Why is it called a 'sea cactus' in Japanese?

Its club-shaped colony resembles a cactus, and it retracts into the sand during the day and extends at night, similar to how a cactus stores water.

Yes—when disturbed it bioluminesces, with the glow spreading across the colony like a wave from the point of contact.

Sea Penウミエラ

Cavernularia obesaNE— Not Evaluated

Cavernularia obesa, known in Japanese as umi-saboten ("sea cactus"), is a colonial sea pen that lives on sandy-muddy seabeds in the warm waters of the Indo-West Pacific, including areas south of Ishikari Bay in Japan. Its club-shaped colony retracts into the sand by day and extends at night to feed on plankton, and it is known for bioluminescing when touched. Its conservation status has not been formally evaluated by the IUCN.

Sea Pen (reference photo) — Photo: opencage / CC BY-SA 2.5 / Source / cropped

Species Data

Classification: Pennatulacea / Veretillidae Source
Size: Forms a club-shaped colony up to about 50 cm long when extended, shrinking to under 10 cm when contracted
Diet: Filter feeder Captures plankton from the seawater with the many polyps arrayed on the colony's surface
Distribution: Found on sandy-muddy seabeds in the warm waters of the Indo-West Pacific, and in Japan south of Ishikari Bay at depths to about 20 m. Source
Sources: aquarium.co.jp

Did You Know?

Its cactus-like colony contracts and hides in the sand by day, then extends at night to open its many polyps and feed on plankton. Source
It is known to bioluminesce when disturbed, with light spreading across the colony in a wave from the point of stimulation. Source

Where to See This Species

Operating facilities registered on this site: Japan 1

Toba AquariumMieOpened in 1955

Source

Conservation Story

Conservation Status

Not Evaluated (NE)

Sources

en.wikipedia.org

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FAQ

Why is it called a 'sea cactus' in Japanese?: Its club-shaped colony resembles a cactus, and it retracts into the sand during the day and extends at night, similar to how a cactus stores water. Source
Does it glow?: Yes—when disturbed it bioluminesces, with the glow spreading across the colony like a wave from the point of contact. Source
How many operating facilities are registered here?: Operating facilities registered on this site: Japan 1 Source

Books & gear

Source: ja.wikipedia.org en.wikipedia.org