GeoLog

bivalve

Geosciences Column: Dating a bivalve

Just as the rings on a tree can be used to determine its age, the bands on a bivalve’s shell can tell us the how long it’s been around for. Warm, food-filled waters lead to greater growth in the summer and low plankton abundance (the principle food source for filter-feeding molluscs) leads to limited growth during the winter months – hence the banding. But pinning down the age of a bivalve may not be a simple matter of counting bands as sudden changes in temperature and food availability throughout the year could cause multiple bands form within a single year.

This is where a little isotope geochemisty comes in. Oxygen naturally occurs as three sable isotopes: 16O, 17O and 18O and the ratio of these isotopes, particularly that of 18O/16O (known as δ 18O) reflects the temperature of the environment. Since 18O is heavier than 16O (by two neutrons), more energy is required to vaporise water containing 18O than 16O. So in the summer, when there’s more energy for evaporation, more water made up of 18O is evaporated and this leaves less 18O in the water (low δ 18O). The converse is true during winter, when there is little energy available for vaporisation and more water containing 16O is evaporated. In short, high δ 18O indicates low temperatures, and low δ 18O indicates high temperatures.

Since bivalves obtain the materials needed for their shells from the surrounding water, their oxygen isotope ratio reflects that of the water column. The seasonal cycling of δ 18O makes this a great way of verifying the bivalve’s age and checking whether we can trust the information gained from band-counting. This is what a team of geochemists led by Joana Cardoso set out to find: by counting the lines on a shell, can you get a reliable estimate of age?

This is a razor clam (Ensis directus).The white arrows point out the annual growth lines. (Credit: Cardoso et al., 2013)

This is a razor clam (Ensis directus). The white arrows point out the annual growth lines (click for larger). (Credit: Cardoso et al., 2013)

To start the investigation, you need to picture a line along the shell from the edge to the umbo (the most primitive part of the shell, where the two valves join). You can then take sections along this line and analyse the isotope ratios within them to see how δ 18O varies over time. The downside: as you approach the younger end of the shell, there is less and less carbonate available for sampling, to the point where there’s not enough material to do isotope analysis. Not to worry, you can combat this by pooling the carbonate sampled from neighbouring sections. Okay, so your data has slightly less resolution for the early stages of bivalve growth, but you can sample all parts of the shell – problem solved!

Cardoso’s results, published in Biogeosciences, found that δ 18O is an especially good measure of age and not only that, it matches the results you get for growth lines on both the inside and the outside of the shell!

Measured and predicted values for δ 18O in the shell. Carbonate is deposited in the summer and as the temperature rises, the ratio of 18O to 16O decreases, hence the troughs in measured 18O in the graph (white circles). (Credit: Cardoso et al., 2013)

Measured and predicted values for delta (δ) oxygen-18 in the shell. Carbonate is deposited in the summer and as the temperature rises, the ratio of oxygen-18 to oxygen-16 decreases, hence the troughs in measured delta oxygen-18 in the graph (white circles). (Credit: Cardoso et al., 2013)

The annual growth lines appear at the start of each growing season, in early summer. These coincide with the drop in δ 18O (remember this corresponds to a temperature rise). Carbonate that’s deposited from June to September (when there is most growth) provides the most detailed δ 18O data. The absence of data during the winter is because there is little or no growth – if no carbonate is deposited then there are no isotopes to analyse!

So, if you’re looking to age bivalves – count the rings – it’s still a sturdy method, and if you’re looking for more detailed data about both age and environmental conditions, oxygen isotope analysis is what you need.

By Sara Mynott, EGU Communications Officer

Reference:

Cardoso, J. F. M. F., Nieuwland, G., Witbaard, R., van der Veer, H. W., and Machado, J. P.: Growth increment periodicity in the shell of the razor clam Ensis directus using stable isotopes as a method to validate age, Biogeosciences, 10, 2013.

Geosciences Column: Larvae, Climate and Calcification

The absorption of atmospheric CO2 by the oceans results in a decline in ocean pH, hence ‘ocean acidification’, and reduces the availability of carbonate. This presents a problem to calcifying organisms (those that deposit calcium as either calcite or aragonite as hard parts) because they cannot produce their shells, valves (in the case of bivalves), or tests (in the case of diatoms) as readily.

To explain this, we need a little chemistry. When CO2 dissolves, it combines with water to form carbonic acid (H2CO3). This then breaks down to form bicarbonate (HCO3) when one hydrogen ion is lost, and then carbonate (CO32-) as the other hydrogen ion is lost. This carbonate is the important stuff, as it combines with calcium to form the calcium carbonate (CaCO3) used by bivalves to produce shells. If something (such as the ocean) is more acidic, there must be more hydrogen ions available. These hydrogen ions interfere with the calcification process as they bond with carbonate, meaning there is less available for shell formation.

Calcification: carbonate chemistry in action!

This process is relatively well established for a number of calcifying organisms, although there are exceptions to (the coccolith, Emiliania huxleyi, for example) and the response to elevated CO2 levels is not uniform across species.

Much of current research has focussed on the effect of constant CO2 levels on calcification, but what about animals that live in environments where the CO2 concentration is constantly changing? The availability of carbonate in estuaries is particularly variable as CO2 concentrations vary seasonally (there’s a greater carbon load in the winter as storms wash nutrients into rivers), diurnally and with the tide. The impact of elevated CO2 levels on an organism is also dependant on its life stage; something that is particularly true of bivalves.

Bivalve larvae. Photo credit: Minami Himemiya (source).

Bivalves spend the first part of their life in the plankton, first as a veliger (a relatively amorphous looking ciliated blob) and then as a pediveliger (that same blob, but this time with an identifiable foot) before metamorphosing into a miniature adult. During these larval stages, they are particularly vulnerable to ocean acidification and, until recently, both the reasons behind this, and the longer-term implications of this vulnerability, were unclear.

This is where doctors Christopher Gobler and Stephanie Talmage come in. They took to the lab to tackle why larvae are especially vulnerable to acidification and what this means for them in both the short and long term. It’s impossible to take a look at how all bivalves respond to acidification, though, so to tackle these questions, two bivalve species, the hard-shelled clam (Mercenaria mercenaria) and the Atlantic bay scallop (Argopecten irradians) joined the team.

The Atlantic bay scallop, Argopecten irradians. Photo credit: Rachael Norris and Marina Freudzon (source).

Using their RNA:DNA ratio as a proxy for growth and the uptake of a radioactive calcium isotope, 45Ca, to estimate calcification, Gobler and Talmage found that growth in the presence of elevated CO2 results in individuals of a smaller size. This is because there is less calcium available for uptake. Their findings, revealed that high CO2 concentrations, not only affected size, but also negatively impacted bivalve physiology, as individuals reared in these conditions were found to have thinner shells. Shells are an important defence against predators and the reduction in shell thickness (and hence strength) may put them at greater risk from predation.

The higher the CO2, the slower the calcium uptake: calcium uptake rates of larval Atlantic bay scallop, Argopecten irradians, under different CO2 concentrations over a 12-hour period (Gobler and Talmage, 2013).

When transferred from a high CO2 environment to an environment with an ambient CO2 concentration, larvae grew faster than those in ambient conditions throughout the whole of their development. However, this higher growth rate doesn’t compensate for the low calcification rate during larval stages, as their final is still smaller than individuals reared in ambient conditions at all life stages. This “legacy effect” presents a significant problem for adult bivalves, due to the detrimental impact of reduced calcification on their defences.

By Sara Mynott, EGU Communications Officer

Reference:

Gobler, C. J. and Talmage, S. C.: Short- and long-term consequences of larval stage exposure to constantly and ephemerally elevated carbon dioxide for marine bivalve populations, Biogeosciences, 10, 2241-2253, doi:10.5194/bg-10-2241-2013, 2013.