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Early life stages are not always the most sensitive: heat stress responses in the copepod Tigriopus californicus

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Because of their complex life histories, different life stages of many marine invertebrates may be exposed to varying environmental challenges. Ultimately, the life stage that is least tolerant of its environment will determine the species' abundance and distribution. The intertidal copepod Tigriopus californicus lives in high intertidal pools along the Pacific coast of North America. Unlike most other invertebrates, the different life stages of T. californicus all share the same tidepool habitat. To determine physiological tolerances of various life history stages of this species, we examined responses to acute heat stress in nauplii, copepodids, and adults from 6 populations along a latitudinal gradient. Results show that early developmental stages (nauplii and copepodids) are generally more tolerant than adults. Our results contrast with the widely accepted generalization that larval forms are more sensitive to physical stressors than adults. As previously observed in adults, nauplii and copepodids from southern populations survive higher temperatures than those from northern populations. Acute heat stress was found to delay development but did not affect adult size. We hypothesize that variation in the thermal tolerance of early life stages among intertidal species reflects ecological differences in larval habitats: where larvae remain in the intertidal zone and experience the same high temperatures as adults, selection will favor high larval thermal tolerance, while in species with planktonic larvae, the buffered temperature regime of the water column might relax such selection, and thermal tolerance will be highest in the more exposed intertidal adults.

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