So for part of my PhD I would like to look at sunfish diet. As I mentioned in my last post, the ocean sunfish has often been described as an inactive, lazy sort of fish which happily eats only jellyfish (called obligate predation). However, new research has suggested that these long held beliefs need significant revision!
Only recently, a new study revealed evidence that juvenile sunfish are also feeding from the seabed known as benthivory (Syväranta et al., 2012) and this PhD aims to examine how and why ocean sunfish might change their diet as they grow.
You might well ask:
Answer: Yes it is true we really can discover what wild sunfish are feeding on without needing to see or follow them(!), using a technique called stable isotope analysis. Time for a tiny bit of chemistry (don’t worry this is far better than any chemistry you remember from school!)
The term “isotope” is used to describe when there is more than one form of the same chemical element (same number of protons, different numbers of neutrons). This means the two (or more) isotope forms will react in almost identical ways but can still be identified. The different numbers of neutrons results in a tiny change in mass and this means isotopes of the same element react differently during chemical reactions. Lighter isotopes require less energy to react than heavier isotopes.
Isotopes of different elements can be used to describe different aspect of animals’ diets:
-Nitrogen isotopes can help us assign animals to a trophic level (i.e. position in the food chain such as producer, herbivore, carnivore etc.). This is because Nitrogen isotopes become enriched as an animal move up the food chain. Animals preferentially retain the heavier isotope and excrete the lighter isotope. This helps us to understand where an animal belongs and what it is feeding on within the local ecosystem.
-Carbon isotopes remain relatively unchanged by trophic level, but they vary due to biogeochemical factors in the local environment. Carbon isotopes become heavier offshore to nearshore, pelagic (open water) to benthic (sea floor) and from higher to mid latitudes. This can help locate where an animal has been swimming or feeding.
Using a combination of different isotopes we can construct “signatures” of each food item and then with some clever computer modelling trickery we can estimate what the sunfish have been eating!
All these food items found in sunfish stomachs*
(*maybe not the burger…)
We can even calculate the relative amounts of each species they have eaten, what trophic level they occupy and if they change diets, the time since this occurred. Clever stuff!!
If you would like to read more about stable isotope analysis and how this relates to sunfish try these references:
Syväranta et al. J Fish Biol 80 225-31, 2012
Xia et al. Aqua En In 3 177-86, 2013
Anderson Aust Ecol 26 32-46, 2011
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