The pea urchin

This is a small urchin that feeds on detrusis and algae on the bottom of the sea. It is well camouflaged by its color, and live to a depth of 200 meter. It cleans the sand by putting sand in its mouth and cleans the sand from organic material.

The pea urchin has a remarkable strong scelet. The scelet is all that remains after a storm push them the beach. Rarely living pea urchins are also washed ashore. They color light green if they are stressed.

pea urchin reproduction

During the sommer the pea urchin drops eggs and sperm into the plankton were they fertilize. So the first stage of live of the pea urchin is a plankton stage: They swimm in the plankton for months and then quickly transform into a small pea urchin . The plankton stage is called meroplankton: these are the larval stages of benthic organisms that spend the first period of their lifecycle in the pelagic stage before settling on the benthos. Their early larvae have bilateral symmetry, but they develop fivefold symmetry as they mature.
The pea urchin is only centimeter large and has the color of sand. The little spines can be slowly moved to crawl over the sand of the sea bottom.

The meroplankton stage of pea urchin.

Important groups within the meroplankton are the larvae of starfish and sea urchins (echinoderms). There can be so many of them that they block the inlet of water storage systems. The filter can get clogged because of the long spines that the larvae of urchin carries. .

Enemies of the pea urchin

Pea urchin is eaten by fish, especially by dab and haddock.
Sea urchin, sea star and sea lilly and a britlle star on an illustration of Brehm.

meroplankton of the North sea. Urchin, seastar and brittle star have large thorns in the meroplankton stage.

1 chaetate larva of the annelid Platynereis agassizi
2 zoea of sand crab Emerita analoga
3 cyphonautes larva of bryozoa
4 tadpole larva of sessile tunicate
5 pilidium larva of nemertean worm
6 advanced pluteus larva of sea urchin
7 fish egg with embryo
8 trochophore larva of scaleworm
9 veliger larva of snail
10 pluteus larva of brittle star
11 nauplius larva of barnacle
12 cypris larva of barnacle
13 medusa of hydroid.