Chance of survival of young molluscs determined by their size
28-12-2012 Even small differences in body size have a significant impact on the chances of survival of young molluscs. This is the conclusion Ms Henrike Andresen draws in her PhD thesis. Ms Andresen studied predation by crustaceans on bivalve molluscs during their early bottom-dwelling stage. She will defend her thesis at the VU University Amsterdam on 8 January 2013.
Background
The number of bivalve molluscs in the Wadden Sea shows high fluctuations from year to year. These fluctuations mainly arise in the first few months of the molluscs' lives, when they are still tiny and settling in the Wadden Sea floor.
In summers following cold winters the number of surviving molluscs turned out to be larger than after a mild winter. After a cold winter, the number of predators living on small molluscs, such as brown shrimp and shore crabs, is lower and they arrive at the Wadden Sea later in the year. The young molluscs thus get the opportunity to grow larger, and they are therefore less likely to be eaten by these shrimp and crabs.
Consequences of Climate Change
The bivalve molluscs make up a large part of the Wadden Sea benthic life. They are also an important source of food for migratory birds. It seems that as a consequence of climate change, the crustaceans arrive in the Wadden Sea earlier and earlier in the year, which means that the molluscs are more likely to be eaten. This may also have a significant impact on the migratory birds in the Wadden Sea area.
PhD Project
Henrike Andresen's research project comprised an examination of the role of size in this predator-prey relationship. She combined several methods of research.
A mathematical model based on size-dependent predation by shrimp could predict prey-size distribution of young molluscs that largely corresponded to the actual size distribution observed on the mud flats. This implies that predation by shrimp is likely to be the cause of the loss of molluscs.
In a large-scale field experiment in the Dutch and German parts of the Wadden Sea, molluscs in the sea floor were stained with fluorescent dye. In some areas, these were protected from predators by putting cages over them. The markings were used to monitor for the first time the growth of the tiny molluscs. Once the effect of growth on the size distribution was established, the loss of tiny individual molluscs could be demonstrated.
The circumstances for predation were very similar in the year when the field experiments were conducted in the two research areas on the far ends of the Wadden Sea (the Dutch western Wadden Sea and Sylt, Northern Germany). In different years, a single area may show considerable differences between the sizes of molluscs and the moment when shrimp and crabs arrive. Data from other research projects indicate that in some years, the young molluscs are eaten in abundance, while in other years these molluscs are already too big by the time the shrimp and crabs arrive.
The consumption of molluscs of various sizes and different levels of density by shrimp was studied in aquarium experiments. The number of molluscs eaten within a short time was extremely high. Even if density levels were low, the shrimp could easily find their prey. However, molluscs that were a fraction of a millimetre larger ran a considerably smaller risk of being eaten.
Some sub-studies show that it is normal that young molluscs of a certain size are eaten on a large scale. These are mostly the smallest ones. The enormous effect of a tiny difference in size was remarkable.
Henrike Andresen
Henrike Andresen (Hamburg, Germany, 1976) studied Biology in Rostock and Bremen (Germany). Her PhD research project at the Royal Netherlands Institute for Sea Research (NIOZ) was funded by NEBROC (Netherlands-Bremen-Oceanography) and the Netherlands Organisation for Scientific Research (NWO). Part of the experiments were conducted at the Alfred-Wegener Institute on the island of Sylt in Germany. Ms Andresen has now moved to New Zealand for a period of three years.
Photo's:
Experiment with cages in the field.
Henrike Andresen in the lab.