Geoscience Reference
In-Depth Information
acidii cation on benthic species is that our informa-
tion is heavily restricted to very few species distrib-
uted amongst a few taxonomic groups (crustaceans,
molluscs, echinoderms, i sh, and to a lesser extent
annelid and sipunculid worms). The list is reduced
even further when the focus is restricted to infaunal
species. Although our understanding has been
slightly advanced by the review of the effect of
hypercapnia on burrowing and non-burrowing ani-
mals by Widdicombe and Spicer (2008) there is still
nowhere near a rigorous phylogenetically based
assessment of burrower versus non-burrower
responses to hypercapnia. Interestingly, while it is
now very difi cult to publish a comparative study
without taking phylogeny into account, the extent
to which phylogenetic relationships are important
for the comparison of physiological features is still
debatable. On the one hand it seems that some
physiological traits exhibit little phylogenetic con-
straint (particularly when comparisons are made at
the high taxonomic levels), such that species can
justii ably be treated as independent data points for
analysis. However, there is seldom sufi cient a pri-
ori knowledge to be able to reject a role for phylog-
eny, and thereby avoid explicitly accounting for it in
analyses (Spicer and Gaston 1999). With those
uncertainties in mind it is still valuable to assess
which of the available literature is at least consistent
with some of the features thought to be characteris-
tic of hypercapnia-tolerant animals.
that has not yet been reported in epifaunal urchins
(J. I. Spicer
et al.
unpubl.). Searching the literature
for extracellular protein concentration values as
an indication of regulatory ability shows that
while there are dramatic differences both between
and within phyla (echinoderms have very low lev-
els, crabs have intermediate levels, and i sh
have quite high levels), there are no consistent pat-
terns for burrowing and non-burrowing phyla.
Spangatoid echinoids have marginally lower con-
centrations of extracellular proteins than epifaunal
forms such as
Psammechinus miliaris
( Miles
et al.
2007). The same is also true for bivalves. Similarly,
deep-burrowing thalassinidean shrimps and
infaunal crabs do not have dramatically higher
haemolymph protein concentrations than epifau-
nal crustaceans (Taylor
et al.
2000 ). In fact, some-
times the burrowing species actually show lower
protein levels than their non-burrowing counter-
parts (Atkinson and Taylor 2005).
9.5.2
Higher rates of metabolism
In the sipunculid worm
Sipunculus nudus
, hyper-
capnia was shown to elicit metabolic depression,
via acidii cation of body l uids (Reipschläger
et al.
1997 ; Pörtner
et al.
1998 , 2000 ), and this response
was accompanied by a reduction in protein turno-
ver (Langenbuch
et al.
2006). However, this is a rela-
tively short-term response and is elicited only at
very high (e.g. 10 000 μatm) CO
2
levels. Although
comparisons of metabolic rates between different
studies are notoriously difi cult, there is some evi-
dence that hypometabolism may be a feature of
burrowing forms. For example, the nereid polycha-
ete
Nereis virens
consumes less O
2
when coni ned in
tubes than when living freely in water (Hyman
1932) and the deep-burrowing
Lumbrineris zonata
has a lower oxygen consumption than the shallow-
burrowing
Nephtys hombergii
( Banse
et al.
1971 ). In
echinoderms, where two studies using exactly the
same experimental technique are available, the
infaunal brittlestar
Amphiura i liformis
shows a
hypercapnia-induced (pH
NBS
= 7.7) reduction in
oxygen uptake whereas the epifaunal
Ophiura
ophiura
does not (Wood
et al.
2008 , 2010 ). The former
also generally exhibits a greater rate of oxygen
uptake than the latter. In this case, it is the infaunal
9.5.1
High extracellular buffering capacity
Extracellular buffering of protons can be provided
by bicarbonate or non-bicarbonate protein buffers
and a good ion-regulatory capacity (Seibel and
Walsh 2003 ; see Chapter 8 ). While knowledge of
extracellular acid-base regulation in response to
hypercapnia is growing for epifaunal crabs, mol-
luscs, and echinoderms (Widdicombe and Spicer
2008 , Melzner
et al.
2009) the same is not true for
infaunal species. Two notable exceptions are the
sipunculid worm
Sipunculus nudis
and the lug-
worm
Arenicola marina
where there appears to be
little extracellular acid-base regulation (Melzner
et al.
2009). In spangatoid burrowing urchins in the
longer term (in excess of 2 months) any initial res-
piratory acidosis is compensated for to an extent
