(^) Much remains to be learned about mineralization in marine animals, but their shell-
forming organs typically take up calcium and carbonate by active transport and lay
down mineral. Some forms can sustain shell structure despite less-basic conditions
than “normal” seawater, pH ∼8.1, either by keeping actively secretory tissue near the
mineral surface or by adding impermeable organic coatings. Other animals,
particularly aragonitic forms lose shell even while alive at pH values around 7.6–7.8.
A plethora of experimental research is now being reported on the responses of
particular species to high levels of CO 2 in aquarium headspaces, or simply to acid
added to their water. An early suite of results is reviewed by Doney et al. (2009).
Calcification is reduced in some coccolithophores but not others, reduced in
planktonic foraminifera, mollusks, echinoderms, tropical corals, and red algae, even
in formation of the calcium sulfate hexahydrate (no carbonate at all) statoliths of
larval jellyfish (Winans & Purcell 2010). Particularly strong effects are observed in
larvae such as initial “D-shell” formation in clams and oysters, spicule formation in
sea cucumber and brittle-star larvae. Shells of adult pteropods exposed to modestly
reduced pH begin to lose mass quickly, become friable, and break off at the edges.
Coastal waters can be more heavily impacted than oceanic areas, which is not only an
anthropogenic effect. Water upwelling from great depths has substantially lower pH
from long-term accumulation of carbonate species. Effects are stronger for high-
latitude, cold-water forms, as expected, with attribution of failed spicule formation in
antarctic echinoderms to low pH. Oyster hatcheries in Oregon and Washington States
(northwest USA) now buffer seawater to a higher pH in their culture systems during
the upwelling season in order to sustain D-shell formation.
A Closing Note
(^) As you contemplate all of this and study climate-change effects in other sources, it is
well to keep in mind how very strong the effects of very modest climatic changes can
be. Just one example is offered here. Brander (1997) shows (Fig. 16.26) the effect of
water-temperature variation on two cod stocks, one off West Greenland, one off the
Faroe Islands. The warmer it is in these cold locales (up to about 11°C), the faster cod
can grow. The close tracking of size to temperature implies a causal link of some sort,
and differences of only 1.0 to 1.5°C in temperature produce a two-fold change in the
size of 4-year-old cod. Getting bigger cod faster seems to be a good thing. Remember,
however, that apparently desirable changes near the colder or warmer limit of a
species are likely matched by undesirable changes at the other end of its range. More
on the climate-change effects on fish, squid, and fisheries will be found in the next
chapter.
Fig. 16.26 Mean annual temperatures (squares and dashed lines) and mean weight