Description of biotope or habitat type
Molgula manhattensis with a hydroid and bryozoan turf on tide-swept moderately wave-exposed circalittoral rock
Habitat (physical) description
|Salinity||Full (30-35 ppt)|
|Wave exposure||Moderately exposed|
|Tidal streams||Moderately strong (1-3 kn)|
|Substratum||Bedrock, cobble, sand|
|Zone||Circalittoral - lower, Circalittoral - upper|
|Depth Band||0-5 m, 5-10 m, 10-20 m|
Download comparative physical and biological data. The comparative tables enable a rapid comparison of the species composition and principal physical characteristics between a given set of biotopes.
- Records used to define the biotope (core records)
- Other records assigned to this biotope, marked as 'certain'
- Other records assigned to this biotope, marked as 'uncertain'
- Predicted extent of the level 3 (for sublittoral rock & deep sea) or 4 (for sublittoral sediment) habitat
This biotope is typically found on slightly sand-scoured, tide-swept, moderately exposed circalittoral bedrock and cobbles. It is commonly recorded from the shallower reaches of the circalittoral around depths from 5m to 15m BCD, as it occurs mostly in very turbid waters. From afar, the physical characteristics are usually silted bedrock reefs and cobble, interspersed with patches of clean sand, causing a scour effect on the rock. Dense aggregations of the ascidian Molgula manhattensis form a silty mat on the rock and there is a sparse hydroid and bryozoan turf. A hydroid turf, composed of Nemertesia antennina, Halecium beanii, Hydrallmania falcata, Sertularella gaudichaudi, Tubularia indivisa and Alcyonium digitatum, in varying amounts, occurs at most sites on the tops of boulders and ridges. A bryozoan turf is also present, but not usually dense and includes Flustra foliacea, Alcyonidium diaphanum, Electra pilosa and the crust-forming bryozoan Conopeum reticulum. The polychaete Lanice conchilega thrives in the sandy patches which often occur between the rock ridges. The scour effect tends to reduce the diversity of sponges present with only Halichondria panicea occasionally present. Isolated clumps of the polychaete Sabellaria spinulosa may be present but they do not occur in dense aggregations as in the Sspi.ByB biotope. The anemones Urticina felina and Sagartia troglodytes may occur in cracks between cobbles or on stones buried in the sandy substratum. The anemone Sagartia elegans is more commonly found attached to crevices in the bedrock. Other species such as the hermit crab Pagurus bernhardus, the barnacle Balanus crenatus, the polychaete Sabella pavonia and Pomatoceros triqueter may all be present whilst the crab Pisidia longicornis may be found under cobbles and stones. Records of this biotope are distributed along the south coast of England and the north Wales coast as well as Pembrokeshire near the entrance to Milford Haven.
As this biotope is often recorded on soft rock (chalk), soft rock communities (SfR biotope complex) would be observed in close proximity with species of the genus Polydora and piddocks (Pholas spp. and Barnea spp). Moderately exposed kelp forests may be found within the shallow infralittoral zone.
|Taxon||Relative importance of taxon for defining this community (%)||Typical abundance - SACFOR scale||% of core records where taxon was recorded|
|Halichondria panicea||1||Occasional||21-40 %|
|Tubularia indivisa||4||Occasional||61-80 %|
|Halecium beanii||1||Occasional||21-40 %|
|Hydrallmania falcata||1||Occasional||41-60 %|
|Sertularella gaudichaudi||2||Occasional||41-60 %|
|Nemertesia antennina||2||Occasional||41-60 %|
|Alcyonium digitatum||3||Occasional||41-60 %|
|Urticina felina||4||Frequent||41-60 %|
|Sagartia elegans||2||Occasional||41-60 %|
|Sagartia troglodytes||1||Occasional||21-40 %|
|Sabellaria spinulosa||2||Occasional||41-60 %|
|Lanice conchilega||5||Occasional||61-80 %|
|Sabella pavonina||1||Occasional||21-40 %|
|Pomatoceros triqueter||2||Occasional||41-60 %|
|Balanus crenatus||1||Occasional||41-60 %|
|Pagurus bernhardus||4||Occasional||41-60 %|
|Pisidia longicornis||1||Occasional||41-60 %|
|Alcyonidium diaphanum||11||Occasional||81-100 %|
|Conopeum reticulum||1||21-40 %|
|Electra pilosa||1||41-60 %|
|Flustra foliacea||7||Occasional||61-80 %|
|Asterias rubens||9||Frequent||81-100 %|
|Molgula manhattensis||15||Frequent||81-100 %|