Reproduction
Urticina crassicornis produces by both asexual and sexual reproduction. In the Atlantic populations, eggs and sperm are held and fertilized within the body column. The young are brooded between the mesenteries of the body and are emitted as smallish, well developed, young anemones. Spawning occurs in the spring amongst Puget Sound populations, when eggs (yolky, 0.7 mm in diameter) and sperm are released into the sea for fertilization. Urticina crassicornis's major sperm chromosomal proteins have been found to be two specialized histone H1 proteins which indicate a strong relation to the chromosomal proteins of bird and amphibians. After fertilization, a solid and ciliated blastula is created due to superficial cleavage. Six days following fertilization, a cone-shaped and benthic, larval planula develops. These planula then settle onto small rocks or the empty tubes of some annelid worms and rapidly develop into small anemones. 12 days after settlement, 8 tentacles appear. Further growth is slow – two months after settlement, 12 tentacles appear and the anemone is 0.88 mm in diameter; one year after settlement, the anemone has 35 tentacles and is 10 mm in diameter. Growth is proportional to food intake, not age. When starved, this anemone can stay alive for 9 months but does not grow. Urticina crassicornis is sexually mature with a diameter of 10 – 15 mm, being at least one year old.
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