Visiting the Residents of the Plett Lagoon

Having lived in Cape Town for most of my life, I’ve travelled up the province’s west coast many times, to towns like Langebaan, Paternoster, Elands Bay and Strandfontein – my journeys up the west coast have even taken me as far as Namibe, Angola – but recently Jamie and I spent a week in the garden route, a little taste of South Africa’s East Coast. For Jamie, visiting Plettenberg Bay and the surrounding nature reserves seemed like revisiting a childhood photograph, but it was my first time exploring the warm Agulhas current and I was truly blown away seeing so many flowing rivers and being engulfed in rich green forested ravines.

One natural hotspot in Plettenberg Bay that I thoroughly enjoyed was the Plett lagoon. Besides being my first time swimming around such a large and clear body of water, it was also my first experience being up close and personal with so many of its resident animals – from slugs to bugs, worms, fish and birds – and once I noticed the abundance of hermit crabs living there, my curiosity about animals and animal social interactions spiked and I was hooked for hours observing the little creatures and how they interact with each other and their environment. After spotting my first hermit crab, I began to notice that every single gastropod shell I picked up contained a little crab. Even remnants of once large shells now housed tiny hermits.

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These hermit crabs were such curious little creatures – shy when approached, but tenacious and unreserved amongst each other – with various sizes and colours, in scattered groups, in pairs or alone, I wondered how many different species I was looking at. I now know there are five main categories of hermit crabs; left handed, right handed, land dwelling, deep water and non-gastropod dwelling hermits with each category having hundreds of sub classification. I believe though that we were observing several types of left handed hermits, based on colour differences. Upon doing a little research I found out that availability of usable shells is often the number one factor in determining the size of hermit crab populations, and that one will rarely find an empty shell when there are many hermit crabs around. I considered this fact – that population size and age can be so dependent on available homes – and intuitively this seems to make sense. Hermit crabs differ from other crustaceans in that their behaviours and way of life, from their defences against predators to social interactions with other hermit crabs, are so heavily shaped and defined by their relationship to their shells; resources which they don’t grow, but must find in the environment around them.

Most of the crabs on the shore of the lagoon were pretty small – about the size of a 10 cent coin. For every ten tiny crabs we would see a hermit around the size of a peach pip, and we found one or two hermits in shells roughly as big as apple cores. With there being so many more small crabs than the large older ones, we originally thought it might be breeding season for the little creatures in the lagoon, but now I’m wondering if the scarcity of large gastropod shells has anything to do with the size distributions we saw. This notion of shell availability, combined with my observations of all the differently sized crabs, got me pondering about the growth rates and lifespans of hermit crabs. It turns out that the structural integrity of a shell plays a big role in the quality of the crab’s life. Hermit crabs in small shells tend to have a slower growth rate than if they were in a well fitted shell. Hermits living in shells too small for them may not be able to fully retract in the presence of danger, making them far more vulnerable to certain predators. Furthermore, quality of shell directly effects how hermit crabs behave and interact with one another, where a group of hermits might pick on a crab with a poor shell, but just as equality gang-up against and ostracise a crab with too perfect a shell, in order to steal said perfect shell.

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In researching hermit crab shell selection, I uncovered a lot of incredible facts about hermit crab intelligence, and even about their social behaviours and social hierarchies. The criteria for a ‘perfect shell’ differ from species to species, with some hermits preferring thicker shells, or starkly coloured shells, whilst another species might prefer a thinner lighter shell, or a more sandy coloured camouflaged shell. One thing is certain though, all species of hermit crabs have intricate methods of examining shells – hermit crabs use their eyes to examine shells, they feel both inside and outside the shells, and even detect the calcium level in shells through chemical means – before taking them on as new homes. Research also suggests that crabs have an excellent memory for the shells they have examined, as they will pass right by shells they have previously tested or used and are known to spend more time examining new or novel shell types.

An adorable hermit crab social behaviour I learnt about is known as a “vacancy chain”, and such behaviour occurs when a crab finds an empty shell, but the shell is too large for the crab. The said crab will wait near the empty shell – sometimes for several hours – whilst other crabs come by and try on the large empty shell for themselves. The hermits who are too small for this new shell will size themselves up amongst each other and wait until a “Cinderella” crab comes along who does fit the large vacant shell. When such a crab finally arrives and leaves their shell vacant, all the other crabs swiftly trade up one size, in a mutually beneficial mass shell swap.

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Having spent several hours with the hermits in the Plett lagoon, I could tell that these crabs are quite intelligent and contextually aware, with a rather complex level of social interactions going on between the little creatures. Research into the hermit crabs has done more than just reinforce my suspicions about their intelligence, but I have been thoroughly amazed to learn more about the biological and social complexities of these squishy crustaceans.

While trying to learn more about these adorable little guys, I was quite astonished to see how popular these creatures are as pets. I also found quite a mixed spectrum of opinions as to whether hermit crabs were considered difficult to keep, or easy to look after as pets. Furthermore, various sources differed quite drastically about the expected lifespans of pet hermits, even when accounting for factors such as species life expectancy and ‘pet purchasing age’. I feel like these differences in presented information (like how hard it is to look after a hermit crab, or how long do they live) stem from people’s differing opinions of what the practice of pet ownership entails, or comprehension of what intrinsic influences our human impacts have on a pet’s life versus a wild animal’s life.

After spending a morning with the hermits in the Plett lagoon, as Jamie and I strolled through a local market that afternoon, we thought more than ever about the problems facing the natural ecosystems of our coastlines (from commercial and industrial shell harvesting, to pollution from rapid urban developments in coastal regions) and my concerns for the natural residents of our shores grew; but whilst harsh conditions in the wild might mean we see fewer and fewer old or mature crabs these days, an old wild crab can live up to three times as long as any pet crab, and travel thousands of kilometres, interacting with tens of thousands of different creatures – which says something about the needs of these creatures, and our ability as “pet owners” to meet and fulfil all those needs. Next week Jamie and I will be posting a video and an article about our visits to the Radical Raptors Rehabilitation and Awareness Centre, which further discusses this concept of implicit human impact on the lives of animals around us, but for now, check out the short clip that we compiled about the hermit crabs we met below:

A majority of my information for this blog came from the research paper below, which I highly recommend checking out for some cool insights into hermit crab behaviour and socialisation (as well as some awesome new facts on jumping spiders):
http://ftp.llu.edu/sdunbar/students/Billock%20Dissertation.pdf