Atmospheric concentration of carbon dioxide has been rapidly increasing in the last 250 years from the pre-industrial 280 ppm to 380 ppm, due largely to fossil fuel burning and cement production. This has led to warming and increasing acidity (termed ocean acidification) of oceans through diffusion of CO2 across ocean surface. There is increasing concern upon potential impacts of these ocean environmental changes, whereas we presently know very little about it. We are experimentally investigating how increasing seawater temperature and acidity affect marine animals by rearing different marine animals under simulated future oceanic conditions. Bubbling seawater with CO2-enriched air (1000 ppm, projected highest CO2 concentration by the end of 21st century) reduces seawater pH by 0.3 units, and we have already seen that this small change in seawater chemistry could have deleterious impacts on early development, gonad maturation, and growth of some marine animals.
(1) Experimental exposure of a shallow-water shrimp for 30 weeks to 1000 ppm resulted in nearly 50% mortality as compared with 90% in control shrimps. Growth was suppressed in females, but not males, and egg bearing. Strangely, the antenna length of the experimental shrimps became only 1/3 of the normal after 30 weeks (Kurihara et al. 2008 J. Mar. Biol. Ecol.).
(2) Early development can be severely disrupted by exposure to high CO2 conditions. Our most recent experiment has revealed that incubation in 2000 ppm CO2 completely inhibits early development of Antarctic krill with 0% hatching (Kawaguchi et al. 2010, Current Biology). Similar levels of CO2 exposure caused malformation of larval shells in nearly 100% of oysters (Kurihara et al. 2007 Aquat. Biol.) and mussels (Kurihara et al. 2008 Aquat. Biol.).
(3) However, some animals such as a copepod was found to be robust, showing no significant negative impacts on survival, growth, egg production or larval hatching even after rearing for 3 generations in 2380 ppm (Kurihara et al. 2008 Mar. Poll. Bull.). We are now focusing on combined effects of CO2 and warming. Experiments on sea urchins have demonstrated very strong suppression in gonad development, behavior and some physiological parameters. Ocean acidification research has become one of the hottest issues in biological oceanography. With overseas students and post-doctoral fellows, we are making cutting-edge investigation into this new scientific field.
Only a small number of extant fish groups show adaptations to amphibious lifestyle. Among them, mudskippers stand out as one of the largest groups that volitionally emerge from water and spend a significant portion of time on mudflat surface. Mudskippers show remarkable capabilities forair breathing, osmoregulation, terrestrial locomotion and feeding, and aerial vision. These support a wide variety of activities displayed by mudskippers on exposed mudflats. In contrast to ample information available on surface activities, the subsurface life of mudskippers in burrows has been little studied. The most notable yet crucial feature of their subsurface activities is reproduction in burrows (Ishimatsu et al., 2007, 2009). It has been known that mudskippers excavate burrows, and use them as a refuge from predators, for protection from desiccation and temperature extremes, and for reproduction. Although burrow sequestration aff ords egg protection, the severe hypoxia of burrow water (Ishimatsu et al., 1998a, b, 2000, 2007) would not permit embryonic development. Mudskippers have evolved a highly sophisticated strategy to ensure embryonic development and larval hatching by integrating their aerial respiratory capability into reproduction. Mudskippers could give useful insights into our understanding of land invasion by ancient vertebrates, despite their distant taxonomic position from those animals that gave rise to ancestral tetrapods, and different environmental and ecological settings of the present and the past. Vertebrate invasion to land has arguably taken place in tropical freshwaters, but recent fossil evidence has pointed out that ancestral vertebrates of tetrapods inhabited both freshwater and marine regions, and that land invasion by some of these animals might have occurred in marine habitats.