Marine Biology Chapter 11 - Echinoderms

Echinoderms Overview

• Echinoderms are spiny-skinned animals characterized by radial symmetry and an internal skeleton, without body segmentation.
• Common examples include sea stars, sea urchins, sea cucumbers, and crinoids.

Terminology and Common Names

• The term "sea star" is preferred over "starfish" due to the lack of fish characteristics, such as scales or a backbone.
• Sea stars typically feature five arms radiating from a central body (though some species may have more).

Sea Star Characteristics

• Exhibit radial symmetry, with body parts arranged around a central axis.
• Equipped with tube feet located in grooves on their ventral surface, aiding in movement, feeding, and respiration.
• Possess eyespots at the end of each arm, allowing them to detect light.

Feeding Mechanism

• Sea stars can open bivalves (e.g., clams) using their tube feet to exert a pulling force, tiring the clam's adductor muscles until it opens for feeding.
• Their feeding process highlights the role of tube feet in clinging to surfaces while extracting prey.

Movement

• Movement is powered by water entering the tube feet from the ampulla; muscles contract to create suction, enabling locomotion or opening shells.
• Tube feet consist of suction disks that cling to surfaces, facilitating movement across the ocean floor.

Other Echinoderm Types

• Crinoids: Composed of feathery arms atop a stalk; filter feed by capturing zooplankton.
• Brittle Stars: Noted for their solitary, nocturnal behavior and long, slender arms.
• Sea Cucumbers: Have a soft body and tube feet arranged in five rows; lack arms and hard spines.
• Sand Dollars and Sea Urchins: Rounded echinoderms, with sea urchins having long spines while sand dollars are smooth.
• Feather Stars: Crinoids that use flapping arms for movement.

Specialized Structures

• Ampulla: Bulblike structure at the top of each tube foot, crucial for movement.
• Aristotle's Lantern: A unique five-toothed mouth structure in sea urchins for feeding on algae.
• Sieve Plate: A small filter in sea stars that facilitates water intake into the water vascular system.
• Skin Gills: Ciliated projections on sea stars' skin, assisting in respiration.

Water Vascular System

• A network of water-filled canals in sea stars, integral for various functions including movement, feeding, and gas exchange.