Feeding ecology and diet

Sea urchins feed in a variety of different ways. Regular urchins use their teeth to bite and rasp encrusting algae, sea grasses, and seaweeds (herbivores), while others feed on sessile organisms, carrion, and detritus (ominvores). Foraging for food usually occurs at night to avoid predators. Most sea urchins are slow moving.

Echinoids are formidable grazers and are capable of significantly altering the marine community in which they live. Following a mass mortality of sea urchins (often caused by a parasitic disease), there is a rapid growth in algae that can have devastating impacts to coral reefs. In many shallow marine communities, such species are recognized as keystone because their impact is disproportionately large. Little is known about deep-sea urchins other than they are believed to have similar feeding habitats to shallow-water species.

Irregular urchins are primarily deposit feeders, processing fine organic matter that settles on the seafloor. Heart urchins process sand in bulk to feed on organic particles trapped in sediment. Some species draw down surface detritus into their burrows, via a respiratory funnel, using their tube feet. Sand dollars, however, are more efficient as they sieve fine sediment particles while burrowing. Their spines are dense enough to prevent sand grains from falling through, yet fine enough to allow particles to drop out onto strings of mucus before being transported to the mouth. Typically, sand dollars feed on small diatoms and organic matter that accumulate in sand. Some species can become suspension feeders by lifting the test obliquely out of the sediment (e.g., Dendraster excentricus) to face the prevailing water currents. The tube feet of some sand dollars are specialized for deposit feeding, whereas some heart urchins have tube feet around the mouth specialized in picking up organic-rich detritus from sediment (selective deposit feeding).

Predators of echinoids include sea otters, sea stars, crabs, eels, lobsters, and fishes such as wrasses, wolffish, American plaice, butterflyfish, porcupine fish, and triggerfish. In coral habitats, the urchin, Diadema, is consumed by triggerfish, which blow jets of water at it in an attempt to turn the test upside down. American plaice feed exclusively on sand dollars, which in most cases only live for a few years. By contrast, heart urchins have fewer predators and can survive for up to 12 years. Some deep-sea echinoids have poisonous spines to defend against predators.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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