Echinodermata: Marine Invertebrates

• Echinoderms are marine invertebrates characterized by tough skin and symmetrical body planes.
• The phylum Echinodermata includes organisms with similar genetic, molecular, physiological, or morphological traits.
• A primary trait is their skin texture.
• They possess endoskeletons made of rigid calcium carbonate plates.
• The word "echinoderm" originates from the Greek term for "spiny skin".
• Approximately 7,000 echinoderm species inhabit almost all seafloor ecosystems.
• They can be found in intertidal zones, coral reefs, and deep ocean trenches.
• They thrive in both tropical and temperate waters, as well as colder regions.

Types of Echinoderms

• Scientists classify echinoderms into five biological classes: Asteroidea, Ophiuroidea, Crinoidea, Echinoidea, and Holothuroidea.
• A class is a more specific taxonomic grouping than phyla or kingdoms.

Class Asteroidea

• Includes sea stars, also known as starfish.
• There are about 2,000 species of asteroids worldwide.
• Most species have five arms extending from a central disc, but some species have more.
• Examples include common starfish, purple sunstars, and sand stars.
• Sea daisies are also members of this class and are sessile animals that resemble flowers or small sea anemones.
• Sea daisies live on submerged wood and lack arms.

Class Ophiuroidea

• Contains 2,000 species known as brittle stars.
• They possess five flexible arms radiating from a central disc.
• They move by using two arms in a twisting motion.
• The limbs detach easily, regenerate quickly, but the animal will die if all five are lost.
• Examples include the common brittle star, ruby brittle star, and great basket star.

Class Crinoidea

• Crinoids are known as feather stars and sea lilies.
• Sea lilies were most abundant during the Mississippian Subperiod.
• Only a few hundred species exist today after nearly going extinct 252 million years ago.
• They are sessile echinoderms with feathery appendages that emerge from varying height stalks.
• Feather stars have up to 150 arms allowing them to crawl or swim.

Class Echinoidea

• Includes over 900 species of sea urchins and sand dollars.
• These have a hard outer shell called a test that supports spines and tube feet while protecting the internal organs.
• They live at all ocean depths, with sand dollars preferring sandy bottoms and sea urchins preferring rockier substrates.
• Common species include the long-spined urchin and keyhole urchin.

Class Holothuroidea

• Consists of roughly 1,200 species of sea cucumbers.
• Range in size from less than one inch (2.5 cm) to over 6 feet (1.8 m) in length.
• They have toxic internal organs that they eject at predators in self-defense.
• The ejected organs grow back in about six weeks.
• They can project hooklike structures from their skin to prevent attacks.

Echinoderm Traits

• The five classes vary in their movement, feeding habits, and reproduction.

Echinoderm Movement

• Echinoderms are either slow-moving or sessile.
• Mobile echinoderms use a water vascular system to move.
• The tube feet excrete sticky chemicals that help the animals adhere to surfaces.
• Brittle stars move more quickly via serpentine motion in their arms.
• Some sea cucumbers and feather stars can swim.

Echinoderm Skin & Organs

• Most have a thin layer of skin called an epidermis that covers their endoskeleton.
• Bumps and ridges in the endoskeleton create the spiny texture.
• Sand dollars and sea urchins have true spines extending from their endoskeletons.
• Sea cucumbers have tiny spicules in their skin instead of plates.
• The nervous system is loosely organized with a neural net.
• A nerve ring encircles the mouth leading to branching radial nerves in the arms of sea stars and brittle stars.
• Echinoderms do not have brains or actual eyes but can sense light and movement through eyespots.
• They have open circulatory systems and lack organized respiratory systems.
• They use their tube feet to extract oxygen from the water and tiny skin gills to bring in oxygen and release carbon dioxide.
• Digestive organs vary among the classes, as most have complete digestive systems, but sea stars excrete most waste materials through their tube feet and possess digestive glands, unlike many other echinoderms.

Echinoderm Symmetry and Limbs

• Echinoderm larvae exhibit bilateral symmetry.
• Adults exhibit pentaradial symmetry, with five projections or multiples of five extending from a central point.

Echinoderm Reproduction & Regeneration

• Echinoderms reproduce sexually by producing eggs and sperm, or asexually through regeneration.
• Some may produce reproductive cells for one sex, or like some sea stars, they may be hermaphrodites with eggs and sperm.
• Some brood their eggs to protect them from predators.
• Others produce free-floating or swimming larvae that do not receive any parental care.
• Brittle stars and sea stars can regenerate damaged limbs and create entire organisms through fission if divided through their central discs.
• Sea urchins and sea cucumbers also reproduce if divided in half.

Echinoderm Diet

• Echinoderms exhibit a variety of feeding behaviors.
• Sea urchins are grazers that eat almost anything in their path and consume more algae than most other echinoderms.
• Sea lilies and sea cucumbers use feeding tentacles to grab small pieces of food that float past, including plankton.
• Brittle stars collect small pieces of algae and animal matter with their tube feet.
• Sea stars are active predators that pursue sessile or slow-moving prey and use their tube feet to pry open bivalve shells before inserting their stomach to digest the prey.

Role of Echinoderms in Ecology

• Many are keystone species, meaning numerous organisms depend on them for survival.
• Sand dollars and sea cucumbers oxygenate the seafloor.
• Brittle stars, sea stars, and sea urchins are food sources for many species.
• Sea urchins keep algae growth in check through their grazing.
• Sea stars clean the seafloor by consuming dead animals.
• Overpopulation of sea urchins can decimate sea kelp beds.
• Crown-of-thorns sea stars cause severe damage to coral reefs if not controlled.