Echinoderms: München
@ 2004 Taylor & Francis Group,
- Heinzeller & Nebelsick (eds) London. ISBN 04 1536 481 7
Abundance and distribution of ophiuroids off Helgoland, German Bight (North Sea)*
K. Boos & H.-D. Franke
Biologische Anstalt Helgoland, Alfred Wegener Instirute.for Polor antl Ilarine Research, Helgoland, Gernanl
Ophiuroids are thought to significantly influence ben- thlc cornmunities by means of bio rbation, processing of organic matter, and as links in local food chains.
This is mainly due to their abundance, feeding behav- iour and high activity levels (Summers & Nybakken 2000). Apart from variotts factors influencing their occurrence, such as hydro dynamics, depth and tem- oerature, the sediment structure in particular deter- mines the spatial distribution ofthe species, reflecting their lifestyle and feeding behaviour.
Therefore, the objective ofthe present study is to give an overview of the species diversity of ophi- uroids off Helgoland and to investigate whether the local species show a preference towards a specific sub- stratum and how this preference affects their spatial d i s t r i b u t i o n .
ln rotal. 24 sediment samples lVan Veen grab. 0.1 mr1 and additional dredge samples were taken at eight sta- tions rvith different sediment types in the vicinity of the island of Helgoland (Fig. l). The samples were examined for ophiuroids and grain size of sediment.
Only adult specimens wet'e considered and identified to the species level.
Six ofnine species, which were previously recorded lbr the German Bight (Gerdes 1977 ) were confirrned in this study (Table l). All ophiuroids classified as
"common" as well as Acrocnida brachiata, which was graded as "rare" by Gerdes (1977), rvere found.
The absence of Amphiura chiajei, Ophiura alJinis and Ophiura sarsi in our samples, confirms thcse species' scarce occurrence in the German Bight as noted by Gerdes (1977).
The epibenthic Ophiura albida washighly abundant on all types of sediment. Ophiura albida maintains a variety of feeding mechanisms such as predation on benthic prey, as well as carrion and deposit f-eeding (Jangoux & Lawrence 1982). This makes the animal rather unselective towards specific substrata.
Other species showed preferences towards certain substrata. In this regard Ophiura ophiurc was found in low numbers, only on muddy substrata. Our results are in agreement with Ursin (1960). Earlier investigations
* Submitted as an extended absrracr
Figure 1. Sampling stations (1-8) around Helgoland, German Bight (North Sea)
prior to 1930 revealed high abundances of Ophitu"a ophiura on al1 types of substrate, but since then the species probably became rare and concentrated on muddy areas (Ursin 1960). Suggested dependence on the distribution of Sagitta species as food supply seems unlikely concerning the broad and unselecttve feeding behaviour of Ophiura ophiura (Feder I 98 I ).
The fairly high rates of occurrence of Amphiura filiformis on muddy sediments refer to its infaunal lifestyle. Summers andNybakken (2000) point out that, due to its increased grain surface, fine sediment con- tains more organic matter and has higher microbial activity than coarse substrata. Accordingly, fine sedi- ment is more suitable for a burrowing life style.
The "rareness" of Acrocnida brachiatq in the North Sea might be due to a temperature limitatlon (Ursin 1960). Considering its infaunal lifestyle, the low numbers of findings here suggest a preference towards fine sandy or clayed mud. Apart from deposit feeding, suspension feeding seems more obvious for Acrocnida brachiata. Hence, Ursin (1960) suggested
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Table 1
samples and clredge samples (in brackets;
Sediment strltcture F ine sand
shells. stones Coarse sand
shel ls. pebbles Mud & clay blended
with sand
f,',?;o ''.r.0
Stations
J
Ophiura albida Ophitu'tt ophinu A rn p h i u ru.fi I ifot. nt i s Acrocnidu bruchiatcr Ophiotltix;fragiIi;i A m p h i p h o I is .s c1 u tt trr o t ct
l I 0 0 0 0 ( 1 0 ) I
22(176) 0 5 0 0
86(3e2) 6(270) 0(3 ) i ( 3 ) 0 0
3 s ( 1 0 I ) 0 3 ( 1 8 ) t ( 1 ) 0 U
0 ( 1 ) 89( I 0) 0 ( 2 ) 0 1 l 0
0 2 0 0 0
2 0 0 0 0
0 7 0 0 0
a correlation befween the distribution of plankronic communities and the occurrence ot. Acrocnicla brachiata.
. Ophiothrirfingrlrs is known to be very common ancl hi-ehJy abr.rndant rn the cenrral North il;i;d;
partly found on soft bottom, it is mostly Iil;;;ä;:d to shells. large stones or sponges dependin.s ;;;.;
l-'lj'Il:h ':lplv planttonic röo,r tuo.t.nrE
"l-s2i t
rne onJy suitable substr
present in the riere il;Ji?;l} T::X.T'Ji;'#i ,ä
individuals.
According to its preferred habirat. u,hich is mainly in the littoral zone under stones anä bould.;; iü;;
i 960), occurrences of A m p h ip ho k s s;;;';;';r r;' grab samples \\€re rare. AAäitionauy, ;;r;;;;;"i;, i;
dredging could be due to the ,r"
"iäi. .ä,_.iä.iie smaller than the mesh size.
I.n, conclusion. rhe :parrrl distribution of the onhi_
rurords scetns to be affected by sedimenr ,,ru.*n. äna.
additionally, by the species, ieeding b.h;;;;;i";
determines the animals fif.r,yr..- öp-f,ir;;ä, "i;;.
o_phiura atbida) maintaining diilerent ;;ä;;;? üä- mg, seem to be more randomly spread on difibr;;i;;_
',.i:"
:lii rl*e being speciatiz.ä in r".oing il;;;ä;.
, rn orcter to get more detailed informatiJn
"n dr.t;i_
butronai patterns and their causes, further fi.fJ.rä laboratory investigations are requrred.
ACKNOWLEDGEMENTS Many thanks to Reinhard
and Reinhold Mülter for n-,t^1Pl:T.:*',' LJr' Q111s'w
fqru "'*: iF *:l.# l,,"#ft s;'; ,li:r[
for Polar and Varine Rcscrrch lor frnancial ,rOOa"..
REFERENCES
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Gerdes. D. 1977. Zur yefieil
,^ -?_. ul,.,l:n Bucht Dip romllse,l1 j::ii,f:ft.'l'
",
Jangou1,. M._& Lawrence, J.M. (eds.) tSti. Eri'*oan,n n u lr tti on. Rotterdam: Balkema.
Mortensen, Th. 1927. I{undb
B r i t i s h l"/", i
";;;;:' "{;,:lpil:r", (i,r,i{:,:;,' ;{lhi
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S u m n r e r s . a . C . & \ y b a k k e n . J. 2000. Brirrle irar disrribu_
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