Merisiilit

Merisiilit, known in English as sea urchins, are a group of spiny marine invertebrates in the class Echinoidea within the phylum Echinodermata. They range from small, round forms to larger, flattened species and are characterized by a rigid, ball- or disk-shaped shell called a test, composed of fused calcium carbonate plates. The surface bears movable spines and tube feet, and adults typically show pentaradial symmetry, while their larval stages are bilaterally symmetric.

Sea urchins are found in oceans worldwide, from shallow coastal areas to deep-sea environments. They inhabit a variety of substrates, including rocky floors, coral reefs, kelp forests, and soft sediments. As grazers, they feed mainly on algae, biofilm, and detritus, using a specialized feeding apparatus known as Aristotle’s lantern to bite and rasp surfaces. Some species are omnivorous or detritivorous, contributing to the control of algal growth and the turnover of organic matter.

Reproduction in sea urchins is typically sexual, with separate sexes. Most species release eggs and sperm into the water column, where external fertilization occurs. The resulting free-living larvae pass through a staging future known as echinopluteus before metamorphosing into juvenile urchins. Many species have complex life cycles that involve planktonic stages before settlement.

Humans interact with merisiilit in several ways. Certain species are harvested for food, particularly the gonads in some regional cuisines, collectively known as uni. They are also important in scientific research and play a vital ecological role as mid-level consumers in marine ecosystems. Some populations face pressures from overfishing, habitat degradation, and climate change, prompting conservation measures in various regions. The fossil record of sea urchins extends back hundreds of millions of years, highlighting their long evolutionary history.