Kevin Larade, Ph.D
A capacity for long term survival without oxygen is well-developed among
many invertebrate species as well as in selected ectothermic vertebrates.
Anoxia tolerance has been particularly well-studied in various species of
marine molluscs, including both bivalves (e.g. mussels, clams, oysters) and
gastropods (e.g. littorine snails, whelks). These can encounter low
environmental oxygen as a result of multiple factors: (1) gill-breathing intertidal
species are deprived of oxygen when the waters retreat with every low tide, (2)
high silt or toxin levels in the water as well as predator harassment can force
shell valve closure, leading to substantial periods of "self-imposed" anoxia, (3)
animals in small tidepools can be oxygen-limited when animal and plant
respiration depletes oxygen supplies in the water, and (4) freeze-tolerant
intertidal species face oxygen deprivation whenever their body fluids freeze.
Life in the intertidal zone is particularly challenging since, in addition to the
cyclic availability of oxygenated water (each tide cycle lasts 12.4 h), organisms
can also be challenged with multiple other stresses including desiccation,
changes in salinity, and changes in temperature, sometimes including freezing;
all can potentially change rapidly over the course of a single tidal cycle of
immersion and emmersion. For this reason, various residents of the intertidal
zone have been used extensively as model systems of stress tolerance, the
most widely studied species being the sessile bivalve, the blue mussel Mytilus
edulis . Littoral snails that graze on rocks in the high intertidal zone are also an
excellent model system for studies of both anoxia tolerance and freeze
tolerance.
Marine invertebrates as a model system
Littorina littorea is one such gastropod mollusc that lives in the high intertidal
zone. Snails are characterized by a soft body that can be divided into two main
parts: 1) the head and foot region, responsible for neural activity and
locomotion, respectively, and 2) the visceral mass, involved in digestion,
respiration, excretion, and reproduction. Molluscan model systems have been
widely used in a number of disciplines. For example, mollusc models have
proven to be enormously successful in studies of nervous system function,
especially gastropods and cephalopods. Several bivalve species have been
extensively studies as models for the effects of pollution on marine habitats.
Both bivalve and gastropod muscle and mantle tissues (and others) have also
been widely used for studies of environmental stress effects on metabolism
and tissue function (e.g. temperature, salinity, anoxia). The hepatopancreas, a
voluminous structure that occupies the bulk of the visceral mass, performs a
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Kevin Larade, Ph.D
number of important functions in the mollusc, including protein synthesis,
glycogenolysis, gluconeogenesis, as well as specific roles in both digestion
and excretion. It has proven to be an excellent organ for metabolic studies at
the molecular level, including both in vivo and in vitro research on gene
expression profiling, protein synthesis and ribosomal analysis, and organ
culture studies.
Anoxia, characterized by the complete absence of oxygen, is a state that many
organisms experience infrequently. Although many organisms have retained
some capacity for anaerobic metabolism, this strategy is usually just a short
term one that deals with brief interruptions of oxygen delivery. Respiratory
exchange for marine snails , which occurs during tidal immersion, can occur at
a number of places through the skin of gastropods, but it is mainly sited at the
surface of the ctenidium or gill. As inhabitants of the intertidal zone, L. littorea
can be exposed to hypoxic or anoxic conditions twice daily for several hours at
a time and, during aerial exposure, may arrive at a situation where tissue
oxygen is fully depleted and cellular energy requirements can be met only by
anaerobic metabolism.
For a review of anoxia studies in marine molluscs, please refer to the Elsevier
book series "Cellular and Molecular Responses to Stress" Volume III, which
contains a chapter entitled 'A profile of the metabolic responses to anoxia in
marine invertebrates' authored by myself and Dr. Ken Storey.
Updated August 2005
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