The brilliantly colored blue ringed octopus (Hapalochlaena lunulata) hunts using a venom so powerful it can kill a human. © Alex Kerstitch / Bruce Coleman Inc.

The form, function and behavior of marine invertebrates reflect the diverse ways they are adapted to life in water. Water is 800 times more dense than air and 50 times more thick and viscous. This density creates resistance to movement, but also keeps small particles in suspension, creating options for feeding and reproduction not available on land or in air.

Eating
Many marine invertebrates feed by collecting small particles of food suspended in the water around them, something that cannot readily be done in air. Seawater is filled with microscopic plants and algae called phytoplankton, microscopic animals called zooplankton, numerous eggs and decaying bits of plants, animals and fecal matter. Although some marine invertebrates are predators that hunt and chase mobile prey, many feed simply by catching whatever drifts by.

Invertebrates anchored to the bottom wait for food to fall to them or to float past. But others actively collect it. Jellyfish pulse their bodies not so much to move around as to create currents in the water that draw food particles under their bell, where the food can be collected and eaten. These and related anemones and corals use stinging tentacles to capture prey; some have hundreds of limbs to increase their catch. Others, such as crinoids, collect food with enormous nets. Clams, tunicates, sponges and other filter feeders pump water through their bodies, filtering out food according to size.

Other Ways of Feeding
Perched on a sponge, the crinoid, or feather star, extends branching arms to collect bits of food from the water. It passes its catch down grooves in its arms to its mouth using hairlike cilia.

	MANGROVE PERIWINKLE Littoraria angulifera

Leeches are parasites that suck blood from other animals for food.

Snails and periwinkles use a rough, tonguelike radula to scrape algae off of rocks.

Squids seize their prey with two long tentacles and eight sucker-lined arms. They draw their victims to the center of their body and tear them apart with a hard beak.