To feed or not to feed?

Hey guys, I was very happy to recently publish my first paper, in a journal named Marine Ecology Progress Series which is quite popular and influential in the marine science community. The paper was all about arrow worms, a group I’ve been working on for a good few years now. These animals, also called chaetognaths, are gelatinous worms that live all over the oceans. If you look at a net-caught plankton sample, you usually find loads of them inside, and they are more exciting than you may think! Let me convince you why! 🙂

Arrow worms extracted from seawater.
Arrow worms extracted from Arctic seawater.

Arrow worms are really obvious when you look at them through the glass jar containing your plankton sample in formalin (the usual preservative), as they turn white or yellow after they die. When they’re alive however, they are translucent or transparent. This is just one reason why they could be effective and sneaky predators on their favorite prey, copepods. There are a host of other reasons too, including that they don’t need light to detect their prey – they can do that by sensing vibrations, just like how spiders detect flies in their web. They also seem to hover in the water column (maybe in large numbers just above the seabed), meaning they can wait for prey to come along without expending too much energy on finding or chasing it. And… whilst they seem to prefer copepods, they also eat lots of other animals including krill, fish larvae and even other arrow worms! Nasty huh! So we would expect them to always find prey right, and if you can get it you’d eat it right! Well, the reality may be a little different.

Head of the (supposedly) fearsome predator! Full of teeth and hooks for grabbing prey!
Head of the (supposedly) fearsome predator! Full of teeth and hooks for grabbing prey! Reproduced from Bieri & Thuesen (1990).

Weirdly when we look at their guts under a microscope, very few arrow worms seem to contain prey (or even prey remains). This may tell us something about how they live, but it could also be related to how we sample them (nets could stress them out so much that they regurgitate their guts, not a nice thought). One very common arrow worm species living in the Arctic (Eukrohnia hamata) has a vacuole in the middle of its body that is packed with oil. Lots of animals, including seals, polar bears and… us… can store fat from food, and live off it when times get tough and food is hard to come by. Maybe arrow worms feed rarely on prey, contrary to what we might expect from them given their similarities to tigers (ambush mode of feeding, a head full of sharp teeth), and snakes (their shape, their ability to inject poison into their prey)! Or maybe they feed lots at one time of year, and like other animals, including visual feeders, live off their fat supplies during winter. Certainly, we seem to find most prey in their stomachs in summer, and much less in winter. In my last paper, we also showed that arrow worms grew a lot more in summer than winter, which would also indicate seasonal feeding.

An Arctic arrow worm (species: Parasagitta elegans) with a big copepod in its gut. This is a rare finding, the prey must have been consumed just a few hours before capture.
An Arctic arrow worm (species: Parasagitta elegans) with a big copepod in its gut. This is a rare finding, the prey must have been consumed just a few hours before capture. Photo taken in our lab at Laval.
The Arctic chaetognath Eukrohnia hamata contains an oil vacuole right in the middle of its body.
Another Arctic chaetognath Eukrohnia hamata contains an oil vacuole right in the middle of its body. Reproduced from Pond (2012).

Maybe the amount of feeding going on in arrow worms (and what they eat too) depends also on their life stage (how old they are). In our paper, we showed that small arrow worms were most common in near-surface waters during summer – copepods at sizes they could eat were most common there too! Larger worms were in deeper waters – were they there to feed, or just resting in darkly lit waters where their chances of being eaten by predators may be slim to none?

Many zooplankton may pass parts of the year at depths of a few hundred metres, where they may avoid fish predators, like this cod, and gain other benefits too (it's quite a stable environment).
Many zooplankton spend parts of the year at depths of a few hundred metres, where they may avoid fish predators, like this cod, and gain other benefits too. Cold, deep waters offer a stable environment for resting. Reproduced from: http://nature/ca/notebooks/english/atlanticcod_p5.htm

I now want to understand the costs and benefits of different survival strategies in these animals. To eat or to rest, that is the question! When an individual feeds, should it take one big copepod or a few smaller ones, and if some arrow worms live off stored oil supplies, how long can they go before they need to replenish their supplies with a new meal?

I think you’ll enjoy reading the paper. We were very fortunate to be able to collect chaetognath samples every month during a whole year (2008-2009) in Svalbard. With annual time series, we can describe exactly what an animal does between one “birthday” and the next. Our paper documents the timing of reproduction, growth and migrations in the arrow worm Parasagitta elegans. By starting to take these measurements now, we can also document the responses of these interesting animals (and others) to the environmental changes that will rock the Arctic in the years to come. 

It's all change in the Arctic, and all its animals will respond in some way or other...
It’s all change in the Arctic, and all of its animals will respond in some way or another…                   Reproduced from: http://www.redorbit.com/news/science/1112418510/arctic-sea-ice-could-melt-away-by-2015

Check out Grigor et al. (2014) here. Cheers, j 😉

References:

Bieri R, Thuesen EV (1990) The strange worm Bathybelos. American Scientist 78:542-549

Pond D (2012) The physical properties of lipids and their role in controlling the distribution of zooplankton in the oceans. Journal of Plankton Research 34:443-453

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