TankBud

Sea biscuit

Clypeaster subdepressus

Also known as Sand dollar, Brown sea biscuit, Flat sea biscuit, Sea biscuit urchin

SaltwaterInvertebrate

Care at a glance

Temperature
72–81°F
22–27°C
pH
8.1–8.4
Adult size
7.9 in
20 cm
Minimum tank
53 gal
200 L
Temperament
Peaceful
Social behavior
Solitary
Tankmates
With caution
Swim level
Bottom
Difficulty
Difficult
Breeding
Hard
Rarity
Rare
Reef safe
Reef safe
Family
Clypeasteridae
Origin
Western Atlantic sand flats, North Carolina and the Caribbean to Brazil

Overview

The sea biscuit is a peaceful, burrowing urchin for the deep sand bed of a big, mature tank. It is for experienced keepers.

Did you know

A sea biscuit flipped onto its back takes about an hour to right itself. Once dug up, it is back under the sand in six to twelve minutes, which suits a deposit feeder that needs to stay covered. The sand dollar Dendraster excentricus stands on edge to filter feed instead.

Caring for the sea biscuit

Keep the sea biscuit only in a big, old, lightly cleaned tank with at least 53 gal (200 L) of footprint. It needs a wide bed of mature, coarse aragonite live sand at least 7 to 10 cm deep, with gentle flow. Keep frags on rock, because it will plow under any that sit on the sand.

Hold the water at 72 to 81°F (22 to 27°C) and a pH of 8.1 to 8.4, and keep nitrate low. Never use copper. Drip acclimate it slowly, and never lift it into the air.

The sea biscuit is a full-time deposit feeder that eats organic matter, diatoms and bacteria sorted out of coarse sand. A home sand bed does not hold enough food for it, so it usually starves. Losses tend to come months after purchase, once the bed is picked clean.

It is peaceful and reef safe, since it cannot hurt coral or fish. Keep out sand-sifting gobies, sea cucumbers, sea stars and crabs, which compete for the same food.

Breeding it is hard. Larvae have been reared in the lab and settle after 16 to 28 days, but nobody breeds it for the trade.

Good to know

Clypeaster subdepressus is an irregular urchin in the family Clypeasteridae, order Clypeasteroida. It belongs to the same group as the true sand dollars but is thicker, with a low domed top and a flat underside. Its test is covered in a velvet of tiny spines, and the largest records come near 30 cm. It pushes food along mucus-lined grooves to a mouth in the center of its underside. Its main predator is the king helmet snail, Cassis tuberosa.

In the wild

The sea biscuit lives buried just under the surface of open sand and shelly bottom, with little or no seagrass. It ranges from North Carolina through the Gulf of Mexico and the Caribbean to Rio de Janeiro. It plows forward at a shallow angle, with its front edge under the sand.

Common questions

What size tank does a sea biscuit need?

A sea biscuit needs at least 53 gal (200 L). It spends most of its time in the bottom.

What temperature and pH does a sea biscuit need?

A sea biscuit is comfortable at 72–81°F (22–27°C) with a pH of 8.1–8.4.

What does a sea biscuit eat?

A sea biscuit is a full-time deposit feeder, eating organic matter, diatoms and bacteria sorted out of coarse sand. A home sand bed does not hold enough food for it.

How big does a sea biscuit get?

An adult sea biscuit reaches about 7.9 in (20 cm).

How does a sea biscuit breed?

Sea biscuits are hard to breed. Larvae have been reared in the lab and settle after 16 to 28 days, but nobody breeds them for the trade.

Related species