity stress experiments. Hypersaline conditions in seven natural tidepool populations were associated with a decrease in the proportion of the population composed of juveniles, and an increase in the proportion of adult females, suggesting that juveniles experienced higher mortality. Unlike previous studies of benthic organisms outside tidepools, I found that higher osmotic and desiccation stress occurred in T. californicus tidepool habitats in areas exposed to high wave energy, not in protected areas. The effects of abiotic conditions on life history traits and age-specific mortality suggest the selection exists for local adaptation to different wave exposure regimes.