Carter during aerial exposure when the tides are

Carter (2014) also reported in her
study that mud clams in the northwestern Santa Isabel, Solomon Islands of the
South Pacific can be found in the seaward mangrove fringe where it experiences
its longest aerial exposures during the spring tides. Morton (1975), on the
other hand noted the opposite in the mud clams of Southeast Asia. They are
found in the landward fringe and are only covered by waters during the spring
tides. Mud clams in the Southeast Asia are adapted in a semi-terrestrial way of
living (Nagelkerken et al., 2008). In addition, Hiong et al. (2004) said that Polymesoda spp. spends portions of their
lives exposed aerially during the low tides. They usually inhabit the high
shores where high tides only occasionally reach the area. This suggests that
they can be found in the landward area of mangrove forests, specifically in the
high intertidal areas (Clemente and Ingole, 2011). Due to this, they are buried
in mud and are usually hidden in between mangrove roots when the tides are low.
They may also be seen in small water pools, which form at the bases of mangrove
trees (Cremades, 2014). Since the mud clams are only seldom submerged or
exposed to water as they are only occasionally being reached by high tides,
this therefore suggests that these mud clams are highly tolerant to aerial
desiccation during aerial exposure when the tides are low. Clemente et al.
(2013) said that this ability of the mangrove clams to withstand prolonged
periods of aerial exposure is an extreme adaptation to a semi terrestrial mode
of life. This may imply that mud clams must have the ability to retain water
somehow within their tissues in order to live. This is where their large size
and their characteristic hard shells come into action. This does not only
provide protection for the clams against erosion but is also responsible in
allowing the bivalves to keep a large volume of water within their shells
specifically at times when the tides are low and they are exposed. This will
keep the mud clams’ tissues able to survive as they are being maintained in an
environment where there is enough and adequate water (Carter, 2014). This
explains why the Polymesoda clams,
specifically the shells are large. Growth is focused more on the growth of the
shell rather on the growth of the tissues. Nevertheless, tissue growth at low
rates is very vital for the mangrove clam’s survival under extended exposure
(Gimin et al., 2004). The need and the ability to contain water in relation to
the habitat of the mangrove clam are very critical for their survival. It would
be detrimental for the mangrove clam to have a continuous growth of the soft
tissue inside the shell and occupy a large space within it. This would lead to
lessened capacity to hold water inside the shell leading to insufficient water
to support the necessary metabolic needs in case the clam’s tissues continue to
grow (Gimin et al., 2004).

            Since
mud clams remain more exposed to air than being submerged in water, they are
also able to undergo aerial respiration. Aerial respiration in the mud clam is
accomplished in the mantle cavity and happens during emersion (Clemente and
Ingole, 2011) or aerial exposure. In most bivalves, emersion could restrain a
number of metabolic function (Hiong et al., 2004). Some of these include
respiration, feeding and even excretion. This suggests that oxygen consumption
in many bivalves is greatly reduced when they get exposed to air. As a response
to this increasing demand for oxygen due to the ongoing shortage, most of the
energy requirements for these bivalves are met through anaerobic strategies.
The lower intertidal and subtidal bivalves utilize this strategy. In order to
shift to anaerobic metabolic pathways, these organisms usually close their
shells. In contrast to the lower intertidal and subtidal bivalves, Polymesoda spp. open up their shells
posteriorly to expose their mantle margins at times of aerial exposure. The
adduction of the shells of the valves that immediately follows the emersion and
at periodic intervals ventilates the mantle cavity and on the mantle, gas
exchange occurs (Hiong et al., 2004). This is how Polymesoda spp. maintains aerobic respiration to survive the long
periods of exposure to air. In the study conducted by Hiong et al. (2004), they
were able to show that species of the mud clam that is the P. expansa, does not encounter lack or even shortage in oxygen
during aerial exposure. They were able to observe that after 17 days of
exposing the clams to air, there was no significant increase in the alanine
contents of within its tissues. Alanine is one of the major end-products of
anaerobic glucose catabolism, which is the anaerobic metabolic pathway that
most bivalves shift into as they close their shells when they get exposed to
air. They also mentioned that the absence of increase in the alanine contents
may be related to the capability of the P.
expansa to maintain normal oxygen utilization rate during emersion. Their
results therefore show that mud clams do not readily shift to anaerobic
respiration but utilize the aerobic respiration instead when they are exposed
to air.

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            Marine
bivalves as mentioned earlier are often the dominant inhabitants of mangrove
ecosystems characterized by the muddy environment. In relation to their
habitat, most of these marine bivalves are either deposit feeders or suspension
feeders (Lopez, 1988) which may also be called as a filter feeder. According to
Argente et al. (2014), the mud clam P.
erosa is a filter feeder, which is also termed by others as a suspensivore.
In the study of Lopez (1988), it was reported that the P. erosa filters particles of subterranean or underground waters
specially those that remains in the burrows of burrowing animals including
crabs at low tides. It has its bio-filter mechanism that is carried out by
their gills. It therefore has the ability to remove suspended particles from
the water column in which the outcome is the formation of biodeposits. These biodeposits
are mucus-coated particles that become the source of nutrients needed by the
mangrove clam. Assimilation or the processing of these biodeposits are
accomplished within the mantle cavity and after which, ejected in the siphon or
along the ventral mantle margin. Feeding of the mud clam only takes place
during the spring tide (Clemente et al., 2013). This is in relation to its
habit of living towards the landward area in the highest high tide level region
causing the mud clam to be left sometimes without food.

            Mud
clams, also, are economically important species. According to Carter (2014),
the genus Polymesoda is one of the
main sources in the Indo-Pacific region that contributes to marine resource
economies. This therefore means that a large fraction of marine resources is
greatly attributed to the mangrove clams. They are highly exploited by
fishermen in Indonesia (Nuryanto and Susanto, 2010), in India (Clemente and
Ingole, 2011) and even in the Philippines (Dolorosa and Dangan-Galon, 2014a)
and is commonly sold in local market stalls as they are being utilized as a
food resource especially by communities near coastal areas. Moreover, the mud
clams have considerably large size which implies a more fleshy body (Nuryanto
and Susanto, 2010) making them more suitable for consumption. Moreover, mud
clams are also reported to contain high amounts of protein (Nasution and
Zulkifli, 2014). According to Cremades (2014), seafood that mainly includes
fishes, shrimps and a variety of many other invertebrate species such as
shells, serve as one of the major food sources for the native population on the
mangrove coastal zones of Southeast Asia. This is made possible by mangrove
conditions that are highly advantageous that favor the recruitment of a wide
array of edible species. Mangrove inhabitants such as the mangrove clams
therefore grow well under favorable environmental circumstances, which on the
other hand become more beneficial to fishermen and other consumers. Moreover,
the P. erosa has a high potential for
aquaculture and mariculture (Dolorosa and Dangan-Galon, 2014b) which could
greatly help in better production.

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