A hydroid (Diphasia alata)

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



Diphasia alata forms a robust feather-like colony. It is red to chocolate brown in colour and is generally 10 cm to 15 cm in height and 6 cm in width. The main stem is thick and straight with alternate side branches. These side branches give the colony a triangular shape with the branches at the base being longer than those at the apex. The feeding polyps are born on secondary branches (hydrocladium) that run roughly parallel.  The wall of the cup around each polyp (the hydrotheca) is about three-quarters fused to the hydrocladium and arranged opposite each other.

Recorded distribution in Britain and Ireland

Records of Diphasia alata are scattered through Britain and Ireland. Most records are from Northern Ireland with 'The Maidens' (a skerry off the north-east coast) being a stronghold for the species.

Global distribution

Diphasia alata is generally recorded in the northern hemisphere with records in west Norway, north west France, and the Azores.  


Diphasia alata is found in relatively deep water on wave-exposed coasts and is commonly associated with bedrock and stable boulders.

Depth range

30-580 m

Identifying features

  • Robust colony, roughly 10-15 cm in height and 6 cm in width.
  • Red to brown in colour.
  • Triangular in shape.
  • Thick stem with alternate branches that are longer at the base.
  • The sides of the branchlet containing hydrothecae are roughly parallel.
  • The wall of the hydrotheca are roughly three-quarters fused to the branchlet and are arranged by opposite branching.

Additional information

Populations of Diphasia alata within Britain and Ireland are scarce. There have been rapid declines numbers with one of the major threats being indirect exploitation. For example, Diphasia alata is commonly associated with the scallop Pecten maximus, which is targeted by fisheries via dredging. Dredging can negatively impact the population structure of Diphasia alata and can cause mild, moderate and even severe disturbances to the benthic community (Goodwin et al., 2011a).


  1. Goodwin, C., Edwards H., Breen, J. & Picton, B., 2011a.  Rathlin Island - A Survey Report from the Nationally Important Marine Features Project 2009-2011. Northern Ireland Environment Agency Research and Development, no. 11/03. DOI https://doi.org/10.13140/RG.2.1.2611.0962
  2. Goodwin, C., Picton, B., Breen, J. & Edwards, H., 2011b. The Maidens — Report from the Sublittoral Survey Northern Ireland project. Northern Ireland Environment Agency Research and Development, no. 11/02.
  3. Cornelius, P.F.S., 1995b. North-west European thecate hydroids and their medusae. Part 2. Sertulariidae to Campanulariidae. Shrewsbury: Field Studies Council. [Synopses of the British Fauna no. 50]

  4. Hayward, P.J. & Ryland, J.S. (ed.) 2017. Handbook of the marine fauna of North-West Europe. Oxford: Oxford University Press.

  5. Lindeboom, H.J. & de Groot, S.J., 1998. The effects of different types of fisheries on the North Sea and Irish Sea benthic ecosystems. NIOZ Report 1998-1/RIVO-DLO, Report C003/98, p. 404., The Netherlands: Netherlands Institute for Sea Research.

  6. Picton, B.E. & Morrow C.C., 2005. Encyclopedia of Marine Life of Britain and Ireland http://www.habitas.org.uk/marinelife/species.asp?item=D10920, 2008-01-08

  7. Porter, J., 2012. Seasearch Guide to Bryozoans and Hydroids of Britain and Ireland.   Ross-on-Wye: Marine Conservation Society.


  1. NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.

  2. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2024-06-25


This review can be cited as:

Nash, R.A., 2021. Diphasia alata A hydroid. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25-06-2024]. Available from: https://marlin.ac.uk/species/detail/2335

Last Updated: 23/02/2021