Journal of Micropalaeontology, 14: 96. 0262-821X/95 $07.00 0British Micropalaeontological Society. MICROPALAEONTOLOGY NOTEBOOK Preliminary observations on living Krithe praetexta praetexta (Sars, 1866), Sarsicytheridea brudii (Norman, 1865) and other marine ostracods in aquaria ' Department STEFAN MAJORAN' & STEFAN AGRENIUS* of Marine Geology, University of Goteborg, Earth Sciences Centre, S-41381 Goteborg, Sweden. 'Kristineberg Marine Research Station, S-450 34 Fiskebackskil, Sweden. More than fifty years ago, Elofson (1941) showed that it is fully possible t o maintain living cultures of marine ostracods in aquaria. H e concentrated particularly on determining the generation length of several species. In this study, wc provide some preliminary observations on the mode of life and morphological variations of marine ostracods kept in aquaria. They derivc from a water depth of 4 0 m in the Gullmar Fjord (58"17'N and 11"29'E), west coast of Sweden. T h e dominant species are Krilhe praetexta praerrxru (Sars, 1866) and Sar.ricyrheridea brudii (Norman, 1865). Other species housed in the aquaria are: Jonesia ucuminatu (Norman, 1865). Palmoconcha gutfafa (Norman, 1865), Pulrnoconcha lueuala (Norman, 1865). Cyrheropreron hfissirnrrm (Norman, 1865). Prerygocythereis jonesii (Baird, 1850), Acanrhocvthereir drrnelmensi~ (Norman, 1865). Kohertsonites tuhercularus (Sars, 1866), Elofk~nelluconcinna (Jones, 1857) and Argrlluecia conoidea (Sars, 1923). MATERIAL AND METHODS T h e study was carried out at the Kristineberg Marine Research Station, west coast of Sweden, from July of 1992 t o June of 1994. Sediment from a depth of 40 m in the Gullmar Fjord was sieved t o remove the macrofauna and frozen, then thawed to constitute a 10-20mm thick sediment layer in two 501 aquaria. T h e sediment consisted of 8 % sand ( X i 3 pm), 44 % silt (>3.Y p m ) and 49 % clay (13.9 p m ) , and with a water content of 71 % k 5% (u= 2.4). Ostracods from the 2 2 5 0 p m sieve fraction of the dredge sample (from a depth of 40 m) were added to the aquaria. They were kept in a continuously flowing, open system, pumping water from the intermediate watermass (Svanson, 1984) of the fjord (from which the ostracods originate). 'This system is intended to reproduce approximately the natural variation in physico-chemical conditions. During the study, the salinity varied between 32-34%0, the temperature between 4-15°C and the oxygen content between 4 (measured a t a depth of 40 m in the Gullmar Fjord by the and 7.3 ml I Swedish National Pelagic Monitoring Program). The vertical distribution of living ostracods in the sediment of the aquaria were estimated in each of 16 samples. A millimetre-graded hollow cylinder with a diameter of 8.7 cm was pressed through sediment to efficiently isolate a small volume. A siphon was passed over the sediment surface within the cylinder to remove thin layers which were sieved through 250 and 125 p m and picked for living ostracods. ' RESULTS AND OBSERVATIONS Notes on reproduction and ontogenetic development. Most individuals of S. bradri, K. pruerexra praerextu, A . conoidea and J . ucuminuru were juveniles (A-l to A-4) at the end of the experiment. W e also recorded living juveniles A-2 and A-I of R. rubercularrrs and A. diinrlmensk, respectively. This implies that ostracods moult, grow and reproduce in the aquaria. Elofson (1941) estimated the total lifespan for S. hrudir, R. tuberculatus and A. dunelmensis t o b e 2-3 years. T h e generation length of K. prarrexta praerrxta is unknown. O u r infrequent sampling is insufficient for such estimates, although the many juveniles of this species in June 1994 could hardly have remained unchanged since July 1992. T h e population density (c. 1000 individuals per m2) and species composition of living ostracods at the end of the experiment were similar t o the natural environment at 4 0 m in the fjord. Life position in the sediment of the aquaria. Of a total of 59 living specimens of K. pracwrtu praetcxta and S. brudir. all except one individual of S.hradii, were found at depths below 2 mm in the sediment and down t o the bottom of the aquaria ( 5 2 0 m m ) . A total of 14 specimens of A. conoidea and a few specimens of R. tuberculurus were found at the surface and several millimetres down in the sediment. Most of the individuals of the remaining species were found at o r near the surface of the sediment. O u r observations are largely consistent with Elofson (1941) who listed J . acuminatu, P.grrrfara and P. larvara among forms predominantly living on the surface of the sediment, whereas K . prurfexra prurrcwri. S. bradii and R. ruhercrr1altr.r were listed as endobenthic o r infaunal species. Elofson (1941) regarded A . conoideu as mainly infaunal. although the prescnt study indicates that this species is equally common close to the sediment surface. 96 Size variation. T h e mean length and height of the left valve (LV) of two adult females of K. praetexra praetextu recorded living after 22 months in the aquaria (in May-June 1994) are 695 and 3 6 0 p m , respectively. T h e corresponding values for three adult males are 675 and 3 2 3 p m , respectively. These dimensions are significantly smaller than those of adult specimens obtained from dredge samples of May 1988 (McKenzie ef al., 1989), and April and July of 1992. T h e mean length and height of the LV of adult females from this database ( N = 7 7 ) are 779 (*S.l; 95% confidence interval) and 400 (&6.9) p m , respectively. The corresponding values for males ( N = 38) are 772 (*7.3) and 359 ( + 4 . 6 ) p m , respectively. A few living adults of R. ruhercularus and P. Xurfutu, respectively, after 22 months in the aquaria were also significantly smaller in average than specimens of the dredge sample of July 1992. A corresponding size reduction was not observed in S. hradii. DISCUSSION T h e oxygen penetration rarely exceeds lOmm in sandy shallow water sediments (Revsbech el a / . , 1980; Rasmussen & Jorgensen, 1992). It is dependent on the porosity of the sediment, the diffusion coefficient of oxygen in the sediment and the oxygen concentration at the sediment surface; there is also an inverse relationship between oxygen penetration and oxygen consumption in the sediment (Revsbech & Jorgensen, IY86). Representatives of K . praercwa praerexru and S. bradii were found at depths > 10 mm in the sediment of the aquaria. This could be explained by the high porosity (water content) of the sediment which may allow a greater penetration of oxygen. T h e intensity of bioturbation may also influence the oxygen penetration and thc vertical distribution of species in the sediment. A juvenile, infaunal macrofauna was inevitably enclosed along with the ostracods at the beginning of the experiment, which together with a supply of pelagic larvas through the pumping system have developed into an actively bioturbating fauna during the course of the cxperiment. Infaunal burrowing polychaetes (mostly Diplocirrus glaticus) were common. O n e possible explanation to the size reduction observed in K. pruefextu praefexra, R. fnberculufus and P.gurfutu is that the temperature became higher in the aquaria during the summer than in the fjord. Another explanation is possibly reduced food supply. Despite having an open circulating systcm without any kind of filter. the supply of plankton is considerably lower in the aquaria than in the natural environment (Granmo, pers. comm.). The nutritive value of the aquarium sediment may differ from the natural environment involving a difference in the microbiota. T h e size reduction in three species but not in S. brudii is possibly explained in terms of different susceptibility to thermal variation and/or different feeding strategies. ACKNOWLEDGEMENTS We are grateful t o Kristinebcrg Marine Research Station for facilities to perform the study; t o D r T. Correge, Bordeaux, for encouraging us to present the results; to Michal Kucera and t o Ursula Schwarz for laboratory assistance; to Dr R.A. Reyment, Uppsala, D r K.G. McKenzie, Melbourne, Dr Bjorn Malmgren and D r D.J. Horne, London, for comments on the manuscript. Financial support: grant (G-PD 6656-302) from the Swedish Natural Science Research Council. Manuscript received September 1994 Manuscript accepted April 1995 REFERENCES Elofson. 0. 1Y41. Zur Kenntnis der marinen Ostracoden Schwedens mit besondercr Beriicksichtigung des Skageraks Zoologrko Btdrog f r m U p p s a b , 1 9 211-534. McKenzie, K . G., Majoran. S . , Emami, V. & Keyment. i R. A. IYXO. 'The Kruhe problem hrst test of Peypouquet's hypotheais. with a rcdescription of Krirhe pmerexra ~ pruerexxui (Cructacea, Ostracoda). PulaeugeoEruptiy. P u l u e u c l i i n a t o l ~ i ~Pulaeoscolog~. ~, 74: 341-354. Kasmussen, H. & Jiirgensen, 8.B 19Y2. Microelectrode siudirs of seasonal oxygen uptake in a coastal wdiment. role of molecular diffusion. Mnrme Ecology Progress Series, 81: 2X'>-303. Krrsbcch, N. P. &i Jiirgenxn, B. B. 1986. Microelectrodes rhelr use in Microbial Ecology. I n Marshall, K. C. (Ed.). Aitwnces i n Mrrmhiul E c o l o g y . 9 293-352. Kevshech, N. P.. SiirenPen. J. & Blackburn, T. H. IYXO. Distribution of oxygen in manne scdirnriits measured with microclectrodes. Limnolugy & Ocrunography,25: 403-41 I . Svansson. A . 1'2x4. l l y d r o g m p k y of rhr Gullmor F,jorrl. Fisher Border Sweden. Institution of Hydrographical Kesearch. Series N o 23 (mimeo).