Senckenbergiana maritima

I

36

[

(2)

I

99-107

I

Frankfurt am Main 29.09.2006

Micro-habitats of brackish water ostracods from Poel Island, southern Baltic Sea coast DORTHE BORCK & PETER FRENZEL

with 5 Figures and 5 Tables Keywords- Ostracoda (Crustacea), ecology, behaviour, movement, speed, north-eastern Germany

Abstract [BORCK, D. & FRENZEL, P. (2006): Micro-habitats of brackish water ostracods from Poel Island, southern Baltic Sea coast. - Senckenbergiana maritima, 36 (2): 99-107, 5 Figs., 5 Tabs., Frankfurt a. M.]

Five different micro-habitats were checked for ostracod distribution in a brackish shallow water site in the mesohaline salinity range. Ostracods were counted semi-quantitatively and their speed of movement and behaviour were observed. ~he micro-habitats are sandy and muddy sediment (epifaunal and shallow infaunal) as well as filamentous green algae, Fucus vesiculosus (macro-algae) and Mytilus edulis (mussel) colonies. Within the nine documented ostracod species are five species dominating the different microhabitats: Leptocytherepsammophila and Cyprideis rotosa clearly dominate the sediment (L. psammophila shallow infaunal and C. rotosa infaunal to epifaunal). In most sediment samples, Loxoconcha el@tica was found in small numbers, but it is frequent on all other substrates, especially the mussel colonies. Xestoleberis nitida is typical for mussel colonies and Semicytherura nigrescemfor macro-algae.

Kurzfassung Fª unterschiedliche Mikrohabitate wurden in einem mesohalinen Flachwassergebiet an der sª Ostseekª vor der Insel Poel aufihre Ostrakodenfaunen untersucht. Die Ostrakoden wurden semiquantitativ in ihrer H~iufigkeit erfasst und ihr Verhalten sowie ihre Geschwindigkeit in den Mikrohabitaten beobachtet. Bei den fª untersuchten Mikrohabitaten handelt es sich um sandiges und schlickiges Oberfliichensediment (flach infaunal und epifaunal), f'~idigeGrª Blasentang und Miesmuschelkolonien. Unter den neun nachgewiesenen Ostrakodenarten sind sechs Arten hiiufig und f'ª verschiedene Mikrohabitate charakteristisch: Leptocytherepsammophila und Cyprideis torosa sind die dominierenden Ostrakodenarten des Sediments. Loxoconcha elliptica wurde ebenfalls in den meisten Sedimentproben gefunden, wenn auch in geringer Zahl. Dafª war sie auf allen anderen Substraten hiiufig, besonders in den Miesmuschelkolonien. Xestoleberisnitida ist fª die Miesmuschelkolonien und Semicytherura nigreseemfª den Blasentang typisch.

Introduction Most ostracod studies along the southern Baltic Sea coast are based on bulk sediment sampling without distinguishing the different micro-habitats of individual species. Relying on a

great number of publications (compare FRENZEL & VIEHBERG 2004) we have a good picture of the general distribution of ostracods from the southern Baltic Sea. However, we need information about the micro-habitats of ostracod species for the palaeoenvironmental analysis of fossil and subfossil brack-

Authors' address: D6RVHr BORCK,PE*ERFPdr Department of Marine Biology, University of Rostock, Albert-Einstein-Str. 3, 18051 Rostock, Germany; corresponding author: .

9 E. Schweizerbart'scheVerlagsbuchhandlung(N~igeleu. Obermiller),2006, ISSN0080-889X

100

ish water associations anda deeper understanding of ostracod ecology. Our knowledge of Baltic Sea coastal ostracods is still based on general descriptions given by KLIE (1938) and ELOFSON (1941). Furthermore, there are only a few papers studying phytal dwelling European brackish water ostracod associations (e. g. WaCNER 1957, MaRINOV 1964, YASSlNI1969). ROSENFELD'S (1977) important work on the ostracods of the Baltic Sea mainly describes substrate preferences for deeper water ostracods and is therefore only partly usable for the shallow water environment and the discrimination of micro-habitats. The present study provides new data about the preferences of ostracods for different micro-habitats in the shallow water of the southern Baltic Sea. We wish to thank the staff of the department of Marine Biology at the University of Rostock for support in many ways. Kerstin Rieder (Rostock) kindly provided environmental data and supported the sampling. MartŸ Feike (Rostock) helped us with technical details of the laboratory setup. Gª Arlt (Rostock) and Ian Boomer (Birmingham) gave us critical comments on our manuscript. The manuscript was further improved by comments of Alan R. Lord (Frankfurt) and an anonymous reviewer. The present work was supported by the German Federal Environmental Foundation (DBU).

Material and methods The study area is situated in the southern Baltic Sea at the coast of the Poel Island close to a nature reserve on the small island ofLangenwerder and dose to the village Gollwitz (Fig. 1). The sample site is not directly exposed to, but is strongly inŸ by, the Baltic Sea. It is a shallow waterway of about 100 m width a n d a maximum water depth of about 1.2 m between both islands. The ground consists of sandy sediment (median 157-184 pm, sorting 0.51-0.76, silt and clay fraction 0.6-7.0 % ' C o r g 0.1-0.6 %; personal communication Kerstin Rieder) with a few large isolated glacial erratic boulders and is partly covered by filamentous algae and macrophytes especially around the boulders. Large parts of the ground are un-vegetated sediment. Mussel colonies (Mytilus edulis LINNA~US) are clustered locally. Depending on wind direction and speed, marginal parts of the study area may become dry sometimes. However, the area is not influenced by tides, which are very weak in the inner Baltic Sea. Wave energy and slight currents ofbipolar changing directions through the study area may cause some sediment resuspension and form ripple marks. The mean salinity is 12-13 psu, but vades remarkably, as does temperature. In August 2004 we took 12 samples at about midday from about 0.3 m water depth. During sampling the salinity was 11.3 psu, the water temperature 22.1~ The sediment was sampled with acrylic tubes of 10 cm diameter up to about

~'ULANU

I

500 km

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GERMANY

Fig. 1: The sampling site on the southern Baltic Sea coast and in more detail between Poel Island and the small island of Langenwerder. 15 cm sediment depth. Each of the four habitats (un-vegetated sediment, sediment with filamentous algae, sediment with mussel colonies, sediment at a large boulder with attached macro-algae - compare Tab. 1) was sampled three times, except the un-vegetated sediment, which was sampled twice. Macroalgae (Fucus vesiculosus LINNAEVS) were taken from the large boulder and placed into the tubes belonging to it. We transported the tubes with micro-habitats in natural orientation to the laboratory immediately after sampling. "Ihese were kept at room temperature under natural light conditions and the water was ventilated with air. To prevent salinity increase by evaporation, the microcosms in the transparent sample tubes were monitored by daily conductivity measurements, balanced by addition of distilled water and refilled with habitat water if necessary. During the three weeks following the sampling campaign, the microcosms were observed for ostracods and other taxa using a translucent light stereoscopic microscope with a cold light source anda vŸ device for documentation. Speed measurements ofostracods were carried out using video recordings. The time of observations comprised about four hours per subsample within one or two days. When thoroughly observed, the algae and mussels were taken successively from the microcosms and washed over a 100 •m sieve. Then, algae, mussels and sieve residues were checked in a Petri-dish in detail. The upper 1 - 2 cm ofsediment from nine microcosms were sieved over a 125 lam sieve and checked for dead and living ostracods. No ostracods were stored from this study.

Table 1. Overview on sampled habitats. Habitats Sample

Filamentous algae FA1

FA2

FA3

Mussel colonies MC1

MC2

MC3

Boulder with macro-algae BA1

BA2

BA3

Un-vegetated sediment S1

$2

101

It should be noted that the investigated micro-habitats are represented by only three samples each and within one small study area from one sampling day only. Seasonal preferences and the study of regions in other salinity ranges may modify the picture presented here. Our taxonomic work refers to ATHERSUCH et al. (1989) where further details on systematics and taxonomy are given.

Results We observed ostracods within all micro-habitats. Their movements were steady and straightforward in most cases. Their speed differed remarkably between different species and substrates (Tab. 2).

Sediment with filamentous algae (series FA; Fig. 2) Only a few upper millimetres of the sediment (muddy fine sand) were lightly coloured. Al1 sediment below h a d a black colour, disturbed by numerous lighter coloured burrows made by polychaetes. ~fhe macrofauna was poor: some Nereis diversicolor (LINNAEUS) (Polychaeta), Hydrobia spp, (Gastropoda) a n d a few small decapod crustaceans. The filamentous algae "covered the ground only scarcely. We found four ostracod species in all three subsamples: Cyprideis torosa (JONES, 1850) Leptocytherepsammophila GUILLAUMZ,1976 Loxoconcha elliptica BRAMO, 1868 Xestoleberis nitida (CosTA, 1847) Observing the sample tubes for living ostracods, all species were rare (Tab. 3). C. rotosa moved and burrowed very slowly and almost exclusively within the superficial sediment. Often it dug into the sediment and sometimes it climbed over mucous nets produced by polychaetes onto the filamentous algae. The climbing individuals were exclusivelyjuveniles. If an adult C. torosa was illuminated for observation, it tried to escape into the sediment. When placed on coarse sand or glass in a petri dish adults were almost unable to move around. L. elliptica climbed skilfully and fast on algae and was sometimes also on the sediment surface, but more slow. X. nitida occurred on algae only. It moved more slowly than L. elliptiia (Tab. 2).

Fig. 2: Distribution of ostracod taxa within the microcosm with filamentous algae (sample series FA, schematic).

L. psammophila could not be observed within the sample tubes on algae or the sediment surface - we found it first living and dead during sediment sieving (Tab. 4). All the other species from the former observations were found within the sediment except X. nitida. The most abundant species were C. torosa and L. psammophila, both of which were found alive and dead.

Sediment with mussel colonies (series MC, Fig. 3) The uppermost centimetre of the sediment (muddy fine sand) was lightly coloured. The sediment below was black and disturbed by polychaete burrows as in the sample series FA. Each subsample bore a single Mytilus edulis colony of about 10 cm diameter. The remaining sediment surface was empty.

Table 2: Speed (mm/s) ofostracods on different substrates. Up to three measurements per substrate type and species were done. smooth surfaces (mussel shells, macro algae)

filamentous structures (algae, byssus)

on top of sediment

within superficial sediment

C. rotosa (juv.)

0.26

not found

0.11

0.07

C. rotosa (adult)

not found

0.16

not found

0.05-0.12

L. psammophila

0.18-0.20

not found

0.15

0.09-0.14

X. nitida

0.32- 0.37

present

0.17

not found

L. elliptica

0.21-0.52

0.20-0.45

0.34-0.58

not found

E. baltica

not found

0.33

present

not found

Ostracoda

102

Loxoconcha ell.iptica

Xestoleberis

~ ~ ,..:;:,.!

(~

ni~Oa

Leotocythere psammophila

")'~,...~~

! -',: ":::'~ :, ; ..':'_2".'.",'[ " "~ 7 : :

:'-"

Fig. 3: Distribution ofostracod taxa within the microcosmwith Mytilus edulis colonies (sample series MC, schematic).

Acari, isopods and balanids were observed on the mussel shells. Some Cerastoderma lamarcki (PoIRET) (Lamellibranchiata) lived within the sediment beside the polychaetes. Five ostracod species were found: Cytherura gibba (O. E M• 1785) Elofionia bahica (Hn~scnMaNN, 1909) Leptocytherepsammophila GUILLAUME,1976 Loxoconcha elliptica BRaDY, 1868 Xestoleberis nitida (CosTa, 1847) The most abundantlyobserved ostracods were X. nitida and L. psammophila (Tab. 3). Both moved over the rnussel shells in high numbers. However, X. nitida preferred the mussds and L. psammophila the sediment surface as well as trapped sediment between and on the mussel shells. X. nitida often moved along the openings of the mussel shells and even over the pal-

lium margin. L. elliptica was confined to the mussels. This species moved very fast and skilfully over the shells, whereas L. psammophila crawled slowly and unsteadily by comparison (Tab. 2). E. bahica on the mussels and C. gibba on mussels and on the sediment were very rare, The sieved sediment yielded many dead and living L. psammophila as well as some L. elliptica and X. nitida, the latter dead only (Tab. 4).

Boulder with macro-algae (series BA, Fig. 4) The sediment beside the boulder was completely muddy, contrasting with the other habitats studied. This black mud showed a 1 cm thick lighter coloured surface layer, indicating

Table 3: Observationof livingostracods within the microcosms.The sample series from the boulder also comprises specimensfrom the washed macro-algae. The abundanceis givenin increasingorder semi-quantitativelyas one specimenonly (o), 2-5 specimens(X), 5-10 specimens(XX) and more than 10 specimens(XXX). Habitats

Filamentousalgae

Samples

FA1

C. torosa

X

FA2

FA3

Mussel colonies MC1

MC2

Boulder with macro-algae

MC3

BA1

BA2

BA3

o

C. gibba E. bahica

o

C. fuscata

not observed in microcosms

H. viridis

not observed in microcosms

L. psammophila L. elliptica

X

X

X

XXX

XX

XX

X

XXX

X

XX

XX

S. nigrescens X. nitida

X XX

XXX

XXX

X XX

X

X

Un-vegetated sediment $1

$2

103

the presence of at least some oxygen. Burrows of polychaetes crossed the sediment core. The side of the boulder was densely covered by Fucus vesiculosus. Many green algae, hydroids, balanids and bryozoans were attached to the macro-algae. Between the thalli and the sessile epifauna moved acari, nematods, nudibranchs and Corophium volutator (PALLAS)(Crustacea). Six ostracod species were found: Cyprideis rotosa (JoNES, 1850) Hirscbmannia viridis (O. E M• 1785) Leptocytherepsammophila GUILLaUME, 1976 Loxoconcha elliptica BRADV, 1868 Semicytherura nigrescens (Baim), 1838) Xestoleberis nitida (LILJEBORG, 1835) Despite the dense epifauna on the macro-algae, ostracods were rarely observed. We found some S. nigrescens and one slowly moving L. psammophila. S. nigrescens climbed over all surfaces including bryozoans and tubes of Corophium. Rinsing the macro-algae yielded more S. nigrescens and L. elliptica as well as some L. psammophila and X. nitida (Tab. 3). Only L. psammophila occurred in all sediment samples. Surprisingly, C. torosa was restricted to one subsample and in a small number. H. viridis, L. elliptica and X. nitida showed a few specimens only (Tab. 4).

Un-vegetated sediment (series S, Fig. 5) The upper two centimetres of the muddy fine sand were lightly coloured, the sediment below was dark grey and disturbed by many polychaete burrows. Nereis diversicolor and Hydrobia spp. were abundant.

Fig. 4: Distribution ofostracod taxa within the microcosm with Fucus vesiculosus from a large boulder (sample series BA, schematic).

Table 4: Observation of living and dead ostracods from sediment of the microcosms after sieving. The abundance is given in increasing order semi-quantitatively as one specimen only (o), 2-5 specimens (X), 5-10 specimens (XX) and more than 10 specimens (XXX). '12 indicates living specimens and 'D' empty valves or carapaces.

Habitats

Filamentous algae (sandy)

Samples

FA1

FA3

C. torosa

XXX L+D

XXX L

Mussel colonies (sandy) MC2

MC3

Boulder with macro-algae (muddy) BA1

BA2

BA3

Un-vegetated sediment (sandy) S1

X L

c. gibba

not found in sediment samples

E bahica

not found in sediment samples

C. f~scata

oD

H. viridis XX D

X L

XX L+D

L. elliptica

XD

XL

XL

X. nitida

XD

XL

L. psammophila

S. nigrescens

$2

X D

XX L

X L XL

not found in sediment samples XD

XL

X L

XXX L+D oL

oL

104

Leptocythere psammophila

,

": "'2,2-"

;:...~._.,:, ..

.

,

~...v

....., .

~,r

"-,

-~

,

".';

.'.~'_.,'

.

.

,.~

.

,.

,*,,~IK~~

,"

'

,

.'..-."'

9

..

. ~ ' , , ~ . ' , * '

....

~

Z

.

.,

~

,

Fig. 5: Distribution of ostracod taxa on and in un-vegetated sediment (sample series S, schematic).

We found five ostracod species: Cytheromorphafuscata (BKaDY, 1869) Elofionia baltica (HIRSCHMANN, 1909) Hirschmannia viridis (O. E M• 1785) Leptocytherepsammophila GU]LLaUME, 1976 Loxoconcha elliptica BRADY, 1868 The un-vegetated sediment surface was normally barren of ostracods. Only one specimen of E. baltica moved over the ground and one L. psammophila burrowed in the superficial sediment (Tab. 3). The sieving of sediment yielded many living and dead L. psammophila a n d a few dead specimens of II. viridis, L. elliNica and C. fuscata (Tab. 4).

Discussion We checked five different micro-habitats for ostracods in a shallow water site in the mesohaline salinity range. These should be the main micro-habitats in the studied locality except for deeper sediment (>2 cm sediment depth) and dense crops of filamentous algae. Less important in terms of covered areas are substrates such as pebbles or reworked shell layers, till, chalk and biogenic structures such as worm tubes etc. Special micro-habitats may include biogenic structures, as HORNE (1982) points out for Sabellaria reef dwelling ostracods in a normal marine environment, Seven ostracod species were observed within the microhabitats. ~his is a much lower diversity in the brackish water than reported from ostracod associations on plants and animals in fully marine environments (e. g. WHaTLEY & WALL I975, KAMIYA1983, NOHARA& TABUKI 1990). The most abundant ostracod species in our microcosms was Leptocytherepsammophila. It occurred mainly within the uppermost sediment layer in all studied habitats and often moved on the sediment surface too. Also, L. psammophila was relatively abundant within the mussel colonies. However, it

was mainly restricted to trapped sediment between the shells. This preference ofsandy sediment is confirmed by earlier investigations (Gu~LLat3ME 1988, ATHERSUC~tet al. 1989, HORNE & BOOMER 2000). Our observation showed fast moving and burrowing animals on the sediment surface, but slowly and unsteadily moving specimens on smooth and small surfaces such as shells and plants (Tab. 2). ~he second abundant ostracod species on and in the sediment was Cyprideis torosa. This is clearly the most common ostracod species in the shallow water of the Baltic Sea. It is typical of muddy ground in brackish shallow water (AxtlE~SUCH et al. 1989, MEISCH 2000). Surprisingly, C. torosa only dominated the sediment beneath the filamentous green algae. We expected it to be dominant in the muddy sediment but it was oumumbered by L. psammophi/a, This fact could be partly explained by specimens of the latter species originating from trapped sediment on the macroalgae and the boulder above as the relatively low number of ostracods within the sediment supports. We assume that the frequent sediment redeposition in our study area is most problematical for the slow-moving C. rotosa in contrast to other affected ostracod species and causes the pattern of non-dominance of C. torosa in most stations. Another reason is our sampling within the reproduction time of other species, whereas C. torosa reproduces only once ayear with larvae first appearing in March (HEIP 1976). Highest ostracod densities and diversities were found within the mussel colonies. This corresponds with our unpublished observations on ostracod associations from Dreissenapolymorpha (PALLAS)(a freshwater bivalve) colonies in the oligohaline Oderhaff, an estuarine lagoon of the river Oder at the southern Baltic Sea coast between NE Germany and N W Poland. In the more sheltered environment, the large surface of this microhabitat as well as oxygen and food supply by filter activities of the mussels could explain this pattern. Xhe shallow infaunal L. psammophila was abundant within the Mytilus edulis colonies, but mainly on sediment trapped between the shells. This special micro-habitat of trapped sediment is distinguished by

105

HARTMANN(1975) from other plant surfaces within the phytal habitat. L. psammophila has an advantage over C. torosa in this micro-habitat because of its smaller size, an elongated, slim carapace suited to ah interstitial mode of life and more skilfui movement. The other dominant ostracods from the mussel colonies were Xestoleberis nitida and Loxoconcha el@tica. X. nitida was also found in our study on Fucus, but it is said to be typical ofZostera and green algae (ATrtERSUClaet al. 1989), micro-habitats that we have not been able to study. FI~NZeL et al. (2005) documented X.. nitida asa dominant phytal species from the nearby Boiensdorfer Werder, a small penŸ about 2 km west of the study area. Here it was mainly found on the sediment but also in high abundance in dense Chaetomorpha crops. The smooth and posteriorly pointed carapace of X. nitida enables it to slip through small spaces between thalli or shells (compare KAMIVa 1983: 317). L. el@tica was the second most dominant species in the phytal zone at Boiensdorfer Werder (FRENZELet al. 2005). In the present study it occurs in practically all micro-habitats but is typical and most abundant on the plant surfaces beside the mussel colonies. We assume that its outstanding climbing ability and fast movements enable this species to use the given space of different epifaunal micro-habitats extensively. Our observations of L. el@tica correspond with references describing it as typical for the phytal zone, but as moving on the sediment too (ELovSON 1941, "ScttaFER 1953, MUUS 1967, VESVtR 1975, ARLT 1977). Semicytherura nigrescens occurred on Fucus only, where it climbed

over the algae and sessile animals. KLIE (1938), ELOFSON (1941) and ATHERSUCHet al. (1989) mark it as phytal species, without indicating special preferences. Fe,eNZEL et al. (2005) found it in the phytal zone of Boiensdorfer Werder too, but only one specimen. We do not know why it is not present within the mussel colonies and filamentous green algae. It may prefer macro-algae, as our results suggest. ROSENFELD (1977) described it as both epiphytal and sediment-dwelling. "Ihe distribution of the more rare species Cytherura gibba, Elofionia baltica, Hirschmannia viridis and Cytheromorpha fuscata of our study cannot be reliably interpreted. C. gibba and E. baltica were often found within the phytal zone (KLIt 1938, ELOFSON 1941) but this habitat is not typical for both ofthem (ATI~ERSUCH et al. 1989) and they are not reported climbing on plants. In accordance with these references, we observed both species on the sediment and the mussels only. II. viridis is given by ATHERSUCt~ et al. (1989) as preferring weed-rich littoral fringes and by other authors as a typical phytal species (KLIE 1941, ELOFSON 1941, YASSlNI 1969). However, we found it with a few specimens in sieve residues from the sediment only. Following ROSEI,aFELD (1979), /-/. viridis should have one of its instar maximums in the sampling time (reproduction time May to July). "Iherefore we infer, the poor record of this species is caused by a different preferred micro-habitat then those sampled by us. FRENZEL et al. (2005) found it in similar low numbers at Boiensdorfer Werder around Chaetornorpha and Fucus. C. fuscata was found a s a few dead speci-

Table 5: Comparison of ostracod species, their morphology, abundance and speed in different micro-habitats based on observations of living animals. Abundance is given as very abundant (>10 specimens), abundant (>5 specimens), common (>2 specimens) and rare (1-2 specimens, speed as very fast (>0.5 mm/s), fast (>0.2 mm/s), slow (>0.1 mm/s) and very slow (_<0.1 mm/s). Inferred typical micro-habitats in brackets give conclusions from literature. Species and their habitus

C. torosa (adult): large, blunt, smooth C torosa (juvenile): small to medium,

Filamentous algae

Mussd colonies

comrnon, slow

--

__

Macro algae

Sediment

--

very abundant, slow on top and very slow within

rare, fast

Typical micro-habitat

infaunal to epifaunal

blunt, smooth

L. psarnrnophila: small, elongated,

--

very abundant, fast common, fast very abundant, fast

little ornamented

C. gibba: medium size, alae, ornamented

shallow infaunal to epifaunal

--

rare

--

--

epifaunal

smooth

--

rare, fast

--

rare

epifaunal

C. fuscata: medium size, +blunt, little ornamented

.

E. baltica: small, ventrally fiattened,

L. el@tica: small, flattened, smooth

common, fast

S. nigrescens: small, elongated, smooth X. nitida: small, elongated and pointed, smooth

li. viridis: small, flattened, smooth

rare --

.

.

.

(sand)

very abundant, very fast

abundant, very fast

rare, very fast on top

phytal and epifaunal

--

abundant

--

phytal

very abundant, f a s t

common

rare, slow on top

phytal

--

rare

(phytal)?

--

106

mens only in the sediment from the un-vegetated sediment station. The relatively high salinity and the muddy fraction within the fine sand may have hampered its settlement. FRENZEr (1996) reported that it avoids muddy ground and prefers sandy substrate within the Greifswalder Bodden (a large mesohaline brackish water lagoon east of the study area).

Conclusions We have observed the following characteristic distribution pattern within the sampled microhabitats: Xestaleberis nitida is typical of mussel colonies, in association with other species. This special micro-habitat shows highest abundance and diversity of ostracods. Semicytherura nigrescens is a characteristic ostracod species of the Fucus vesiculosus samples. Loxoconcha elliptica was found in small numbers in most sediment samples but is frequent on all other substrates, including plants but especially within the mussel colonies. Leptocytherepsammophila and Cyprideis torosa dominate the sediment. Analyzing subfossil brackish water ostracod associations from shallow water sandy sediments of the mesohaline salinity range one should interpret the dominant L. psammophila and C. torosa as inhabitants of un-vegetated sediment; however, high C. torosa abundance points to higher organic content within the sedimentas ir is typical between submerged macrophytes. High proportions of species such as X. nitida, L. elliptica and S. nigrescens indicate phytal habitats of animal constructed habitats such as mussel colonies (Tab. 5). An adaptation of ostracods to micro-habitats may be expressed through their morphology (carapace forro and ornamentation, morphology of appendages) and behaviour (speed, burrowing and climbing ability, light avoidance) as we have seen in our study (Tab. 5). However, foraging strategies may play ah important role too, but were not investigated here. Concerning these different adaptation features, ir is hard to determine micro-habitats of extinct species from shell morphology alone. Ir should be stated additionally, that there could be shifts in behaviour during ontogeny, as indicated by Cyprideis torosa. Furthermore, there could also be seasonal shifts in preferred micro-habitats, e. g. by changing feeding patterns, what ate not recognizable within a single-period study such as ours.

References ARLT, G. (1977): Verbreitung und Artenspektrum der Meiofauna ira Greifswalder Bodden. - Wissenschaftliche Zeitschrift der Wilhelm-Pieck-Universitiit Rostock, mathematisch-naturwissenschaftliche Reihe, 2 : 2 1 7 - 222. ATHERSUCH,J., HORNE, D.J. & WmTTAK~R,J.E. (1989): Marine and brackish water ostracods. - In: KERMaCK,D.M. & BAI~aES, R.S.K. (Eds.): Synopses of the British Fauna (New Series), 4 3 : 359 pp.; Leiden, New York, Kobenhaven, K61n (E. J. Brill). ELOFSON, O. (1941): Zur Kennmis der marinen Ostracoden Schwedens mit besonderer Berª des Skageraks. - Zoologiska Bidrag fr~inUppsala, 19: 215-537. FVa~NZEL, 12. (1996): Rezente Faunenverteilung in den Oberfl~ichensedimenten des Greifswalder Boddens (sª Ostsee) unter besonderer Berª der Ostrakoden. - Senckenbergiana maritima, 27 (1/2): 11-31.

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Manuscript received: 20 February 2006 Revised version accepted: 10 July 2006