Physical factors affecting the life of shellfishes

In nature, shellfishes are very crucial both in ecological and economical point of view among calcifying species. They play an important role in ecological balance and thus they are termed as engineer of the ecosystem, governing energy and nutrient flows in ecosystems, providing shelters and abodes for many benthic organisms and constituting an important food source for birds, crabs, starfishes, fishes and as well as for dairy and poultry. In Bangladesh, shellfishes have great utility in financial openings through export, ornamentation and food value.

Shellfish filter large volumes of water and trap particulate matter and dissolved substances suspended in the water as a source of food. Therefore, they are totally dependent to their environment as well as environmental parameters also. Consequently, if the environment in which they are grown and its characteristics are fluctuated, then the shellfish may fall in difficulty which will ultimately affect its life and therefore production also. Depending on their distribution and habitats, coastal and especially marine pelagic organisms are experienced in various levels of different parameters fluctuations both seasonally and daily. These parameters have great effect on the life of shellfish. In this feature, it is tried to describe that how are the fluctuation of environmental physical parameters viz. light, temperature, turbidity, depth, water current, substrate affecting the life of shellfish.

Light:

The extent and intensity of day light and artificial lighting at night might have substantial effects on the biology and ecology of molluscans species such as mussels and oysters in the wild.

Blue mussels have light-sensitive eyes and thus, light intensity as a strong limiting factor, may have an effect on their behaviour. Indeed, periods of light and dark have been shown to reduce and increase feeding activity and/or growth in blue mussels respectively (Robson et al., 2010). Furthermore, several scientists have recently been reported to increase their feeding activity at night. Together with reports of a significant increase in the activity of wild subtidal and intertidal molluscs at night, it would appear that molluscs can exhibit higher activity levels during the hours of darkness. Constant illumination at night of shellfish has been found to produce a weak tendency for shell-gape to be greatest during the night time.

Day light may increase phytoplankton production for planktivorous molluscs thus the day light intensity is an important factor limiting their abundance. The production of phytoplankton is driven by photosynthesis and dependent on the availability of specific wavelengths of light passing through the water column. Turbidity reduces light penetration and diminishes photosynthesis. Production of phytoplankton results in removal of dissolved nitrogen and phosphorus from the water column (Robson et al., 2010). Some authors reported that intensity of day light or photoperiod may have pronounced effect on calcareous plankton production.

In shallower areas (<~3m) there is often sufficient light (a region termed the euphotic zone where photosynthetic active radiation can support photosynthesis) to allow benthic microalgae (also termed microphytobenthos) to grow at the sediment water interface. During daylight these microalgae can intercept a large proportion of inorganic nutrients before they are released back up into the water column. These benthic microalgae are a crucial food resource for many mobile and sessile benthic animals or shellfish (Gardner and Maguire, 1998).

As the most of the shellfish are benthic dwellers, therefore, light intensity has great effects on the life history of shellfish. Shellfish are phytoplankton feeder and phytoplankton production is directly related to light intensity. With the increasing of photoperiod, phytoplankton production is increased. It has both positive and negative effects on shellfish development. One- sufficient food will produce with the increasing of light intensity while another one- biological oxygen demand will also increase which ultimately reduce the availability of dissolved oxygen. Not only that, light intensity affects greatly on shellfish during their spawning behavior.

Light intensity also affects on some toxic producing phytoplankton in marine environment which will subsequently affect shellfish population. Growth rate of Protogonyaulax tamarensis decrease with the decrease of light intensity and therefore the amount of toxin production increase concomitantly.

Water temperature:
Water temperature plays an important role on the life history of shellfish in determining their biological reaction. Shellfish have a range of water temperature in which they function best. Outside this range, they do not function as well. The suitable ranges of water temperature of some shellfishes required their reproduction and favourable growth are given below in the table 1.

Table 1: Suitable ranges of water temperature of some shellfishes required their reproduction and favourable growth

Water temperature determines the rate of biochemical reactions especially metabolism (i.e., biochemical breakdown of food to energy). When it increases, biochemical reactions become faster. As shellfish are cold blooded animals (poikilothermic), their body temperature fluctuates with that of the environment and thus their metabolism is directly influenced by water temperature.

Increasing water temperature increases metabolic rate, while decreasing temperatures will decrease metabolic rate, affecting both growth and reproduction of shellfish. At the upper and lower extremes of temperature tolerance, these biochemical processes will cease, resulting in diminished growth, poor health, or death.

Temperature also affects water quality. For example, the solubility of gases decreases with increasing temperature. Therefore, the amount of oxygen dissolved in water decreases by about half as the temperature is raised from 0°C to 30°C. Since oxygen is a requirement for aerobic metabolism, at high temperatures it becomes a challenge for shellfish to obtain sufficient quantities.

Water turbidity:
Turbidity is often used as a general term to describe the lack of transparency or “cloudiness” of water due to the presence of suspended and colloidal materials such as clay, silt, finely divided organic and inorganic matter, and plankton or other microscopic organisms. Water turbidity has great effect on shellfish as sensitive or threatened shellfish species may be lost when turbidity exceeds 100 ppm.

Dissolved oxygen (DO) levels also decline when turbidity is caused by organic particles due to natural degradation of the materials by microbial populations. Many microorganisms use organic carbon as a source of energy for respiration and consume oxygen in the process. Additionally, nutrients often leach from decaying organic materials into surface water. These nutrients may contribute to over-stimulation of algal growth and production (algal blooms). Increased levels of algae are often associated with very low concentrations of DO during dark periods (especially at dawn) due to respiration, often resulting in extensive shellfish kills. Turbid waters with a high load of mud and silt are tolerated by Anadra, but not by Mytilus and Pinctada.

Water depth:
The optimal depth of water depends on the requirements of the specific species of shellfish and the method of culture to be applied. Thus, for the culture of Anadra species the preferred areas are the zero tide levels, while Crassostrea species are better suited in areas exposed during low tide for few minutes or hours.

Mytilus viridis has a wider range of vertical distribution. Mytilus smaragdinus settles on a 10 m bamboo stake from the low tide mark down to the seabed. For Pinctada species, deeper waters are essential for optimal growth in the production of pearl. In the case of P. maxima an optimal water depth of 30–40 m is required.

Water current:
Water current is another important factor to be considered in shellfish farming. Being filter feeders, shellfishes thrive best in areas where the availability of food is continuously replenished through the intrusion of water currents and daily tidal water transport. Also, the supply of oxygen required by the shellfish and the elimination of its metabolic wastes can also be enhanced with the water current.

Substrates:
The type and nature of substrate determine the methods of culture to be applied. For clams, since they are grown directly on the seabed, this becomes even more important. Muddy or sandy-mud substrates should be selected and not hard or rocky seabed.

For the culture of species of Mytilus, Crassostrea and Pinctada, the nature of the substrate will determine the method of culture to be applied. Thus, for the construction of bamboo plots a muddy bottom is essential, while floating rafts or suspension lines can be sited over any form of substrate.

In next feature, different types of chemical factors affecting the life of shellfishes will be described. The most important chemical factors are salinity, pH, dissolved oxygen etc.

Key References and further readings:

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