Reproduction

While some asexual reproduction can occur in echinoderms, the majority reproduce sexually, with separate

male and female individuals. Sea Stars, Brittle Stars, and Sea Cucumbers are capable of regeneration; in

fact, if an arm of a Sea Star is removed with a portion of the central disk intact, an entire new sea star can

be regrown. Sexual reproduction usually occurs in late winter through fall and while some engage in the

practice of brooding (most common in Crinoids and Brittle Stars), most echinoderms are known as

broadcast spawners. Broadcasters release their gametes simultaneously into the water and fertilization

occurs within the water column.

As mentioned in the Biology section, echinoderms are actually classified as bilateria because their larvae

show bilateral symmetry, but this is not the only reason. During early embryonic development, the recently fertilized egg

will have concentric layers of cells known as germ layers. Animal embryos will have two or three of these germ layers

depending on the phylum; true radiata like Cnidarians (jellyfish) have two layers and are known as diploblastic.

Chordates, on the other hand (like humans), have three germ layers and are known as triploblastic. These germ layers

are what will eventually become the skin, skeleton, and internal organs. Echinoderm larvae have three germ layers,

which adds to the justification of their classification as bilateria.

Larva of Dendraster excentricus (the Pacific Sand Dollar) showing its bilateral symmetry.

Photo from http://darkwing.uoregon.edu/~remlet/publications.html

The larvae are generally classified into two types based on how they receive their nutrition. A

planktotrophic larva is one that derives its energy from the capture of organic particles, in other words, they

eat (primarily algae cells. It is most common in Sea Stars, Brittle Stars, Sea Urchins, and Sand Dollars. A

lecithrotrophic larva is a non-feeding type. Its energy requirements are met by a yolk reserve supplied from the egg.

The of lecithrotrophs are, naturally, larger than planktotrophs, and as a result fewer numbers of them are shed during

spawning. At some point, the larvae will settle out of the water column onto a substrate of some type and begin their

metamorphosis into their adult forms with their radial symmetry.