Meeting Abstract
Environmentally cued hatching is present in diverse phyla in response to biotic and abiotic factors including temperature, tidal inundation, and predator cues. Echinoderms have been shown to delay hatching in response to salinity reductions during early development. In the sand dollar Echinarachnius parma, embryos exposed to low salinities early in development can delay hatching up to early pluteus larval stages. Embryos that delay hatching appear to continue normal development, but confinement within the fertilization envelope prevents larval arms from growing as long as those of hatched individuals. The fertilization envelope also acts as a barrier, preventing larvae from ingesting the algal cells they might otherwise consume. As a result, costs of delayed hatching could result from shorter arms, delayed access to algal food, or both. To distinguish between these alternative possibilities, we conducted an experiment with five treatments, each containing sibling embryos that had: 1) hatched at 18 hours post fertilization (hpf) in normal salinity (~33 ppt) with food present, 2) hatched at 18 hpf in normal salinity with delayed availability of food, 3) hatched at 18 hpf in low salinity (26 ppt) with food present, 4) hatched at 18 hpf in low salinity with delayed availability of food, and 5) hatched at 38-40 hpf at low salinity with food present (n = 4-5 replicates per treatment). Five larvae from each replicate were photographed weekly to record arm and body lengths. Juvenile size, spine number, and spine length were also measured after settlement. The strongest treatment effects were related to pre-hatching salinity. Larvae at lower salinities were more likely to be asymmetrical than their siblings but, counterintuitively, exhibited a higher degree of competence when challenged to metamorphose.
