Abstract
Anthropogenic activities are increasing ocean temperature and decreasing ocean pH. Some coastal habitats are experiencing increases in organic runoff, which when coupled with a loss of vegetated coastline can accelerate reductions in seawater pH. Marine larvae that hatch in coastal habitats may not have the ability to respond to elevated temperature and changes in seawater pH. This study examined the response of Florida stone crab (Menippe mercenaria) larvae to elevated temperature (30°C control and 32°C treatment) and CO2-induced reductions in pH (8.05 pH control and 7.80 pH treatment). We determined whether those singular and simultaneous stressors affect larval vertical movement at two developmental stages. Geotactic responses varied between larval stages. The direction and rate of the vertical displacement of larvae were dependent on pH rather than temperature. Stage III larvae swam upwards under ambient pH conditions, but swam downwards at a faster rate under reduced pH. There was no observable change in the directional movement of Stage V larvae. The reversal in orientation by Stage III larvae may limit larval transport in habitats that experience reduced pH and could pose challenges for the northward dispersal of stone crabs as coastal temperatures warm.
Highlights
Increasing atmospheric CO2 concentrations are warming the atmosphere and the ocean, and are causing a decline in ocean pH
Our results suggest that low pH may change the vertical movement direction in some larval stages, which could alter dispersal in highly stratified waters
A negative geotaxis response in stone crab larvae is consistent with other crab species including Callinectes sapidus [40], Rhithropanopeus harrisii [37,41] and Hemigrapsus sanguineus [36] and positions newly hatched larvae in relatively shallow depths [26]
Summary
Increasing atmospheric CO2 concentrations are warming the atmosphere and the ocean, and are causing a decline in ocean pH. Some coastal habitats are experiencing increased runoff and eutrophication, which amplifies pH variability [3,4,5,6,7,8,9] In combination, these stressors can affect the development, behaviour, growth and survival of marine species, during sensitive larval stages [10,11,12]. Our study is the first to test the hypothesis that elevated temperature and reduced pH conditions will alter the vertical swimming behaviour of larval crustaceans using the Florida stone crab as a model example. The experiment measured changes in larval vertical swimming behaviour after rearing larvae in the treatment conditions (table 1). All statistical analyses were performed using R v.3.6 [39]
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