Chemical factors affecting the life of shellfishes

In previous feature, different kinds of physical factors affecting the life of shellfishes were described. The most important physical factors are light, temperature, turbidity, depth, water current, substrate etc. In this feature, different types of chemical factors (viz. salinity, pH and dissolved oxygen) affecting the life of shellfish will be explained.

 

Salinity:

Salinity is a major environmental factor determining the distribution of bivalve molluscs. Changes in salinity bring about a broad range of physiologic responses in bivalves and have been shown to influence filtration rate, oxygen consumption, electrolyte balance and the rate of particle transport over the gills. Clearly, salinity may have a major impact on growth and survival of cultured bivalves.

Most species in temperate waters show seasonal restricted period of spawning. In tropical conditions, spawning in bivalves is influenced by the changes in salinity over the animal beds. Naghabhushanam and Mane (1975) reported that increase in salinity initiated spawning in Crassostrea madrasensis from the east coast, and Merefrix meretrix and Kafelysia opima from the southwest coast respectively.

Salinity profoundly influenced the growth and breeding of clams, M. merefrix and K. opima (Ranade, 1973). Shau-Hwai Tan (1997) reported that larvae of Perna viridis could thrive in 16-30 ppt. Similarly, responses were reported for the species Mytilus viridis and in Myfilus edulis. It was reported that a salinity range of 28-35ppt is good for normal development of larvae of Mytilus californianus (Young, 1941).

Low salinity negatively affected mechanical properties of shells of the juveniles, resulting in reduced hardness and fracture resistance. Therefore fluctuating salinity may jeopardize the survival of bivalves (especially oysters) because of weakening of their shells and increased energy consumption and making them more prone to predators, parasites and other mechanical damages. Brackish waters have lower alkalinity and less buffering capacity compared with open ocean waters, leading to lower pH of the brackish waters. Low salinity also results in major changes in water chemistry, such as reduced Ca²⁺ concentrations and total inorganic carbon (Hofmann et al., 2009), which– in conjunction with changes in alkalinity, buffering capacity and pH– may affect metabolism and bio-mineralization in marine calcifiers.

The favourable salinity range for the growth and development of some marine algae is 28-30 ppt and these algae have great food value for some marine molluscans. These algae (2-3 µm) were easily filtered and consumed by molluscans larvae for their development. It was observed that 90% algal cells were consumed in salinity 30 ppt and only reduction occurred in low salinities. Some scientists were also reported algal death due to low or high salinity which will ultimately destroy the life of shellfish and decline overall production

Salinity or the total quantity of dissolved salts in water is a useful indicator of a water body. Salinity is a major environmental factor determining the distribution of bivalve molluscs. Changes in salinity bring about a broad range of physiologic responses in bivalves and have been shown to influence filtration rate, oxygen consumption, electrolyte balance and the rate of particle transport over the gills. Clearly, salinity may have a major impact on growth and survival of cultured bivalves.

The range of salinity tolerance for shellfishes differs from species to species. Anadara species, for example, can tolerate low salinity from 0.5 ppt during low tide to as high as 35 ppt during high tide. Species of Crassostrea are capable of closing their shells for days to avoid fresh water during the flood season, but the same is not true with Mytilus. Pinctada species prefer a consistently high range of salinity, and are readily affected by the lowering of salinity.

Table 1: Favourable salinity (ppt) range of different shellfish organisms

Phylum/Class	Group	Representative species	Favourable salinity range (ppt)
Crustacea	Prawn	Macrobrachium rosenbergii	0 – 2
	Shrimp	Penaeus monodon	10 – 25
	Lobster	Panulirus polyphagus	30 – 35
	Crab	Scylla serrata	0 – 6
Mollusca	Gastropod	Pila globosa	0 – 2
	Bivalve	Crassostrea sp.	27 – 33
	Mussel	Lamellidens sp.	10 – 12
	Clam	Anadara sp.	0.5 – 35
Cephalopoda	Cuttlefish	Sepia sp.	25 – 35
	Squid	Loligo sp.	25 – 35
	Octopus	Octopus sp.	32 – 38

pH:

pH (potential hydrogen or power of hydrogen) is a measure of the concentration of hydrogen ions (H+) in water or other liquids. In general, waterbody should have a pH between 6.5 and 8.5. Water with pH below 7.0 is termed “acidic” and water with pH above 7.0 is termed “basic” whereas pH 7.0 is “neutral”.

Marine calcifying organisms (such as mollusks, echinoderms and corals) that build calcium carbonate (CaCO₃) skeletons are susceptible to changes in seawater carbonate chemistry because both biomineralization and CaCO₃ dissolution can be directly affected by reduced pH (Kleypas et al., 2006). Early life stages of calcifying organisms are generally considered to be more sensitive to environmental disturbances (Raven et al., 2005).

Increased atmospheric CO₂ and reduced ocean pH could impact fish and shellfish (reef building organisms, some calcareous plankton, molluscs, crustaceans and echinoderms) through two distinct pathways:

• direct physiological stress manifested as reduced rates of growth and survival through a disruption of inter-cellular transport mechanisms; and
• indirectly through reduced abundance of calcareous plankton due to the decreasing availability of carbonate ions (CO₃²⁻) driven by increasing pCO₂ (partial pressure of atmospheric CO₂) that are prey for fish and shellfish

 

Calcite and aragonite have been identified as the main CaCO₃ mineralization form in molluscs larval stages (Medakovic, 2000). As aragonite is 50% more soluble than calcite, these aragonitic larval stages (corals, pteropods) are expected to be more sensitive to ocean acidification than calcitic organisms (oraminifera, coccolithophores, crustaceans, echinoderms). Molluscs and echinoderms larvae produce mainly calcite calcium carbonate during the first 2–3 days of development and aragonite in the following days. As the solubility of calcite calcium carbonate is 30 greater than that of aragonite, early larval stages should be much more vulnerable than older larval stages and adults that precipitate aragonite and calcite.

In acidified seawater, an increase in energy consumption required for carbonate sequestration and mineral deposition may incur a significant energy cost to marine calcifiers. This may result in trade-offs of limited energy resources between different biological processes, including homeostasis, growth, reproduction, development and biomineralization (Sokolova et al., 2011).

About one third of anthropogenic CO₂ emission (from fossil fuel, cement production and land-use changes) has been stored in the oceans since the industrial revolution (Sabine et al., 2004). Ocean acidification (OA) is expected to reduce ocean pH levels by 0.2 to 0.5 units, from present levels of 8.0, by 2100. It may have profound impacts on marine biota. Brackish waters can experience large fluctuations in seawater pH and carbonate chemistry compared with open ocean waters with higher salinity because of a lower buffering capacity, acidic inputs from land-based sources, and biological CO₂ production (Pritchard, 1967; Burnett, 1997; Ringwood and Keppler, 2002).

Crustaceans require calcium and bicarbonate ions for the mineralization of their exoskeleton after molting and may, therefore, be particularly vulnerable to ocean acidification. Fertilization rates of sea urchins and egg production rates of copepods decreased and nauplius mortality rate of copepods increased with decreased pH. Although the calcification process in crabs is poorly understood, a significant decrease in pH or undersaturation of calcite will likely influence the growth and morphology of early life stages and the reproduction of crab.

Most of the marine algal species grew at pH range of 6.5-8.0 (Kinne, 1971). Davis (2000) indicated the possibilities of algal death due to pH variations therefore it would be a great vulnerability for the life of shellfish.

Although recent experiments (Ries et al., 2009) have suggested positive effects on calcification rates in some cases in low pH levels, a negative impact of ocean acidification on the growth of these species would, therefore, not only have major consequences for coastal biodiversity and ecosystem functioning and services, but will also cause a significant economic loss.

Marine calcifying organisms (such as mollusks, echinoderms and corals) that build calcium carbonate (CaCO₃) skeletons are susceptible to changes in pH because both biomineralization and CaCO₃ dissolution can be directly affected by reduced pH (Kleypas et al., 2006).

Table 2: pH requirement of different shellfish organisms

Phyllum/Class	Group	Representative species	Favourable pH range
Crustacea	Prawn	Macrobrachium rosenbergii	7.00 – 8.00
	Shrimp	Penaeus monodon	7.40 – 8.50
	Lobster	Panulirus polyphagus	6.50 – 8.50
	Crab	Scylla serrata	7.30 – 8.00
Mollusca	Gastropod	Pila globosa	7.00 – 8.00
	Bivalve	Crassostrea sp.	7.00 – 8.50
	Mussel	Lamellidens sp.	7.30 – 7.50
Cephalopoda	Octopus	Octopus sp.	7.50 – 8.00

 

Dissolved oxygen (DO):

In the natural environment, it may be difficult to identify oxygen stress in shellfish. Shellfish (mainly bivalves) have the ability to close their valves in response to hypoxic or anoxic conditions and can keep their valves closed for several days. Long-term responses may include gaping of the valves.

Signs of adverse environmental conditions in juvenile or adult shellfish may go unnoticed because they live buried in the sediment. However, stressed shellfish (especially clams) may rise to the surface of the sediment or fail to bury. These signs are not necessarily specific indications of oxygen stress; they may also be associated with infectious or noninfectious diseases or other adverse environmental conditions such as high temperature and low salinity.

Dissolved oxygen concentrations below 4 milligrams per liter (mg/l) are considered to be unhealthy for many aquatic community inhabitants. When the level of dissolved oxygen falls to 2 mg/l, severe physiological stress to marine organisms occurs and death may result. The dissolved oxygen requirement of different shellfish organisms are listed in table 3:

Table 3: Dissolved oxygen requirement of different shellfish organisms

Phyllum/Class	Group	Representative species	Favourable dissolved oxygen range (mg/l)
Crustacea	Prawn	Macrobrachium rosenbergii	5.00 – 7.00
	Shrimp	Penaeus monodon	4.25 – 8.25
	Lobster	Panulirus polyphagus	5.00 – 8.00
	Crab	Scylla serrata	4.00 – 6.00
Mollusca	Gastropod	Pila globosa	5.00 – 6.00
	Bivalve	Crassostrea sp.	4.00 – 6.00
	Mussel	Lamellidens sp.	5.00 – 6.00
	Clam	Anadara sp.	4.00 – 7.00
Cephalopoda	Cuttlefish	Sepia sp.	5.00 – 8.00
	Squid	Loligo sp.	6.00 – 8.00
	Octopus	Octopus sp.	5.00 – 8.00

 

Conclusion:

Shellfish have a low capacity to compensate for disturbances in ion and acid–base status induced by changes in seawater pH and salinity, and their metabolism is sensitive to disturbances in extracellular and intracellular pH. In molluscs, changes in environmental salinity directly translate into changes in intracellular osmolarity. Thus, salinity and pH stress, alone and in combination, can strongly affect metabolism and biomineralization in these organisms. A negative effect on early life stages may not only be detrimental to recruitment and endanger the species survival, but also result in economic loss due to a collapse in global shellfish aquaculture production. Other factors affecting the life of shellfish include temperature, current, anthropogenic-atmospheric CO₂, dissolved oxygen, anthropogenic disturbance, predation, pollution etc.

 

Key References and further readings:

• Burnett, L. E. 1997. The challenges of living in hypoxic and hypercapnic aquatic environments. Am. Zool. 37: 633-640.
• Costa-Pierce, A., S.R. Malecha and E.A. Laws, 1984. Effects of polyculture and manure fertilization on water quality and heterotrophic productivity in Macrobrachium rosenbergii ponds. Trans. Am. Fish. Soc., 114: 826-836.
• Davassi, L. A. 2011. Survival and Growth of the Freshwater Prawn Macrobrachium rosenbergii in Relation to Different Nutrients Composition. Journal of Fisheries and Aquatic Science, 6: 649-654.
• Davis, M. 2000. The combined effects of temperature and salinity on growth, development and survival for the tropical gastropod veligers of Strombus gigas. I. Shellfish Res., 19(2): 2883-2889.
• Gardner, C. and Maguire, G. B. 1998. Effect of photoperiod and light intensity on survival, development and cannibalism of larvae of the Australian giant crab Pseudocarcinus gigas (Lamarck), Aquaculture, 165(1–2): 51–63.
• Hofmann, A. F., Middelburg, J. J., Soetaert, K. and Meysman, F. J. R. 2009. pH modelling in aquatic systems with time-variable acid-base dissociation constants applied to the turbid, tidal Scheldt estuary. Biogeosciences, 6: 1539-1561.
• Islam, M. J. and Bhuiyan, A. L. 1982. Temperature tolerance and its impact on the distribution of mud crab in the Karnafully river estuary, B. J. of Agriculture, 7: 38-46.
• Kinne, O. 1971. Salinity: animals: invertebrates. In Marine Ecology, Vol. 1 (ed. O. Kinne), pp. 821-996. New York: Wiley.
• Kleypas, J. A., Feely, R. A., Fabry, V. J., Langdon, C., Sabine, C. L. and Robbins, L. L. 2006. Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers. A Guide for Future Research. Report of a workshop held 18-20 April 2005, St Petersburg, FL, sponsored by NSF, NOAA and the US Geological Survey, 88 pp.
• Kleypas, J. A., Feely, R. A., Fabry, V. J., Langdon, C., Sabine, C. L. and Robbins, L. L. 2006. Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers. A Guide for Future Research. Report of a workshop held 18-20 April 2005, St Petersburg, FL, sponsored by NSF, NOAA and the US Geological Survey, 88 pp.
• Medakovic, D. 2000. Carbonic anhydrase activity and biomineralization process in embryos, larvae and adult blue mussels Mytilus edulis L., Helgoland Mar. Res., 54: 1–6.
• Nagabhushanam, R and Mane, U. H. 1975. Reproduction and breeding of the clam, Katelysia opima in the Kalbadevi estuary at Ratnagiri, on the West Coast of India. Indian I. Mar. Sci., 4: 86-92.
• Pechenik, J. A., Eyster, L. S., Widdows, J., and Bayne, B. L. 1990. The influence of food concentration and temperature on growth and morphological differentiation of blue mussel Mytilus edulis L. larvae, J. Exp. Mar. Biol. Ecol., 136: 47–64.
• Pritchard, D. W. 1967. What is an estuary: physical viewpoint. In Estuaries (ed. G. H. Lauff), pp. 3-5. Washington, DC: American Association for the Advancement of Science.
• Rahman, M.M., Corteel, M., Alday-Sanz, V., Pensaert, M. B., Sorgeloos, P. and  Nauwynck, H. J. 2007. The effect of raising water temperature to 33 °C in Penaeus vannamei juveniles at different stages of infection with white spot syndrome virus (WSSV), Aquaculture 272: 240–245.
• Raven, J., Caldeira, K., Elderfield, H., Hoegh-Guldberg, O., Liss, P., Riebesell, U., Shepherd, J., Turley, C., and Watson, A.  2005. Ocean acidification due to increasing atmospheric carbon-dioxide, Policy document 12/05, www.royalsoc.ac.uk, The Royal Society, UK.
• Renade, M. R. 1973. Effects of temperature and salinity on the oxygen consumption in clams. I. Bombay nut. Hist. Soc., 20 (1): 128-146.
• Renade, M. R. 1973. Effects of temperature and salinity on the oxygen consumption in clams. I. Bombay nut. Hist. Soc., 20 (1): 128-146.
• Rice, M.A. and J.A. Pechenik. 1992. A review of the factors influencing the growth of the northern quahog, Mercenaria mercenaria (Linnaeus, 1758). J. Shellfish Res. 11: 279-287.
• Ries, J. B., Cohen, A. L. and McCorkle, D. C. 2009. Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology, 37: 1131-1134.
• Ringwood, A. H. and Keppler, C. J. 2002. Water quality variation and clam growth: is pH really a non-tissue in estuaries. Estuaries, 25: 901-907.
• Robson, A. A., Leaniz, G. C., Wilson, R. P. and Halsey, L. G. 2010. Effect of anthropogenic feeding regimeson activity rhythms of laboratory mussels exposed to natural light. Hydrobiologia, 655: 197-204.
• Sabine, C. L., Feely, R. A., Gruber, N. 2004. The oceanic sink for anthropogenic CO₂, Science, 305: 367–371.
• Shau-Hwai Tan. 1997. Effect of salinity on hatching, larval growth and survival in the green mussel Perna viridis (Linnaeus). Phuket Marine Biological Center, Special Publication. 17 (1): 279-284.
• Shau-Hwai Tan. 1997. Effect of salinity on hatching, larval growth and survival in the green mussel Perna viridis (Linnaeus). Phuket Marine Biological Center, Special Publication. 17 (1): 279-284.
• Sokolova, I. M., Sukhotin, A. A. and Lannig, G. 2011. Stress effects on metabolism and energy budgets in mollusks. In Oxidative Stress in Aquatic Ecosystems (ed. D. Abele, J. P. Vazquez-Medina and T. Zenteno-Savin), pp. 263-280. Boston: Wiley Blackwell.
• Sprung, M. 1984. Physiological energetics of mussel larvae (Mytilus edulis). I. Shell growth and biomass, Mar. Ecol. Prog. Ser., 17: 283–293.
• Wilber, C.G. 1983. Turbidity in the Aquatic Environment: An Environmental Factor in Fresh and Oceanic Waters. Charles C. Thomas Publishers, Springfield, IL.
• Young, R. T. 1941. The distribution of the mussel (Mytilus californianus) in relation to the salinity of its environment. Ecology, 22: 379-386.
• Young, R. T. 1941. The distribution of the mussel (Mytilus californianus) in relation to the salinity of its environment. Ecology, 22: 379-386.

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