Hair-like Structures Filter Fine Materials

a crab with one large orange claw and a green and brown body in brown, muddy sand

Biological Strategy

Fiddler crabs

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Capture, Absorb, or Filter Solids

Some living systems must secure solid particles such as sediment, usually to keep the particles from hindering their health or activity. The most common way in which they do this is through filtering. To be effective, a filtering system must be appropriate to the sizes of solid particles to be captured and must capture only what is needed. It must also be effective in the appropriate media–air, water, or sometimes solids like soil. An example is mangroves, which are trees that grow along ocean coasts. Their root system slows down and settles sediment out of the water, building up soil to support the mangrove ecosystem.

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Crustaceans

Subphylum Crustacea (“crust”): Crabs, barnacles, shrimp, pillbugs

Some crustaceans, like pillpugs and woodlice, live on land, but most are found in the ocean. Their heads are distinctive among the arthropods: they have two pairs of antennae and three pairs of feeding appendages that help them hold and crunch food. Crustaceans also have a unique larval stage called the nauplius (named after the son of Poseidon) which has three pairs of legs and one eye. Most crustaceans are in class Malacostraca, which contains a wide range of species that live in mostly marine environments, like krill, lobster, and mantis shrimp.

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The mouthparts of fiddler crabs filter fine materials from sediment using spoon-shaped setae (stiff hair-like structures) to hold sand grains, while brush-shaped setae brush off diatoms and bacteria for eating.

Fiddler crabs (Uca spp.), are deposit feeders, ingesting organic matter from exposed mud at low tide. “Sediment is sorted within the buccal cavity. The outermost mouth parts, the third maxillipeds, play little active part other than helping to retain sediment and water during the sorting process. The inner surface of the second maxilIipedries carries quite large numbers of long setae, some with spoon tips and others feathery. Facing these on the outer surface of the first maxillipeds is a brush of stiff setae. The sediment is rolled between two maxillipeds. The spooned setae of the second maxillipeds hold sand grains against the brushlike setae of the first maxillipeds, and diatoms and bacteria adhering to the grains are brushed off and moved towards the mouth itself. While this is going on, water is pumped out of the gill chamber into the buccal chamber. This helps the sorting process which takes place essentially in suspension…The mouth parts, too, are adapted to particular sediment compositions. Species feeding predominantly on coarse sandy sediments have more of the long, spoon-tipped setae on the inside of the second maxillipeds, and the tips of the setae are more spoon-shaped, while the setal ‘brush’ on the outside of the first maxillipeds is denser. Where the preferred sediment contains more fine organic particles, extra rows of setae are present at the base of the third maxillipeds to protect the aperture into the gill chamber and prevent the gills from becoming clogged. (Macnae 1968; Miller 1961; Ono 1965).” (Hogarth 1999:94-95)

Last Updated August 18, 2016

References

Book

The Biology of Mangroves and Seagrasses (Biology of Habitats Series)

Oxford University Press | 31/05/2007 | Peter Hogarth

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