7.3 Echinoderms: sea stars, sea urchins, and their kin

Echinoderms are unique marine animals with pentaradial symmetry and a water vascular system. These invertebrates, including sea stars and urchins, play crucial roles in ocean ecosystems as keystone species and bioturbators.

Echinoderms possess remarkable regenerative abilities and have economic value in cuisine and medicine. Their diverse adaptations, from the Aristotle's lantern of sea urchins to the autotomizing arms of brittle stars, showcase their evolutionary success in marine environments.

Echinoderm Characteristics and Adaptations

Characteristics of echinoderms

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• Deuterostome invertebrates closely related to chordates, with embryonic development where the blastopore becomes the anus
• Pentaradial symmetry in adult form, evolved from bilateral symmetry in larvae (sea star, sand dollar)
• Endoskeleton composed of calcium carbonate ossicles connected by connective tissue provides structure and support (sea urchin test, sea star plates)
• Water vascular system is a hydraulic system for locomotion, feeding, and gas exchange consisting of a central ring canal, radial canals, and tube feet (sea star locomotion, sea urchin tube feet)
• Mutable collagenous tissue allows for rapid changes in tissue stiffness enabling movement and flexibility (sea cucumber body wall, brittle star arms)
• Pedicellariae are small, jaw-like structures on the body surface used for defense, cleaning, and capturing prey (sea star, sea urchin)

Morphology of echinoderm classes

• Asteroidea (sea stars) have a star-shaped body with arms radiating from a central disc, tube feet for locomotion and feeding, and evert their cardiac stomach for extraoral digestion
• Echinoidea (sea urchins and sand dollars) have a globular or flattened body covered in spines, Aristotle's lantern for feeding on algae and invertebrates, and tube feet for locomotion and attachment
• Ophiuroidea (brittle stars and basket stars) have a central disc with long, slender arms used for locomotion that can be autotomized for defense, and tube feet for feeding on detritus and small invertebrates
• Crinoidea (sea lilies and feather stars) have a feather-like appearance with arms extending from a central calyx, most are sessile and attached to substrate by a stalk, while feather stars are free-swimming and use arms for locomotion, feeding on suspended particles using tube feet on arms
• Holothuroidea (sea cucumbers) have an elongated, cylindrical body with leathery skin, tentacles surrounding the mouth for feeding on detritus and plankton, and tube feet for locomotion and attachment

Ecological roles in marine ecosystems

• Keystone species control populations and maintain biodiversity (Pisaster ochraceus sea stars in intertidal communities, Diadema antillarum urchins in coral reefs)
• Bioturbators mix sediments, increase oxygenation and nutrient cycling, facilitate breakdown of organic matter, and enhance microbial activity (burrowing urchins, sea cucumbers)
• Food source for various predators like sea otters, fish, and crustaceans that prey on urchins and sea stars, and sea cucumbers consumed by fish and humans (bêche-de-mer)
• Habitat provision with urchin spines and tests providing shelter for small invertebrates and fish, and crinoids serving as attachment sites for other organisms

Regeneration and economic importance

• Many echinoderms can regenerate lost body parts, with sea stars regenerating arms or entire bodies from a single arm, and sea cucumbers expelling and regenerating internal organs as a defense mechanism
• Economic importance includes:
  1. Sea urchin gonads (uni) as a delicacy in many cuisines
  2. Sea cucumber body wall dried and consumed as bêche-de-mer
  3. Echinoderm-derived compounds with potential medical applications like anticoagulants, antimicrobial agents, anti-inflammatory compounds, and biomaterials for bone and tissue regeneration
  4. Urchins and sea stars popular in the aquarium trade
  5. Echinoderms serving as bioindicators of environmental health and pollution