Effects of ocean acidification on sediment fauna - Ocean Acidification - page 186

temperature) in the epifaunal Ophiura ophiura
( Wood et al. 2008, 2010). In both cases net calcii ca-
tion was maintained at low pH NBS (undersaturated
Ω a ) but extensive muscle wastage in low-pH
treatments was found only in A. i liformis (after 40
days).

of the epifaunal oyster Crassostrea virginica survived
much better to elevated CO 2 levels than either the
epifaunal scallop Argopecten irradians or the infau-
nal Mercenaria mercinaria (Talmage and Gobler
2009). However, exposure to CO 2 -induced acidii ca-
tion (650 μatm) resulted in delayed metamorphosis
and a smaller body size in the planktonic stages of
these three bivalve species. Fourthly, we still know
very little about the physiological adaptations that
enable one species to adopt a fossorial existence and
another species to remain epifaunal. Finally, one of
the major assumptions running through much of
what has been presented above is that animals
which already experience hypercapnic conditions
and demonstrate physiological adaptation to such
conditions will be the least affected by ocean acidi-
i cation (e.g. Raven et al. 2005 ). However, this
assumption is dependent on such animals having
much wider tolerance of hypercapnia than they cur-
rently require. But what if these animals are already
working at the limits of their resistance to, and
capacity to cope with, hypercapnia? Then the
assumption is turned on its head, and it is those
species that, comparatively speaking, are currently
most tolerant that may be under greatest threat
from additional ocean acidii cation. An analogous
situation exists in the effect of temperature increase
on intertidal animals. Stillman (2003) showed that,
counter-intuitively, it was the porcelain crabs which
inhabit the upper shore that were most at threat
from the increase in temperature. Unlike those of
the lower shore, the high-shore species were already
living at, or close to, the upper limits of their ther-
mal tolerance, and so were most vulnerable to
warming.
It is clear, as argued by Garland and Adolph
(1994), that care is needed when making deduc-
tions from the 'two species' study approach that
one is continually forced to adopt in reviews such
as this one, where pairs of closely related species or
genera, one fossorial the other not, are compared.
Given that targeted comparative studies are logisti-
cally complex it is worth considering possible alter-
native approaches that might help identify
hypercapnic tolerance in and between infaunal
groups. For example, it is interesting that in the
three main environments where seawater can
become markedly hypercapnic, namely intertidal

9.6 Summarizing the vulnerability of
infaunal organisms to ocean acidii cation

It is clear that pH in sediments is frequently lower
than the minimum pH of surface waters that is pre-
dicted to occur under projected ocean acidii cation
scenarios. In addition, sediments have a high pH
buffering capacity (Leclercq et al. 2002 ; Andersson
et al. 2003). These observations could suggest that
ocean acidii cation will have a lesser impact on
infaunal sediment communities than on communi-
ties that live on the sediment surface or in the
pelagic zone. Indeed, at i rst glance the literature
review above would seem to suggest that during
short-term exposure at least, infaunal organisms
may cope better with hypercapnic conditions than
epifaunal ones. However, there are a number of
important caveats.
Firstly, formal comparisons of the responses of
infaunal species across different phyla are fraught
with difi culties, for example the comparability of
methods and holding conditions as well as the pos-
sible phylogenetic dependence of physiological
responses and functions. Secondly, there may be as
much differential sensitivity across infaunal spe-
cies, even amongst those that possess substantial
CaCO 3 exo- or meso-skeletons, than between infau-
nal and epifaunal taxa. Thirdly, the majority of the
experimental evidence used in this chapter has been
generated in short-term exposure experiments
which only address a limited fraction of an organ-
ism's entire and often complex life history. It is gen-
erally accepted that early life-history stages of
invertebrates are more sensitive to CO 2 than juve-
niles or adults (Dupont and Thorndyke 2009; but
see Kroeker et al. 2010). Since many infaunal species
rely on a planktonic larval stage, the most severe
effects of ocean acidii cation on benthic species
could be on these early life stages. In this respect,
differences between infaunal and epifaunal species
may be far less clear cut. For example, adult forms