Thomas' sea urchin (Prionocidaris thomasi)

Classification (WoRMS)

Full classification as recorded by WoRMS: Kingdom: Animalia · Phylum: Echinodermata · Class: Echinoidea · Order: Cidaroida · Family: Cidaridae · Genus: Prionocidaris.

Accepted name: Prionocidaris thomasi · Authority: (A. Agassiz & H.L. Clark, 1907) · Taxonomic status: accepted · AphiaID: 513493.

Synonyms and superseded combinations recorded for this taxon: Actinocidaris thomasi (A. Agassiz & H.L. Clark, 1907); Leiocidaris thomasi (A. Agassiz & H.L. Clark, 1907); Phyllacanthus thomasi A. Agassiz & H.L. Clark, 1907.

Source: World Register of Marine Species (WoRMS) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=513493 (CC BY 4.0). Accessed 2026-09-23.

Biology and life history (SeaLifeBase)

Measured values published by SeaLifeBase for Prionocidaris thomasi. Only fields the database actually holds for this species are shown; these are database values, not aquarium guidance.

Habitat zone: benthic · Aquarium trade: never/rarely.

SeaLifeBase notes: Mainly found in deeper waters, but may be observed in waters as shallow as 9 m. May host small molluscs in shallower waters and barnacles in deeper waters. Stays under coral slabs and in crevices during the day (Ref. 122680).

Source: SeaLifeBase species summary (SpecCode 169188) — https://www.sealifebase.se/summary/Prionocidaris-thomasi.html (CC BY-NC 3.0 — non-commercial use only). Accessed 2026-09-23.

Distribution and occurrence records (GBIF)

23 georeferenced occurrence records for Prionocidaris thomasi are published through GBIF from 3 countries or marine areas. The map above plots 10 distinct locations from a sample of 23 records, omitting 3 inland records that carry an institution's coordinates rather than a collection site; it shows where the species has been observed and reported, not a modelled range.

Most records by country or area: United States of America (20); United States Minor Outlying Islands (2); New Caledonia (1).

Source: GBIF Occurrence Search (taxonKey 5965639) — https://www.gbif.org/occurrence/search?taxon_key=5965639&has_coordinate=true (records CC0 / CC BY); WoRMS distribution records (MarineRegions) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=513493 (CC BY 4.0); Natural Earth 110m basemap — https://www.naturalearthdata.com/ (public domain). Accessed 2026-09-23.

Description and natural history (Wikipedia)

This urchin is characterised by its fuzzy-looking spines and bright maroon color. The fuzz on its spines is caused by algae growth, and its characteristic maroon color comes from the smaller secondary spines located below the larger, thicker spines. The largest size it can reach is about 5 or 6 in including their spines.

Prionocidaris thomasi uses small tube feet underneath its body to move across the rocks it inhabits. It uses five calcareous teeth, called Aristotle's lantern, to scrape algae off rocks.

Prionocidaris thomasi can reproduce both sexually and asexually. Like most other echinoderms, Thomas's urchin releases gametes into the water so that they can be fertilized during mass spawning events. After a few months of swimming around and eating phytoplankton, larvae of P thomasi will find a suitable area to transform into their juvenile form, which more closely resembles their adult form.

This species of urchins is native to the central and eastern Pacific Ocean. It can be found across the islands within the Papahānaumokuākea Marine National Monument, as well as on Kauaʻi and Oʻahu. It is typically found around 1 100-150 m deep but can be found in shallower areas as well. However, it is occasionally seen in as little as 9 m of water in places like Sharks Cove on the North Shore of Oahu and other protected reefs, such as those around Midway Atoll.

Prionocidaris thomasi, just like many other sea urchins, faces the threat of aquarium collectors, and climate change which causes coral to bleach and die.

Source: Wikipedia: Prionocidaris thomasi — https://en.wikipedia.org/wiki/Prionocidaris_thomasi (CC BY-SA 4.0). Accessed 2026-09-23.