Papagaios, Life & Times on a Speculative Archipelago - Complex Clinker Shrubs and Social Crabs

Transcript from podcast ‘Strange Bedfellows’, episode 217 ‘Clinker Shrubs and their Lodgers’. Hosts Ariadne Royce (AR) and Lex Charlton (LC).

AR: Welcome back to Strange Bedfellows, this week we're on the Papagaios islands exploring the symbiosis between clinker crabs, their bucket-like plant homes and the bacteria they sorta-maybe-kinda farm to become toxic to predators.

Before the break we were discussing the more basal crab-plant symbiosis, but Lex you said there are other species with more complex lives.

LC: Yeah, so there are six or maybe seven species of clinker crab each with its own species of clinker shrub. Most of the crabs are fairly similar to each other, differing in size, coloration, or dietary preferences.

AR: One species are flexitarian?

LC: More like which small invertebrates are the tastiest or which seeds can my claws crack.

AR: Gotcha. You say most of the crab species are similar, I'm guessing one or more are not?

LC: Brilliant deduction, Holmes. Three of the species are... I suppose social is the way to put it.

AR: Like bees?

LC: That's eusocial. A queen laying eggs that hatch into drones and workers and all that. These crabs aren't so specialised. For two of the three species, the biggest difference is that they are more tolerant of others in 'their' shrub-

AR: Classic flatmates, tolerating each other.

LC: -they also share childcare duties. That is to say one crab will monitor the pool with the young in and keep predators out while the others forage or rest.

AR: Are they related? Like those animals who live in groups of siblings plus partners.

LC: It doesn't seem so. Each crab finds the plant independently and so long as it's not full the others allow it. To briefly go back to that last point about the crabs resting, these social crabs bend the inner leaves of their host shrub to create chambers to live in, to hide from predators, or to store food.

AR: That's very cool. How much building goes into this? Do they make a gap then squeeze in between the leaves or is it more advanced than that?

LC: Very good question. They take the long leaves in their pincers and curl or roll them backwards, tucking the ends into the tighter spaces between the leaves' bases. You get either tubes or cones depending on how they go about it.

AR: This is so cool. Imagine pulling a wall of your house and making a new room. Now Lex: there’s something that’s bugging me. You said there were three species of social crabs and we have been discussing two of them. What's the deal with this mysterious third species?

LC: Before we discuss the crabs themselves, here's a picture of the clinker shrub they live in. Could you describe it for the listeners?

AR: That is literally a boat.

LC: It is not literally a boat, but please elaborate.

AR: First up its shape. It has the steeper, more compact back and longer, sloping front that old sailing ships had. You know, it's almost like a baby basket. Then there's this stem sticking out of it that in the boat metaphor would, I guess, be the mast.

LC: These are the original plants that the 'clinker' name came from. It so resembles a boat that it is known as the ‘flagship clinker’ and the species that lives in it is known as the ‘shipwright crab’. You may also have spotted the orange structure on top of the stem. That's a fleshy structure derived from leaves that attracts insects to the flowers which sprout from the end of it.

When it comes to pollination, some flowers work to attract specific species and others are more general. The flagship clinker is at the specific end of the scale, being pollinated by a single species of butterfly, and the crabs help in the process.

AR: Do they keep other insects away, like they keep the nursery pools free of predators?

LC: They actively protect the caterpillars of the pollinating species. Scientists aren’t sure if the plant had one specific pollinator and then the crabs started to help them as a way to make sure they had more homes, or if the crabs tended to the caterpillars for other reasons and their proximity to the plant changed to interdependence. Either way, the crabs fashion chambers for the caterpillars and snip the leaves in a certain way that encourages growth so that they can eat their host without destroying the chamber.

Flagship Clinker in cross-section, showing the different areas created and managed by the crab residents.

AR: That can’t be easy because, from what I remember, leaves grow from the tip. So where do you eat that keeps the leaf alive, but still growing? The sides maybe?

LC: Plants generally grow from meristems which is kinda like an area of stem cells that can become any kind of cell. In the case of leaves, yeah, these meristems are at the tip. Same goes for roots and stems. We call them apical meristems because they're at the apex of the structure.

However, this isn't always the case. Blades of grass, for example, grow from the base of the leaf. These are called intercalary meristems, that is, areas of stem cells between areas of differentiated cells.

It seems the leaves of the flagship clinker shrub have the ability to have develop intercalary meristems when they are cut by the crabs. Researchers tried to replicate this, but cutting the leaves with scissors doesn't lead to growth.

AR: That implies to me that the crabs are doing something to bring about the growth beyond cutting the leaf.

LC: The researchers came to the same conclusion. It is known that the roots if legumes can develop new meristems when infected with a certain kind of bacteria. It might be that the crabs - which we know have a symbiotic relationship with certain toxin producing species - might also be utilising a bacteria that encourages leaf growth. Another alternative is that the gardener crabs themselves are creating some sort of protein and are using it to foster growth.

AR: 'Gardener' sounds like a specialised role. I know you said they weren’t eusocial, but do the crabs have castes like ants or bees do?

LC: Ish. Castes in eusocial insects are determined by genetics and the queen is mother of them all. Shipwright crabs have three types, most often called 'classes' and there is no way to know at birth what class an individual will be part of.

AR: Okay. Let's take it from the top. You're a crab egg and you hatch into a... larva? No zoea.

LC: Crabs have several swimming zoea stages, separated by moults. Then they progress to the megalopa stage, which is also swimming. In clinker crabs all these stages live in the central pool of the shrub. What happens next appears to be down to the size of the megalopa at the point it passes to the next stage: juvenile crab.

AR: So, the smallest megalopa become on class the largest another?

LC: More or less. The biggest megalopa become juveniles like all the other clinker crab species. They become terrestrial and leave the pool. The smaller megalopa become nurse class crabs. They remain in the nursery pool and their role includes protect future zoea from predatory insect larva and keeping the pool clear of algae. As part of this role, they have some unique adaptations. Here's a photo of one.

AR: Oh wow, that's pretty unique. I'll start with the claws, they're very slender and long, like tweezers. Which I guess is a better shape for catching prey in the water. They have less spikes on the carapace too, more like small bumps, one assumes to increase streamlining. And... oh that's weird.

LC: Thought you'd spot that sooner. Go on.

AR: They have four pairs of legs, not including their claws, like all crabs. But the front and back pair are flattened and paddle-shaped.

LC: To help them swim nimbly round the pool, chasing threats and avoiding cashing into egg clusters. They are very good swimmers. Their remaining pairs of legs are like in a normal crab and they can, with some difficulty move on land. Though ideally, they never have to.

AR: We have nurse class in the pool and out of it we have the gardeners?

LC: Not yet. The juvenile crabs that leave the pool are very small and mostly shelter in the chambers and eat food collected by larger crabs. When they moult to their adult forms, size again is a factor. The biggest crabs become warrior class. Here's another photo.

AR: Whoa that is a hench crab. That is my first observation. Its legs are all really thick and one of its claws is enormous. It is also very spiky and said spikes are very long and sharp. The colouration is very bold blue and white stripes which is a major warning pattern too. This crab is going all in on both defence and offence.

LC: The warrior class are the ones most often out and about. They hunt or forage food and bring some home. They stand guard on and around the plant. Generally speaking, they're the crab you're most likely to see when you stumble upon a shrub. And finally, we have the gardener class. These come from the smaller terrestrial crabs.

AR: The crab in this picture is pretty typical looking compared to the previous two. It has spikes, but not massive ones. Its claws are fairly small compared to the warrior, but look thinner and more precise which makes sense for cutting the leaves. What is it doing with its second pair of legs? Has it got a second set of claws?

LC: For those at home, the crab is bending back and leaf and cutting it. What Ariadne has seen is a pointed projection on the foot of the second limb. It is an adaptation for gripping leaves and is most often used to pin them in place or piece the leaf to hold them steady while the claws cut. A clinker crabs foot already has a row of spines so it's not a massive adaptation, but behaviourally it is unusual at first glance.

And so, here we have our three classes of shipwright crab in its home the flagship shrub.

AR: Before we end for this week, how does breeding work in this more specialised set up?

LC: Technically all the classes can breed, but it is most commonly done by the gardeners, probably by virtue of them being the least active class and therefore the one with the most time and energy. However, when a shrub is overcrowded it is the warriors who leave, wandering the forest until they find either a shrub with a smaller population, or a newly grown shrub, ready to be colonised. The warrior will signal to nearby crabs by loudly clicking its claw and hopefully one or two other wandering warriors will join. This small band of two to four crabs will breed and start a new colony.

As there are yet no nurses or gardeners, the warriors do without chambers and try their best to keep pool safe. This is the point when there are most losses of young, but enough will survive to become nurses and gardeners and at this point the new colony is truly established.

AR: Huh. These crabs really have it all figured out. Alright I think that's all from us this week. As ever, if you have a symbiont pair you'd like us to discuss do get in touch and you can follow us on our our socials. We are on...