Slender wart weed (Gracilaria gracilis)

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help



A tough and leathery seaweed, bright red, dull purple or black-brown in colour that may bleach in the summer sunlight to a green or yellow colour. The thallus is composed of irregular branches with pointed tips and bears hard knob-like reproductive bodies. The thallus arises from one or more small, fleshy, circular holdfasts, which may coalesce.  The erect fronds normally grow up to 60 cm in length but may reach 90 cm in some nutrient-rich areas. 

Recorded distribution in Britain and Ireland

Widely distributed throughout Britain with most records on the western coast. Common in the northwest and southwest coasts of Ireland. 

Global distribution

Common in the North Eastern Atlantic from southern Norway to Spain. Also recorded in the Mediterranean and the coasts of Africa, Asia, South America and eastern Australia. 


It is found attached to rocks, stones, pebbles or gravel. It is especially common in sheltered areas with sandy sediment as it can survive burial.  

Depth range

Lower littoral to 5 m

Identifying features

  • Thallus up to 60 cm, occasionally 90 cm, in height.
  • Bright red to dark brown-black in colour.
  • Terete thallus.
  • Tough and leathery in texture.
  • Small fleshy discoid holdfast.
  • Unbranching to irregular branching, 2 mm in diameter with pointed tips.

Additional information

Extreme variability between species makes it a difficult family of red seaweeds to identify by eye.  Accurate identification requires PCR testing to distinguish species. This species is cultivated and collected on the coasts of South Africa and Namibia for use in the production of agar. 

Listed by

- none -


  1. Bunker, F.StD.P., Brodie, J.A., Maggs, C.A. & Bunker, A.R., 2017. Seasearch Guide to Seaweeds of Britain and Ireland (Second edition). Plymouth: Wild Nature Press.

  2. Dixon, P.S. & Irvine, L.M., 2011. Seaweeds of the British Isles. Volume 1 Rhodophyta. Part 1 Introduction, Nemaliales, Gigartinales. Exeter: Pelagic Publishing Ltd.

  3. Fish, J,D., Fish, S., 2011. A Students Guide to the Seashore (Third edition). Cambridge: University press.

  4. Guiry, M.D. & Guiry, G.M. 2015. AlgaeBase [Online], National University of Ireland, Galway [cited 30/6/2015]. Available from:


  1. NBN (National Biodiversity Network) Atlas. Available from:

  2. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. Accessed: 2024-04-21


This review can be cited as:

Mardle, M.J., 2022. Gracilaria gracilis Slender wart weed. In Tyler-Walters H. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 21-04-2024]. Available from:

Last Updated: 17/01/2022