Echinodermata
To date, about 7,000 living species and nearly 13,000 extinct species have been identified in the fossil record. Among invertebrates, echinoderms possess a body organization that is considered more advanced than that of most other groups.
Echinoderms are divided into five extant classes:
- Asteroidea (sea stars): They have a central disc and usually five arms, acting as predators of bivalves and other invertebrates.
- Ophiuroidea (brittle stars): Characterized by long, slender arms distinct from the central disc, they mainly engage in suspension feeding on plankton.
- Echinoidea (sea urchins and sand dollars): With spherical or flattened bodies and movable spines, they function as algal grazers, playing a key role in regulating the structure and dynamics of kelp forests.
- Holothuroidea (sea cucumbers): These have elongated, soft bodies with a reduced skeleton and recycle nutrients by feeding on sediments.
- Crinoidea (crinoids): Including feather stars and stalked sea lilies, they are suspension feeders that capture plankton and play an important role in food webs.
From an evolutionary perspective, echinoderms and vertebrates share a common ancestor, and their embryological similarities provide evidence for this relationship. The unique features of echinoderms—particularly the water vascular system and pentaradial symmetry—offer significant insights into the evolution of bilaterians and the diversity of marine invertebrates.
Echinoderms primarily reproduce sexually through external fertilization, with eggs and sperm released into the water column. Some species are also capable of asexual reproduction; through fragmentation and regeneration, they can repair damage and even form colonies.
