AquaticCare difficulty: IntermediateLegal complexity: Low

St Thomas mushroom

Rhodactis osculifera · also called St. Thomas mushroom, Warty corallimorph, Discosoma sanctithomae, Rhodactis sanctithomae

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Called the warty corallimorph in the field, this Caribbean mushroom has a flat disc bumpy with wart-shaped tentacles that split into forks at the tip. Moderate light and low to moderate flow suit it, as they do most corallimorphs, and hobby forms are chosen for brightly coloured warts. Older names such as Discosoma or Rhodactis sanctithomae are still common, but Rhodactis osculifera is the accepted name today. It damages corals it can reach, so keep a clear gap between it and stony corals, and allow for its spread into mats.

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Quick facts

Size	Flattened disc up to about 9 cm across; one field guide gives 2-8 cm.
Lifespan	—
Social needs	solo
Native region	Florida, the Bahamas and the Caribbean
Origin	New World
Climate	Tropical
Water type	Marine
Family	Discosomidae
Genus	Rhodactis

Part of the Mushroom Corals

Soft, disc-shaped corallimorphs (Rhodactis, Discosoma, Ricordea, and bounce morphs) that carpet rockwork in fluorescent colors. Hardy, low-light, low-flow, and among the best beginner reef invertebrates.

Bounce Mushroom Discosoma (Red Mushroom)Giant elephant ear mushroom Rhodactis Mushroom Ricordea mushroom Ricordea Yuma

Color & pattern variants

Natural variants occur in the wild; selectively bred (man-made) variants were developed in captivity.

Natural

Yellow St Thomas Mushroom →

Named for its yellow warts.

Purple St Thomas Mushroom →

Named for its purple warts.

Strawberry and Cream St Thomas Mushroom →

A hobby colour name that points to red and pale tones. No fuller description is on record.

Orange Cream St Thomas Mushroom →

A hobby colour name that points to orange and pale tones. No fuller description is on record.

Red St Thomas Mushroom →

UncommonIntermediate

A vivid green disc almost buried under red-orange forked warts, thickest toward the rim. Near the centre the green shows through, with a few grey-mauve warts around a lilac mouth.

Orange St Thomas Mushroom →

UncommonIntermediate

A green disc with pale radial streaks, dotted with forked tufts in golden yellow to amber orange. The tufts sit further apart than on the red form, so more of the green shows, and the raised mouth is mauve.

representative

St. Thomas Mushroom →

UncommonBeginner

A forked-tentacle mushroom sold for years as a Discosoma and now placed in Rhodactis. It has a cool blue or green base topped with bright branching growths in warm colours or white.

Habitat & enclosure

Give it moderate light and low to moderate flow, on rock in the lower to middle part of the reef. Wild animals occur singly, in small clusters or in mats at about 3-23 m in the Caribbean, the Bahamas and Florida.

Keep stable reef water: SG about 1.025, 24-27 °C (75-81 °F), pH 8.1-8.4.

Substrate

Place it on live rock or a plug in a sheltered spot. It holds on with its own foot, so no glue is needed if it can settle undisturbed.

Equipment & setup

Moderate reef lighting and low to moderate flow are enough. Basic filtration with a skimmer suits it, and no special calcium dosing is needed for this animal itself.

Diet

Relies on light and on food. Its oral tentacles hold large numbers of symbiotic algae, and at night the rim of the disc folds over prey. Give moderate light and offer small meaty foods such as mysis now and then, placed on the disc in low flow.

Behavior & temperament

In the wild most are green tinged with blue, and the hobby has picked out forms whose warts are red, orange, yellow or purple. The forked tentacle tips are the clearest field mark. It can gather into small groups and mats on rock over time, so give it space to spread.

Health

Damages corals it can reach. In laboratory and field work, under the name Discosoma sanctithomae, it attacked adjacent stony corals with modified marginal tentacles and mesenterial filaments, and every Agaricia agaricites colony transplanted beside it was damaged. Keep a clear gap between it and stony corals, and allow for its spread into mats. If the disc shrinks or stays folded, review light and flow first, since this animal does best with moderate amounts of both.

Tips, DIY & hacks

Check for the forked tentacle tips when identifying it. Start it low in the tank, since it does well in moderate light. It damages corals it can reach, so keep a clear gap between it and stony corals, and allow for its spread into mats. Search under the older names too, because much hobby writing uses Discosoma or Rhodactis sanctithomae.