Ready or Not, Here I Come!

Hide and seek: one of the most popular children’s games. What makes it so fun? For me, the most exciting part comes in the moment leading up to the final second of the countdown, right before the search begins. However, for snails, their lives become at risk as the predator begins its search. Like the seekers in hide and seek, sea stars also search for prey such as snails. Whether the snails are ready or not, hungry sea stars will find their way towards them.

How does sea star predation on snails affect other species’ abundance? Using algae to investigate these indirect interactions, we created 4 food webs:

  • Algae only
  • Algae and snail
  • Algae, snail and sea star
  • Algae and snail in water with crab chemical cues

Indirect interactions occur when a third species mediates the interaction between two species. In our case, sea stars act as the mediator because their predation influences the abundance of snails. For example, with more sea stars preying on snails, the snail population will deplete, ultimately increasing the amount of algae available due to fewer herbivores consuming it. This is an example of a trophic cascade, a phenomenon where the top predator has an indirect effect on lower trophic levels that are not immediately below it in the food chain. Contrastingly, direct interactions occur between two species without mediating species. For example, sea stars directly influence snail abundance and snails directly influence algae abundance.

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Indirect and direct interactions between algae, snail and sea star community (Photo credit: Prabh Sahota)

To further understand the direct interaction of predation between sea stars and snails, we should consider the mechanisms involved in sensing prey. How do sea stars detect the presence of the snail in the tank? One way is by detecting their prey’s odor and moving their legs in that direction. In the lab, I was lucky enough to witness sea star predation! The sea star slowly approached the snail and eventually moved itself on top of it. Why? Because unlike most invertebrates with mouths, a sea star’s mouth is on its lower surface! Unfortunately, the snail could not escape and remained cushioned underneath the sea star, analogous to its position at the bottom of the food chain.

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Sea star preying on snail (Photo credit: Prabh Sahota)

Although we focused on one food chain consisting of sea stars, snail and algae, it is fascinating to consider how changes in abundance can alter the entire interconnecting network of multiple different food chains, aka the food web. For this reason, it’s also important to keep in mind how humans, a dominant consumer may impact the food web too.

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