An enriched Chaetopterus tube mat biotope in theeastern English Channel

E.I.S.Rees*, M. Bergmann, M. Galanidi, H. Hinz, R. Shucksmith and M.J. Kaiser

School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey,Wales, LL59 5AB, UK.*Corresponding author, e-mail: oss058@bangor.ac.uk

Patches of a very dense tube mat biotope were found during ¢sh habitat studies in the eastern EnglishChannel. At three locations in the lows between linear sand banks o¡ the French coast an un-describedsmall Chaetopterus sp. occurred with small Lanice conchilega as an enriched sediment stabilizing biotope.Thisbiotope was distinct though having similarities to other tide swept sub-tidal biotopes dominated byL. conchilega. Using cameras and side-scan sonar it was seen to overlay heterogeneous cobbles and shellhash with intermittent rippled sand veneer. The patchiness of this enriching biogenic feature contributedto the variability in trawl catches of ¢sh.

INTRODUCTION

Classi¢cations of sub-tidal sedimentary biotopes fromnorth-west European shelf seas (Connor et al., 1997,2003) include several categories wherein tube worms areso abundant that they form biogenic mats. Such matshave important ecological functions in trapping ¢ne sedi-ment and in provision of food or shelter for small ¢sh(Auster et al., 1997). A worm tube dominated biotope,di¡ering from those previously recognized, is describedhere from the eastern English Channel. The character-izing species in this case was a small sized, but not yetdescribed species of Chaetopterus. It occurred together withvery large numbers of small Lanice conchilega. The small

form of Chaetopterus has an easily collapsed muddy tubeand is distinct from the widespread large parchment tubebuilding species (Mary E. Petersen, personal communica-tion). Similar specimens of a small Chaetopterus, collectedo¡ Brittany had previously been sent to Dr Petersen byProfessor M. Glemarec. Taxonomic descriptions of thenew species are in preparation by Dr Petersen.

MATERIALS AND METHODS

Habitats were studied at a series of locations in theEnglish Channel, previously ¢shed by the Centre forEnvironment, Fisheries and Aquaculture Science

J. Mar. Biol. Ass. U.K. (2005), 85, 323^326Printed in the United Kingdom

Journal of the Marine Biological Association of the United Kingdom (2005)

Figure 1. Photographs of the Chaetopterus tube mat biotope. Vertical views are at di¡erent enlargement scales. Image (A) 0.3macross, image (B) 0.04 m across.

(CEFAS) for stock assessment purposes.The primary aimswere to compare the diversity and consistency of ¢shcatches with the benthic environments, and to help de¢nethe habitat requirements of three £at¢sh species (Hinz etal., 2003). At each site observations were made by digitalside-scan sonar to indicate ground type mosaics prior todeployment of sledge mounted underwater television andstill cameras. The video system was a Rovtech Seacamcamera pointing obliquely forward and recording on aSony digital 8 tape recorder. The still camera was aPhotosea 1000 system arranged with the camera pointingdirectly downwards, with the lens 0.7m above the bed,and the strobe at 60 degrees to it. Photographs weretaken at 42 second intervals spread along each 30 minutecamera sledge tow. Distances covered by camera sledgeand trawl tows were calculated from shot and haulingpositions derived from Digital Global Positioning Systemsatellite navigation. Spread randomly along each of theprevious CEFAS trawl tow lines a 0.1m72, day grab wasdeployed four times, with extra samples sometimes beingtaken if the side-scan showed obviously di¡erent groundtypes at opposite ends of the tows. These macrobenthossamples were sieved over 1-mm mesh before preservationfor later laboratory processing. Four tows of 5min durationat a speed of two knots were made with a 2m small meshbeam trawl and four tows of 20min duration at four knotswere made with a 4m, beam trawl with a 82mm cod-endmesh. Both trawls had chain matrix. Material from thetrawls was sorted, enumerated and weighed at sea on aPols motion compensated balance accurate to 1g. Thecamera tows and grabs were made prior to using thetrawls at each site.

RESULTS

The Chaetopterus tubes were noted at three of the local-ities in the eastern English Channel sampled during a

cruise on RV ‘Prince Madog’ in August 2002.The stationswith the tube mat (Table 1) were on the French side of theeastern English Channel o¡shore from Boulogne and LeTouquet.

Photographs of the seabed with the Chaetopterus andLanice conchilega tube mat biotope are shown in Figure 1.Di¡ering scales of enlargement from the original 35mmcolour slides are used to illustrate the habitat and thedisposition of the Chaetopterus tubes. Of 83 interpretablephotographs taken at the three localities, 32 showed thedense tube mat biotope with Chaetopterus. Assuming conti-nuity between sequential ¢lm frames during a camerasledge tow, the tube mat patches seen were spread alongan estimated combined distance of 0.38 km. As video

324 E.I.S. Rees et al. Chaetopterus tube mat in the English Channel

Journal of the Marine Biological Association of the United Kingdom (2005)

Table 1. Stations in the eastern English Channel where theChaetopterus tube mat biotope was found in August 2002.Positions of the camera sledge tows, and the limits of trawlingareas within which the 2 m and 4 m beam trawls and grabs weredeployed.

Station code 69 71 72

Camera towsDeployedLatitude 50846.85’N 50839.78’N 50833.62’NLongitude 01833.16’E 01831.00’E 00832.74’E

RecoveredLatitude 50847.12’N 50839.81’N 50833.35’NLongitude 01833.48’E 01830.81’E 00832.73’E

Distance towed km 0.71 0.57 0.50Depths m 19^20 26 16^17

Beam trawl and grab area spreadNorth-east limitLatitude 50847.99’N 50839.67’N 50834.90’NLongitude 01834.27’E 01830.94’E 01833.10’E

South-west limitLatitude 50846.64’N 50838.17’N 50832.12’NLongitude 01832.88’E 01830.33’E 01832.28’E

Table 2. Numerical abundance of the more common species ingrab samples as N/0.1 m2. Columns A, B and C based on the ¢vegrab samples which had 420 Chaetopterus sp. in each.Similarity values are per cent contributions to the similarity ofthe grouped stations analysed by SIMPER after square-roottransformation. Column D has median abundances from sampleswith 520 Chaetopterus but 450 Lanice conchilega.Column E has median abundances from samples with few ornone of either tube building species.

A B C D EMedian Maximum Similarity

Tube building andtube attached oroccupyingChaetopterus sp. 34 609 8 3 0Lanice conchlega 416 2280 22 352 2Eumida sanguinea 101 178 10 53 0Apseudes latreilli 3 571 4 1 0Phtisca marina 17 90 4 6 0

InfaunaNotomastus

latericeus

40 129 6 10 0

Nemerteaindet.

14 136 3 15 6

Echinocyamus

pusillus

31 113 5 6 0

Poecilochaetus

serpens

22 118 5 13 0

Lagis koreni 21 119 3 20 2Echinocardium

cordatum

1 85 10 0

Cirratulidae indet. 20 29 5 9 0Mysella bidentata 3 41 2 3 0Urothoe

brevicornis

7 28 2 0 2

Ensis arcuatus 3 22 2 5 3Sipuncula indet. 1 24 1 0Glycinde

nordmanni

6 14 1 1

Abra alba 1 14 12 0Aonides

pauchibranchiata

2 12 0 0

Mobile epifaunaOphiura ophiura 5 57 1 1 0Abludomelita

obtusata

11 48 2 7 0

Polinices pulchellus 2 15 7 0Asterias rubens 1 6 0 0

image resolution was lower than from the stills camera,the Chaetopterus tubes could not always be reliably distin-guished from those of other species. However, the openapertures of tubes of Chaetopterus could often be seen onvideo monitors as small de¢ned dark spots in comparisonwith the ragged and paler appearance of the fringed topsof the L. conchilega tubes. The underwater television(UWTV) showed a dense tube biotope apparently withChaetopterus and L. conchilega for 41min of a combinedtotal of 108min when the sledge was on bottom at thethree localities. For a further 39min the video showedsparse to moderately dense tubes which appeared to beL. conchlilega. This equates to 0.45 km over the ground forthe dense tube mat. The di¡erence from the still cameraestimate was due to ¢lm running out before the UWTVwas recovered. On the enlarged still photographs, thelengths of the shadows implies that the tubes of the smallChaetopterus species often protruded from the sediment by

only about 0.7^1.2 times their aperture diameter, but somemore exposed tubes were seen lying collapsed on thesediment surface.

The numbers of individuals per 0.1m72 of the moreabundant taxa in the ¢ve grab samples that each containedat least 20 individual Chaetopterus worms are shown inTable 2. This arbitrary cut o¡ was chosen to coincide witha break in the ranked abundance sequence. For compar-ison, the table also shows median abundances of the samespecies in samples with more than 50 Lanice conchilega butfew Chaetopterus. Median abundances in samples from thesame trawl tow locations with very few tubes of eitherspecies are also shown. This indicates how sparse thebenthos was without the in£uence of the worm tubes. Forthe whole 14 grabs, high variance to mean ratios of 460 inthe counts of Chaetopterus and 914 for L. conchilega indicatedthat both species had strongly aggregated distributions.Video observations con¢rmed this patchiness. The grabswere, however, spread over the length of the previousCEFAS trawls and not targeted at where the shortercamera tows were taken. Of the ¢ve grabs, only one seemsto have coincided with the very dense Chaetopterus tube matas seen on the photographs and video.

Table 3 lists the mean wet weights, from the beamtrawls, of the more abundant taxa by di¡erent life-formgroups. Some of these, such as the attached epifauna,mainly came from habitats other than the tube mat. Thetrawls would have swept several habitats, includingpatches of exposed cobbles and shells. On average the 4mtrawl picked up 29 kg of stones and 5 kg of shells per20min tow and 3 kg of stones and 2 kg of shells were in5min tows of the 2m small mesh trawl.

The super¢cial sediment collected by grab fromamongst the Chaetopterus tubes was mainly cohesivemuddy sand (median phi diameter 3.04^3.09). Loss onignition was 2.3^3.1%. In the patches where thetubes were most dense the video showed black patchesaround burrows and black coiled worm casts indicatingorganic enrichment and anoxia under the sedimentsurface.

DISCUSSION

In this part of the eastern English Channel there is acomplex of linear sand banks aligned with the tidalcurrents, with a range of more heterogeneous seabedtypes in the lows between the sand banks. These includecobbles, relict shell beds, and exposures of the underlyingbedrocks (chalk and Kimmeridge clay), together withrippled sand sheets (James et al., 2002). The water here isfairly turbid with moderately strong tides (2 knots on meanspring tides) (Lee & Ramster, 1981). The bed surface adja-cent to the tube mat patches had asymmetrical sandripples, with cobbles sometimes showing through thesand, or there was a lag composed mainly of shelly hash.However, within the tube mat patches the surfaceappeared smooth with only slight mounding. The sedi-ment samples taken by grab indicated that the tube mathad trapped a ¢ner and more cohesive muddy sand thanwould be predicted from the strength of the tidal currents.From the presence of Urticina sp. which attach to solidobjects, the tube mat probably formed only a super¢cial¢ne sediment layer.

Chaetopterus tube mat in the English Channel E.I.S. Rees et al. 325

Journal of the Marine Biological Association of the United Kingdom (2005)

Table 3. Mean wet weights (grams) of fauna caught in 2 mand 4 m beam trawls, at the three stations where the tube matwas found, based on a total of 12 tows with each gear type.Species omitted where mean 510 g in 2 m small mesh trawls and525 g in 4 m trawls.

2m smallmesh

4m ¢shtrawl

Attached epifaunaAlcyonidium diaphanum 60 1248Alcyonium digitatum 836Nemertesia antennina 36 479Hydralmania falcata 11Hydrozoa indet. 581Porifera indet. 291Mytilus edulis 190

Mobile epifauna and infaunaAsterias rubens 1261 6748Liocarcinus holsatus 321 849Hinia reticulata 87Ophiura ophiura 65Crangon crangon 63Maia squinado 45 36Pagurus bernhardus 37 28Diogenes pugilator 32Necora puber 21 82Spisula elliptica 14Echinocardium cordatum 11Psammechinus miliaris 230Cancer pagurus 180Spatangus purpureus 63Liocarcinus depurator 32Ophiura albida 26

Squid and ¢shSepia o⁄cinalis 322 5069Pleuronectes platessa 31 11953Platichthys £esus 2240Limanda limanda 482Solea solea 73 294Microstomas kitt 117Scophthalmus rhombus 39Callionymus lyra 136 557Raja clavata 939Trigla lucerna 181Trisopterus minutus 42Agonus cataphractus 11 25

Lanice conchilega was more widespread and the opentoppedChaetopterustubesoccurredmainlywhereL.conchilegawas also particularly abundant. On some enlarged photo-graphs, theChaetopterus tubes appeared tobe themore abun-dant, but this was not so in any of the grab samples.Individuals of Chaetopterusmay each have several tube open-ings in comparison with the single fringed top usual forL. conchilega which might account for L. conchilega being themoreabundantwhenwormsrather thantubeswerecounted.

Under the system of biotope classi¢cations used forthe European Nature Information System (EUNIS)(undated) there is a sub-tidal, L. conchilega dominatedbiotope, occurring in tide swept sand (EUNIS codeA4.123). The Chaetopterus tube mat has obvious similaritiesto this but seems su⁄ciently distinct to be considered sepa-rate at level 5 in the hierarchy.

Based on side-scan sonar and video evidence, the widerextent of the individual trawl tow tracks previously usedfor ¢sh stock assessment would have run over severaldi¡erent sedimentary habitats. These would have includedrippled sands, with cobbles and shell hash both intermit-tently exposed at the bed surface as well as the enrichedtube mat. Such super¢cial deposits, held together byworm tube mats, are liable to wash away when particularcohorts of key species die out, so the tube mat patchescould be ephemeral. This may explain why the biotopehas not been reported before in a comparatively wellstudied inshore sea area.

In north-west European shelf seas there are severalexamples of biogenic habitats where key species createniches for rich and diverse ranges of co-occurring species.In some cases these biogenic habitats even appear as ‘oases’of higher biodiversity amongst wider areas of ‘semi-desert’, impoverished through sand instability or scour.For example, a tube mat found on the edge of Nephropsnorvegicus ¢shing grounds in the western Irish Sea had asuper-abundance of small bivalves and crustaceansamongst a very dense mat of ampharetid worm tubes(Rees & Holme, 1988). Biologically determined benthichabitats seem to be disproportionately productive andbiodiverse in comparison with those at locations overwhel-mingly in£uenced by physical sedimentary processes. Inthe Chaetopterus biotope described here, nine taxa hadmaximum abundances equivalent to over 1000/m72. ForL. conchilega the maximum was equivalent to over 22,000/m72 and for Chaetopterus sp. over 6000/m72. The existenceof such locally enriched habitats has implications for themeso-scale distribution of several commercial ¢sh speciesincluding plaice Pleuronectes platessa (Kaiser et al., 1999)and silver hake Merluccius bilinearis (Auster et al., 1997). Inthe eastern English Channel the plaice biomass washighest in trawl tows with tube mat somewhere in thesame general location. Spatial and temporal patchinessassociated with biogenic features will in£uence the

variability noted in trawl tows made for ¢sh populationmonitoring (Hinz et al., 2003). Knowledge of where thesebiogenic features occur should be an important factor inecosystem based ¢sheries management and interpretationof stock sampling trawl tows.

We are grateful to the Master and crew of RV ‘Prince Madog’for their skill in deploying and towing the range of gear used inthis project, to Ben Powell and Ray Wilton for assistance withoperating the side-scan sonar. Dr Mary Petersen advised on theidentity of the small Chaetopterus sp. Kerry Howell gave pre pub-lication access to revisions of sub-tidal parts of the Joint NatureConservation Committee Marine Habitat Classi¢cation. Clear-ance to operate inside territorial waters was given by theGovernment of France.

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Submitted 26 April 2004. Accepted 8 December 2004.

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