Stable Photosymbiotic Relationship under CO2-Induced Acidification in the Acoel Worm Symsagittifera Roscoffensis

As a consequence of anthropogenic CO2 emissions, oceans are becoming more acidic, a phenomenon known as ocean acidification. Many marine species predicted to be sensitive to this stressor are photosymbiotic, including corals and foraminifera. However, the direct impact of ocean acidification on the relationship between the photosynthetic and nonphotosynthetic organism remains unclear and is complicated by other physiological processes known to be sensitive to ocean acidification (e.g. calcification and feeding). We have studied the impact of extreme pH decrease/pCO2 increase on the complete life cycle of the photosymbiotic, non-calcifying and pure autotrophic acoel worm, Symsagittifera roscoffensis. Our results show that this species is resistant to high pCO2 with no negative or even positive effects on fitness (survival, growth, fertility) and/or photosymbiotic relationship till pCO2 up to 54 K µatm. Some sub-lethal bleaching is only observed at pCO2 up to 270 K µatm when seawater is saturated by CO2. This indicates that photosymbiosis can be resistant to high pCO2. If such a finding would be confirmed in other photosymbiotic species, we could then hypothesize that negative impact of high pCO2 observed on other photosymbiotic species such as corals and foraminifera could occur through indirect impacts at other levels (calcification, feeding)

Visually Elicited Escape in Crabs

A review is given on field and laboratory studies, including intracellular recordings from the optic lobes, on the visually elicited escape response of a mangrove and a rocky shore crab. The crabs flee when the angular size of an approaching object has expanded by a critical value. Neurons which respond specifically to this stimulus were encountered in the lobula (internal medulla). Illusiory expansion produced by rotating spirals evokes no reaction. The results suggest a hypothesis on the underlying mechanism for looming detection. The direction of the crab’s escape can be understood as a compromise between a tendency away from the approaching object and towards a probable shelter since it can be described by a weighted vectorial summation of both components