Algae: “Thank You, Seastar”

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Trophic cascade relationships (PC: nature.com)

Most people are well aware of the direct interactions such as predation, herbivory, and competition that occur within different organisms that form food chains. As those food chains mingle amongst each other, they form a larger food web. However, there are different types of indirect interactions that affect the abundance of particular species thereby altering food web structure. Indirect interactions occur when an interaction between two species is mediated by a third species. These effects can be shown via trophic cascades. One of the well-known trophic cascades involve sea otter-> sea urchin-> kelp. Sea otters have an indirect positive effect on kelp by consuming or altering the behaviour of sea urchins thereby reducing the number of urchins feeding on kelp. If the increase in kelp population is due to sea otters eating the sea urchins, it is a density-mediated indirect effect; whereas if it is strictly the presence of the sea otter that changes the urchin’s behaviour and prevents them from feeding on the kelp, it is a trait-mediated indirect effect.

We were curious to see if a similar trophic cascade existed between Littorina littorea (snail) which grazes on Ulva lactuca (algae); and the snail’s two potential predators: Evasterias troschelii (sea star) and Metacarcinus magister (crab).

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Seastar preventing the snail from feeding (PC:Juhae Oh)

We set up an experiment involving 4 different environments:

  • Ulva
  • Ulva + Littorina
  • Ulva + Littorina + Evasterias
  • Ulva + Littorina + Metacarcinus cue water
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removal of snails and algae after 7 day exposure (no seastar/crabwater) (PC:Juhae Oh)

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Evident grazing with no predator present (PC:Juhae Oh

We compared the weight of Ulva before and after being exposed in each environment for 7 days. We predicted the snails to graze on the algae but with the presence of the sea star and the scent of crab, they would reduce their amount of feeding or even get consumed in environments with the sea star. As we had predicted, the snails were able to graze more without the presence of predators. They ate significantly less algae with the presence of the sea star, although the crab water was not as significant. Moreover, the sea stars tended to not eat the snails.

Through our results, we were able to see a positive indirect effect of algae and the sea star. Although the sea stars didn’t eat the snails, we still saw a significant decrease in the amount of grazing, meaning the snail’s behaviour was affected by the presence of the sea star. Therefore, we can categorize this effect as trait mediated.

Watch this youtube video for more information about trophic cascades: https://www.youtube.com/watch?v=hRGg5it5FMI

Climate Change: Should Hermit Crabs be Concerned?

Global warming has been a critical issue in the past and still remains a concern today. Not only does it affect us humans, but many other organisms are also affected. With human activities increasing green house gas concentrations, air and water temperatures increase, causing rise in sea level and decrease in salinity. Climate change is already evident in many oceans, but it is expected to deteriorate in our near future. Since climate change affects multiple factors such as temperature and salinity, we examined the affects of these stressors together and in isolation to visualize a potential scenario.

 

 

The species we investigated were the hermit crabs, Pagurus hirsutiusculus. We investigated their behavior by manipulating an environment they will most likely be exposed to in the future: an ocean with decreased salinity and increased temperature.

 

We exposed hermit crabs to 4 treatment levels for 3 days:

1)A control with optimal temperature (31ppt) and salinity (11°C)

2)Low salinity (18ppt)

3) High temperature (14°C)

4)Low salinity (18ppt) & high temperature (14°C)

 

We then performed 3 different experiments that we predicted would show behavior changes when exposed to the two climate change-associated abiotic stressors. We first counted the number of antennule flicks in 30 seconds, then we examined how long a hermit crab spends on feeding in 10 minutes, and lastly, we pretended to be a predator and scared it out of its shell and examined how long it took for it to return to its shell and seek refuge.  We predicted:

1)temperature would increase metabolic rates and show increased speed of behaviours

2)salinity may disrupt osmoregulation and show decreased speed of behaviours

However, our results for all experiments were insignificant.

 

While doing the experiments, one thing I found really cute were the extremely small shells on some of the individuals. This shell which is supposed to function as protective shelter for its soft, exposed abdomens, only covers a part of its body, leaving the uncovered areas vulnerable to predators. Thinking about the reasoning behind this left me sympathetic. Having such shell probably means that it has lost the vigorous competition to find a suitable shell or perhaps got its shell stolen in a shell fight. This made me consider taking this weak individual into an environment with no competition. Hermit crabs are fairly easy pets to care for once their tank is properly set up. As long as you can bear some sporadic chirping, they are dainty critters that can be found at beaches. Here is a link with more information about how to raise saltwater hermit crabs.

 

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Hermit crab removed from its  unsuitably small shell (PC: Juhae Oh)

Although our experiment doesn’t represent the whole population, I am relieved that through our experiment we could predict Pagurus hirsutiusculus will tolerate the decrease in salinity and increase in temperatures we will most likely face in the future. However, we must remember that it may be a strong concern for other organisms, including ourselves. Click here to learn more about what we can do to save our environment!