Thorny Sea Star (Protoreaster Sp.)

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The Thorny Sea Star is a striking marine invertebrate recognised by its five-pointed shape, thick arms and distinctive raised knobs or spines across its upper surface. Members of the genus Protoreaster include colourful species such as the Red Knob Sea Star (Protoreaster linckii) and Chocolate Chip Sea Star (Protoreaster nodosus). Exact identification depends on the specimen's colour and markings. 

These sea stars are fascinating additions to large marine aquariums, where they move slowly across the substrate in search of food. They are typically found in shallow tropical habitats, including sandy lagoons, seagrass beds and sheltered reef areas.

Although visually appealing, Thorny Sea Stars are not suitable for most reef aquariums. Depending on the species, they may consume soft corals, coral polyps, sponges, clams, tube worms and other invertebrates. They are better suited to a carefully planned fish-only aquarium with open sand and plenty of room to move. 

Successful long-term care requires stable marine water conditions and a reliable feeding routine. They should not be assumed to survive solely on leftover food or natural aquarium growth, as inadequate nutrition is a recognised problem in captivity. Acclimate slowly and avoid exposing the animal to air during handling.

Quick Care Guide

• Scientific Name: Protoreaster sp.

• Common Name: Thorny Sea Star

• Family: Oreasteridae

• Type: Marine Invertebrate — Sea Star

• Adult Size: Up to approximately 30 cm, depending on species

• Temperament: Generally peaceful, but predatory

• Care Level: Moderate to Difficult

• Minimum Aquarium: 300 L+ recommended; larger is preferable for adults

• Temperature: 24–27°C

• pH: 8.1–8.4

• Salinity: 1.023–1.026 SG

• Diet: Carnivore / Omnivore, depending on species

• Food: Suitable chopped seafood and species-appropriate meaty foods

• Tank Level: Substrate and open aquarium surfaces

• Substrate: Open sand bed preferred

• Reef Safe: No

• Suitable For: Large, established fish-only marine aquariums

• Handling: Avoid exposing to air and handle gently

• Important: Sensitive to sudden changes in salinity and water chemistry

Frequently Asked Questions

Is the Thorny Sea Star reef safe?

No. Protoreaster sea stars can eat corals and other sessile invertebrates, so they should not be kept in a mixed reef aquarium. 

How big does a Thorny Sea Star grow?

Some species can reach approximately 30 cm across. Adult size varies depending on the exact species. 

What does a Thorny Sea Star eat?

Depending on the species, it may consume small marine invertebrates, sponges, coral tissue and other meaty foods. Offer appropriate food directly and monitor that it is eating.

Can it live with marine fish?

It may be compatible with suitable fish in a large fish-only aquarium. However, small, sleeping or weakened fish may be at risk with some specimens.

Does it need sand?

A spacious open sand bed is recommended because these sea stars naturally forage across sandy or muddy habitats and can struggle to navigate tightly packed rockwork.

Is it suitable for beginners?

It is not the best choice for beginners. Correct identification, careful acclimation, stable water quality and consistent feeding are important for long-term success.

Common Mistakes

• Adding a Thorny Sea Star to a reef tank with corals and ornamental invertebrates.

• Assuming every Protoreaster species has identical care requirements.

• Providing insufficient food and allowing the sea star to starve.

• Acclimating too quickly to changes in salinity or temperature.

• Handling roughly or exposing the sea star to air.

• Keeping a large adult in a small aquarium with limited open substrate.