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Biology

Beyond the shell: What are hermit crabs really after?

Paper: Laidre, M. E. and Greggor, A. L. (2015). Swarms of swift scavengers: ecological role of marine intertidal hermit crabs in California. Mar. Biol. 162, 969–977.

Intro

Hermit crabs are one of the most recognizable animals that beachgoers love to catch in rocky tide pools. A common sight here on the east coast, most people know the basics of life as a hermit crab: as they grow larger, they’ll get new “houses” (shells of other animals) to protect their soft bodies from predators (Figure 1).

Figure 1 - Hermit crab without and with a shell

Figure 1 – Hermit crab without and with a shell

 

 

 

 

 

 

 

That process of switching shells has interested marine scientists since the 1950s, but scientists were often so focused on the shells that they didn’t look at other aspects of life as a hermit crab. This paper’s goal was to investigate the ecological importance of hermit crabs in their tide pool habitats.

Previous studies that looked into hermit crab behavior around dead animals suggested that hermit crabs were attracted to decaying animals that could provide a shell for them. The idea was that hermit crabs could smell certain chemical cues in the breakdown of shelled animals called gastropods, and those chemical cues let them know that there was an empty shell available for them to use. The researchers in this study put forth a different hypothesis: what if the hermit crabs were headed towards the decaying material not because it had that “new house” smell, but because it smelled like dinner?

Methods

The researchers conducted their study in the field in actual tide pools so as to get the most ecologically relevant results. They went out to the beaches of northern California, found some tide pools, and noted whether there were hermit crabs in them. If there were, the researchers would watch the tide pool for 10 minutes, observing their behavior and the behavior of other animals in the tide pools (fish, snails, starfish, whatever was in there). After those 10 minutes, the researcher would put one of four things into the tide pool: a) a newly smashed snail, b) a live snail, c) a newly smashed mussel, or d) a live mussel (Figure 2).

Figure 2 – Smashed mussel (left), smashed snail (right)

Figure 2 – Smashed mussel (left), smashed snail (right)

 

 

 

 

 

 

 

The goal of adding just smashed (and therefore dead) animals to the tide pool was to see how long it took the hermit crabs to arrive on the scene, and whether they would respond differently to the mussel or the snail. Mussels don’t provide a suitable shell for the hermit crabs but the snails do.

After the animal (smashed or live snail/mussel) was added to the tide pool, the researcher observed the tide pool for 10 more minutes, noting how long it took the hermit crabs to get to the new addition. The researchers did this experiment 80 times, and also did the same general experiment 20 more times with a time of 30 minutes before and 30 minutes after the animal was dropped into the pool.

Results and Significance

The researchers found that hermit crabs were much more attracted to the smashed animals than the live animals, and that there was no difference in how attracted the hermit crabs were to the smashed mussel and the smashed snail (Figure 3).

Figure 3 – Hermit crab response to the introduced animal: smashed animals attracted a larger number of hermit crabs than alive animals, and the crabs were equally attracted to mussels and snails

Figure 3 – Hermit crab response to the introduced animal: smashed animals attracted a larger number of hermit crabs than alive animals, and the crabs were equally attracted to mussels and snail

 

 

 

 

 

 

 

 

Hermit crabs were also equally as speedy to show up to either a smashed mussel or a smashed snail: the average response time was 2.87 and 2.9 minutes, respectively.

 

 

 

 

 

 

 

 

These results suggest that hermit crabs are attracted to these dead animals not because of the prospect to move into a larger shell, but because it represents a food source for the crab. Previously, hermit crabs were thought to be primarily deposit detritivores, which means that they were filtering the sediment around them, looking for small pieces of food to eat. Now, we can think about hermit crabs as a larger contributor to the whole tide pool food web. Animals that eat other dead animals are important to nutrient cycling. They ensure that the nutrients that the dead animal contained don’t get washed away with the tides. Hermit crabs are more important to the ecosystem than we knew!

Engage

Do you like to go tidepooling at the beach, by yourself or with your kids? Have you found hermit crabs? What’s the strangest animal you’ve ever seen at the beach?

Discussion

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