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species which has the greatest oxygen uptake, in
line with our original prediction. It is concluded
therefore that support for the 'increased metabo-
lism' hypothesis as a way of differentiating infauna
from epifauna is currently equivocal.
study by Batten and Bamber (1996) was there dis-
ruption of the burrowing behaviour. Similarly, bio-
turbation by the brittlestar
Amphiura i liformis
is not
affected on exposure to a pH
NBS
of 7.7 or 7.3 for 40
days (Wood
et al.
2009). Finally, for the polychaetes
Hyalinoecia tubicola
and
Diopatra cuprea
, which
inhabit tubes in soft sediments, exposure to hyper-
capnia (pH
NBS
= 7.5) causes no signii cant differ-
ences in the frequency or duration of respiratory
irrigation (Dales
et al.
1970 ).
9.5.3 Greater tolerance to hypercapnia in
terms of survival and behaviour
As mentioned above, tolerance to hypercapnia is
assumed to be a feature of infaunal organisms.
Certainly the spatangoid heart urchin
Echinocardium
cordatum
shows no mortality in sediments exposed
to acidii ed water (7 weeks at pH
NBS
= 7.5; Dashi eld
et al.
2008 ). Widdicombe
et al.
( 2009 ) noted the per-
sistence of capitellid worms in mesocosms exposed
to extreme hypercapnia (pH
NBS
< 6) for over 1
month. The same was true for
Nereis
virens
in previ-
ous acidii cation experiments (Batten and Bamber
1996 ; Widdicombe and Needham 2007 ) with no
observed deleterious effects observed at pH
NBS
as
low as 6.5. However, these few observations of
apparently CO
2
-tolerant burrowing species should
not be extrapolated to the entire infaunal commu-
nity. Using data extracted from Ries
et al.
( 2009 ) it
can be shown that there is no pattern in CO
2
-related
mortality when comparing infaunal (
Mya arenaria
and
Mercenaria mercenaria
) with epifaunal (
Mytilus
edulis
and
Argopecten irradians
) bivalves. Admittedly,
such a comparison was not the aim of their study
but the data still call for some explanation. It is
interesting, in this connection, that Knoll
et al.
( 2007 ;
see also Chapter 4) supported their hypothesis of a
link between the end-Permian extinction and hyper-
capnia with geological evidence that the extinction
of epifaunal bivalves was double that of infaunal
species. The underlying assumption is that infaunal
bivalves were already used to higher CO
2
in their
burrows than their epifaunal counterparts.
The number of behavioural studies is limited, but
it seems that hypercapnia does not affect the behav-
iour of some infaunal organisms. For example, there
was no effect of seawater acidii cation (5 weeks,
pH
NBS
= 7.21-7.30) on the burrowing activity or the
mortality of
N. virens
, a polychaete worm which
creates a semi-permanent U-shaped burrow
(Widdicombe and Needham 2007). Only at the
extremely low pH (pH
NBS
< 6.5) used in an earlier
9.5.4 Reduction in reliance on calcii cation for
skeleton or shell construction
There is little doubt that some infaunal soft-bodied
species can show a high level of tolerance to hyper-
capnia. Studies by Batten and Bamber (1996) and
Widdicombe and Needham (2007), both on the bur-
rowing polychaete worm
N. virens
, suggest that an
infaunal organism can tolerate seawater with a
pH
NBS
down to 6.5. Similarly, the nemertean
Procephalothrix
, which inhabits coarse sand or lives
beneath stones in the intertidal zone, is tolerant of
pH
NBS
in the range 5.0 to 9.2, at least over short (96
h) time periods (Yanfang and Shichun 2006). Are
there infaunal species or groups with closely related
epifaunal representatives where there is a reduction
in reliance on calcii cation for producing skeletal or
defensive structures? There does not appear to be
any such reduction in bivalves, although it could be
argued that the tests of infaunal echinoids are con-
siderably thinner and less substantial than epifau-
nal forms. A recent study (Ries
et al.
2009 ) has
compared rates of net calcii cation in a large number
of animal groups including infaunal and epifaunal
forms, one of the i rst studies of this kind. The results
are equivocal and no dei nitive pattern between
infaunal and epifaunal species can be discerned. The
response of the rate of calcii cation under elevated
CO
2
concentrations of the infaunal clams
Mya are-
naria
and
Mercenaria mercinaria
is different. The
former exhibits a negative rate, the latter a threshold
response. Of the two epifaunal species,
Argopecten
irradians
shows a negative rate and
Mytilus edulis
shows a neutral response (see Table 1 in Ries
et al.
2009). Finally, in the infaunal brittlestar
Amphiura
i liformis
, arm regeneration increases at low pH
whereas no such effect is detectable (at comparable
