Preliminary observations on living Krithe praetexta praetexta (Sars, 1866), Sarsicytheridea bradii (Norman, 1865) and other marine ostracods in aquaria

More than fifty years ago, Elofson (1941) showed that it is fully possible to maintain living cultures of marine ostracods in aquaria. He concentrated particularly on determining the generation length of several species. In this study, we provide some preliminary observations on the mode of life and morphological variations of marine ostracods kept in aquaria. They derive from a water depth of 40m in the Gullmar Fjord (58°17′N and 11°29′E), west coast of Sweden. The dominant species are Krithe praetexta praetexta (Sars, 1866) and Sarsicytheridea bradii (Norman, 1865). Other species housed in the aquaria are: Jonesia acuminata (Norman, 1865), Palmoconcha guttata (Norman, 1865), Palmoconcha laevata (Norman, 1865), Cytheropteron latissimum (Norman, 1865), Pterygocythereis jonesii (Baird, 1850), Acanthocythereis dunelmensis (Norman, 1865), Robertsonites tuberculatus (Sars, 1866), Elofsonella concinna (Jones, 1857) and Argilloecia conoidea (Sars, 1923). MATERIAL AND METHODS The study was carried out at the Kristineberg Marine Research Station, west coast of Sweden, from July of 1992 to June of 1994. Sediment from a depth of 40 m in the Gullmar Fjord was sieved to remove the macrofauna and frozen, then thawed to constitute a 10–20 mm thick sediment layer in two 501 aquaria. The sediment consisted of 8 % sand (>63 μm), 44 % silt (>3.9 μm) and 49 % clay (<3.9 μm), and with a water content of 71 % ± 5% (σ = 2.4). Ostracods from the ≥250 μm sieve fraction of the dredge sample (from a depth of 40 m) were added to the aquaria. They were kept. . .


MATERIAL AND METHODS
The study was carried out at the Kristineberg Marine Research Station, west coast of Sweden, from July of 1992 to June of 1994. Sediment from a depth of 40 m in the Gullmar Fjord was sieved to remove the macrofauna and frozen, then thawed to constitute a 10-20mm thick sediment layer in two 501 aquaria. The 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 and 7.3 ml I ' (measured a t a depth of 40 m in the Gullmar Fjord by the 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. We 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 to be 2-3 years. The 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. The population density (c. 1000 individuals per m2) and species composition of living ostracods at the end of the experiment were similar to 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 to 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 or 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 or 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. 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
The 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. The 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.
One 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. The 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.