S4E4 - Engineering Fiddler Crabs

Fiddler crabs, with their one giant claw, are considered ecosystem engineers in mangrove environments. Herre de Bondt gives us a glimpse into the world of these incredible crustaceans.

Recorded: 27 November 2023. 

 Featured: 

• Fiddler Crabs by Jochen Zeil et al. 
• Burrow-Morphological Characters of the Fiddler Crab and Ecological Correlates in a Lagoonal Beach on Pulau Hantu, Singapore by Shirley S. L. Lim and C. H. Diong.
• Effects of Fiddler Crab Burrows on Sediment Properties in the Mangrove Mudflats of Sungai Sepang, Malaysiaby Mohammad Mokhtari et al. 
• Contrasting Effects of Two Burrowing Crabs on Sediment Composition and Transport in Estuarine Environments by F. Botto and O. Iribarne.
• State of the World’s Mangroves by the Nature Conservancy. 
• Bigger is Better by National Geographic. 

Credits:

• Claudia Hirtenfelder, executive producer and host 
• Herre de Bondt, co-host and co-editor
• Christiaan Mentz, sound editor and producer 
• Rebecca Shen, content producer and designer (logo and episode artwork)
• Gordon Clarke, bed music composer
• Learn more about the team here. 

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Animals in Politics, Law, and Ethics researches how we live in interspecies societies and polities.

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Chapters

• 0:00 Introduction
• 1:11 Journey to a Mangrove
• 2:46 Fiddler Crabs are Ecosystem Engineers
• 10:59 Crab Museum
• 12:38 Credits

Transcript

 00:00 – Introduction

• Season 4 of The Animal Highlight: Animals and Waste. 
• Co-host this season: Herre de Bondt has done research on rats in Amsterdam, crows in Tokyo, and gulls in The Hague. His work has now brought him to London where his PhD project is concerned with urban bird feeding practices. From hanging up fatballs for chirpy robins to tossing seed to flocks of ‘flying rats’, Herre is determined to investigate the inherently multispecies practice of bird feeding. He is particularly interested in the ways non-human animals inform and shape the contemporary city in collaboration with – and in defiance of – humans. You can connect with Herre via Twitter (@HerreBondt). 
• In this episode we look at fiddler crabs. 
• Herre visited Crab Museum in Kent, England 

01:11 – Journey to a Mangrove

• Let’s go on a journey! 
• Imagine a place where the ocean meets the sea. From a boat on the water, trees seem to have surfaced straight out of the water, forming a lush forest. The fresh water from further inland dilutes the salty seawater, creating a buffer zone in which mangrove trees thrive. Not many plants can handle the level of salinity in the water, creating an opportunity for vast forests of mangrove to flourish. 
• As low tide begins we move our boat closer. We marvel at  the mesh of roots branching out from the stems of the mangrove trees. They resemble octopus arms, reaching out in an umbrella-like pattern to plant themselves firmly in the ground. As the water recedes further we see strange formations protrude from the water. They look like dark-brown, wooden, fingers reaching to the sky. 
• Mangrove trees shoot these roots out of the thick, muddy ground - somewhat like snorkels - to suck up oxygen. We avoid these breathing tubes as we wade through the muddy water towards the treeline. The low tide reveals a large mudflat behind us and a labyrinth of roots and trees in front of us. But, if we look closely, there’s something else there, a flash of bright red sticking to the roots.  

02:46 – Fiddler Crabs are Ecosystem Engineers

• Coming closer, we see that it is a fiddler crab. No more than five centimeters across. 
• The body of the fiddler crab is small and darkly coloured. Two eyes on top of stalks protrude upwards from the front of his body. This crab is one of the few species of crab who can climb trees and is clinging on to the mangrove root with his eight spider-like legs. 
• This specific crab is a male, easily recognizable by his right claw which absolutely massive and brightly coloured red. His large, right claw is nearly as big as his body, and dwarfs his left claw which, honestly, just looks silly in comparison. 
• While you’re imaging this crab nestled in the mangrove forest, imagine a yellow, plastic hard hat in between his eyes and a rolled up blueprint in his left claw because the fiddler crab is a known ecosystem engineer. 
• In previous animal highlights we talked about waste in relation to humans. We mostly talked about human waste and how particular animals either intrinsically know how to deal with it, or how they learned to do so through their own inventiveness in proximity to humans. 
• But both in presence and in absence of humans, organic waste finds itself in a continuous flow between organisms, places, and states. Leaves from trees, dead animals, fecal matter, and whatever food people or animals don’t finish end up being processed. 
• Today I want to focus on the way fiddler crabs are considered keystone species as they engineer their way through the mangrove forest. 
• While the crab might not actually have a real blueprint or masterplan, their entire way of life works in symbiosis with the mangrove forest and adjacent mudflats. The big, red claw that the male fiddler crabs walk around with are mainly used for fighting other males, for communication, or for mating displays. 
• Imagine a large crowd of crabs, synchronously waving their brightly coloured claws at you in a gesture that both humans and crabs interpret as “come over here”. They synchronise in large numbers mainly to have their waving be more visible to distant females. 
• Aside from doing synchronized dancing, fiddler crabs also use this claw to move and eat dead mangrove leaves that have accumulated on the floor. This not only feeds them, but also prevents the build-up of excessive organic waste which would negatively influence other species. Essentially, crabs are the clean-up crew of the forest. 
• This becomes even more apparent if we consider how both males and females use their smaller claws. They scoop claw after claw of mud into their mouths, which, according to some, looks like they play their large claw as a fiddle, hence their name. 
• Eating mud has a benefit for the ecology of the mangrove forest. As the tide lowers on the mudflats, the water leaves behind tons of sediment, algae, and organic detritus from the plants further in the forest. By putting this in their mouth, the fiddler crab quickly filters through the material and extracts anything that is edible. They afterwards deposit a small, spherical chunk of sand that is often enriched with nutrients and can further stimulate growth of marsh vegetation. 
• In this intertidal zone, the soil is incredibly dense and low in oxygen. Rivers from upstream carry with them miniscule pieces of sand, making the soil sludge-like with no space for oxygen. But the fiddler crab has engineered a solution. 
• Small crustaceans like them would easily be swept away by the changing tides, and to prevent this the fiddler crab digs burrows in the sand. The burrows are so small the crab has to enter it sideways, although their hideouts open up toward the end as they often take the shape of the letter J. 
• But the forest also benefits from this burrow. Holes in the dense soil allow oxygen to aerate not only the mangrove tree roots, but also the bacteria living inside the soil. These bacteria, just like the crabs themselves, break down organic matter, further oxidizing the soil for the benefit of everything that still is alive. 
• But wait, there’s more! 
• Holes in the sediment significantly slow down the water as the tide rises or lowers, which prevents water from sweeping away large parts of the mudflats that fiddler crabs live in. 
• And so whether you call fiddler crabs ecosystem engineers or clean-up crew, the fact remains that they are keystone species to the mangrove ecology. They manage the soil, oxidate trees, prevent erosion, and form a crucial part of the diet of other animals in the mangrove forest such as birds, fish, sharks, monkeys, hawks, and raccoons. 
• Even though I made it sound like humans were going to be completely absent from this episode, fiddlers' management of mangrove forests helps us as well. The complex tangles that mangrove roots form keeps mud in place so effectively that the trees protect the inland areas – and those living in it –from storm surges, waves, and high tides. Mangrove forests reduces risk to the lives of more than 15 million people per year and prevent more than 65 billion in property damages each year. 
• It is strange to think about how big of an effect all those tiny claws - and bigger claws - can have on such a big scale. Who would think that eating mud and digging holes could prevent ecological disasters? It almost makes me think we should rename the butterfly effect to the fiddler crab effect. 

10:59 – Crab Museum

• In Kent and it is run by people who are passionate about crabs. 
• The museum has a political perspective. 

12:38 – Credits 

• The Animal Highlight is produced and hosted by Claudia Hirtenfelder. This season was co-hosted with Herre de Bondt. The editing was done by Claudia Hirtenfelder, Herre de Bondt, and Christiaan Mentz. The episode artwork was done by Rebecca Shen and the best music by Gordon Clarke.