Crustecean Behaviour Flashcards
When did crabs evolve?
The earliest true crabs occur in deposits from the Jurassic (see International Geological Time Scale), but the main radiation of crabs started in the Cretaceous, about 120 Ma ago. Although they are called crabs, the horseshoe crabs are in a different sub-phylum from the true crabs and evolved much earlier, in the Cambrian period.
How do crabs influence the supply of oxygen and water to the mangrove roots?
Crabs burrow in the mud around the roots and snorkels of the mangrove trees, forming a labyrinth of burrows. At low tide the burrows channel oxygen from the air and fresh rain water direct to the roots. When the tide comes in salt water enters the burrows but is often diluted by the freshwater already present and this lessens the amount of salt that the mangroves take up.
How do crabs influence the supply of nutrients to the mangroves?
Crabs feed on organic material, particularly bacteria, present in particles of soil. They take balls of soil down into the burrows as the tide comes in and effectively recycle the organic matter for the mangroves. Some crabs gather up leaves – an astonishing 80% of fallen leaves – and store them in the burrows, where decay releases nutrients that the mangrove can utilise.
Outline the features of the mangrove ecosystem in Sundarbans.
It is an area characterised by mud, formed from the fine particles of silt carried down to the sea by the river, which then precipitate out of suspension. The tide covers the silt and mangrove roots with salt water twice a day. Sulfurous fumes are produced by bacteria. No oxygen gets into the soil.
Outline the larval stages of the crab.
Female crabs release fertilised eggs into the water where they hatch, releasing zoea larvae. The larvae go through growth stages of megalopa post-larval stage in the water, until finally they moult into miniature crabs which crawl ashore into the intertidal zone.
Define autotomy and give examples.
The process of casting off a limb or appendage when an animal is under threat or direct attack. For example, fiddler crabs shedding their large claw or lizards shedding their tail.
Suggest behaviours that might be influenced by visual signals, such as fiddler crabs waving their large claw.
Species recognition, mate recognition, mate choice and male–male rivalry are obvious behaviours influenced by signals.
Outline crab activity patterns throughout the day.
They are diurnal active just a few hours each day during low tide. They descend into burrows when the tide comes in. Burrows are also used during low tide to escape the threat of predation and to provide access to water needed to keep the respiratory surfaces moist.
What do fiddler crabs feed on?
They extract nutrients from the detritus in the mud, cleaning sections of mud of all organic matter and abandoning nutrient free balls of mud as they go along.
Outline the duration of mating challenges in fiddler crabs.
Short frequent contests which are essentially ritualistic demonstrations of potential and occasional longer contests which constitute actual fights with the potential for damage. The longer contests are rarer.
What would be a consequence of a male fiddler crab losing the large claw in a fight?
The male would lose the visual signal that it was male and would be pursued as a female.
Outline the different methods employed by fiddler crabs to achieve mating.
Standard gambit: standard surface courtship at the entrance to female burrows, female compliance is required & they can refuse to mate.
Attraction by waving and suitable burrows: females willingly enter male burrows to mate, males plug burrows.
Herding: males attempt to herd females into their own burrow where they plug her in to force mating.
Dig out: males literally dig females out of their own burrows, but some degree of compliance is required as the female isn’t trapped and so can still refuse to mate.
Define osmoconformers.
Aquatic organisms whose osmotic pressure tracks that of the external environment.
Define osmoregulators.
Organisms that regulate internal osmotic pressure to an optimal value.
Think back to the Sundarbans and the crabs living around the mangroves. What is the salinity of the medium they are bathed in likely to be?
The mud is covered by water when the tide is high. The mangroves are part of a major river delta, so freshwater will be flowing down to meet the sea. There will be a varying salinity, and how much it varies will depend upon the location of the mud in which the crabs are burrowing
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