Hydrobiologia 143 : 1 1 9 -121, (1986)© Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands

On the relationship between the Bosmina taxa coregoni and thersites (Cladocera), asindicated by subfossil remains

Wolfgang HofmannMax-Planck-Institut fzir Limnologie, Abt. Mikrobenokologie, Postfach 165, D-2320 Plon, F.R.G .

Keywords: lake sediments, Bosmina coregoni f. thersites, long term morphological variation,polymorphism, taxonomy

Abstract

A succession of morphs from Bosmina coregoni f. coregoni to B. c. f. thersites, which vary in shell andantennule characters, occurred in an undated 176 cm long sediment core from the Neversdorfer See (N . Ger-many) .

Introduction

Shells and head shields with first antenna of theplanktonic cladoceran Bosmina are preserved inlake sediments . On the basis of these remains directobservation of long term morphological variationis possible. This may document the evolution of thevarious morphs in the subgenus Eubosmina andmay clarify their taxonomic rank (Frey, 1964) .

The simplest explanation of the results relatingto Bosmina coregoni Baird obtained from NorthGerman lakes was that morphological transitionfrom kessleri to coregoni morphs occurred bymucro reduction (Hofmann, 1977, 1978, 1984) .

The mucroless B. coregoni f. coregoni is the mostcommon species of the subgenus Eubosmina in eu-trophic lakes in North Germany (Schleswig-Holstein). However, in some highly eutrophic lakesan easily recognized form with a backward-archedshell occurs. This is considered a subspecies : B.coregoni thersites Poppe (Flossner, 1972; Lieder,1983) . Analysis of Bosmina remains in a sedimentcore from a thersites lake has been carried out toobtain further information on the relationship be-tween the taxa coregoni and thersites.

Material and methods

The Neversdorfer See is a highly eutrophic lake

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(84 ha, maximum depth 10 m, mean depth about4 m) about 60 km southwest of Kiel, N. Germany.Two sediment cores of 150 and 176 cm, respec-

tively, were taken from sites, approximately 2 kmapart (5 m water depth) using a 'Klein-Rammkolbenlot' (Zullig, 1969) . The cores were cutinto 5 cm slices . Preparation of the sediments fol-lowed Hofmann (1978) .Bosmina shells which represent the range of vari-

ation in a sample were drawn by means of a cameralucida in a scale of 160 :1 . Lengths of the first anten-nae were measured and numbers of the segmentswere counted (both distal to the triangular scale) .The relative abundance of chydorid and Bosminaspecies were determined .

Results and discussion

Results according to Bosmina morphology andto the percentages of the cladoceran taxa were alikein both cores. The following data refer to core Afrom the eastern lake basin .

In the cladoceran assemblage Chydorus sphaeri-cus predominated (40-64%) throughout the core,followed in abundance by Bosmina longirostris(mostly between 20 and 30%), whereas Bosminacoregoni mostly ranged between 4 and 11% . The re-maining chydorids had frequencies below 10%, themost abundant species being Alona rectangula,

1 20

Alona quadrangularis, and Leydigia quadran-gularis. The cladoceran assemblage as a whole dis-tinctly reflects the eutrophic conditions (Whiteside,1970; Flossner, 1972) . Although the lake was rathershallow the planktonic elements (Bosmina, Chydo-rus sphaericus) clearly predominated, accountingfor more than 90% of the cladoceran remains andindicating a high planktonic/littoral ratio (Alho-nen, 1970) .

A plot of the shapes of the Bosmina coregonishells against sediment depth shows that only theform coregoni occurred in the lowermost layerswhile at 112 cm high-arched gibbera morphs and at67 cm back-arched thersites morphs appeared,producing a high degree of polymorphism, becausethe coregoni forms were still present (Fig . 1) . In theuppermost sediments Bosmina was only represent-ed by typical thersites specimens. Hence a mono-morphic coregoni phase was followed by a phase ofpolymorphism and finally by a uniform thersitesstage .

A steady increase in the number of the segmentsof the first antenna was discernible: In the zone172-122 cm the number mainly ranged between 12and 16 (mean about 14) . At 112 cm there was a sig-nificant shift to higher values (mostly 16-22,mean: 17-18) and in the 17 cm sample a third in-crease occurred resulting in values largely above 20segments (maximum: 28, mean : 22-23) (Fig . 2) .

Similarly, there was an enormous increase in an-tennule length : 172-122 cm: means 157-194 µm ;117-57 cm: means 205-274 µm; 52-2 cm: means322-416 µm (Fig . 3). As with the number of seg-

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Fig. 1 . Neversdorfer See, Bosmina coregoni: shape of the shellsof 10 representative specimens from different sediment layers .

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26

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Fig. 2. Neversdorfer See, Bosmina coregoni: number of anten-nule segments in relation to sediment depth .

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ments this change was due not only to increasingrange of variation but also to a displacement of therange, i .e. by the disappearance of short antennuleswith low segment numbers .

There was no overlap between individuals fromthe lowermost and the uppermost sediment layerswith respect to antennule length and number ofsegments (Fig . 4) .

The morphological changes in both the shell andthe first antenna clearly show a development fromcoregoni to thersites morphs, the latter beingcharacterized by back-arched shells and long firstantennae, with a high number of segments (Floss-ner, 1972; Lieder, 1957) .

It is however not clear whether polymorphismand subsequent transition to thersites morphs oc-

Nm

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24 T 28number of ontennule segments

Fig. 4. Neversdorfer See, Bosmina coregoni, specimens from2 cm and 172 cm sediment depth : number of antennule seg-ments against antennule length .

curred within the original population or if it was in-duced by immigration of a second population . Themost significant change occurred between 120 and115 cm sediment depth, as indicated by the appear-ance of high-arched shells and an increase in anten-nule length and segment numbers .

Three different pathways for this developmentare possible :

1 . The morphological changes occurred within asingle Bosmina coregoni population, induced bychanging environmental conditions . This impliesthat the morphological forms observed weremorphs of B. coregoni, which suggests a close rela-tionship between ecological conditions and Bosmi-na morphology.

2. Morphological variation was due to immigra-tion of another population and so reflects species

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succession. This implies a species rank of B.coregoni relative to gibbera and thersites.

3. Immigration of gibbera morphs and subse-quent hybridization with the original populationled to polymorphism and, under the influence ofchanging environmental conditions, to the predom-inance of thersites morphs. This supports the sub-specific status of the taxa involved .

Closer observation of the transitional phases anddata from other thersites lakes are required to an-swer these questions . Further analysis should helpto elucidate the taxonomical relationships betweenthe coregoni taxa as well as the influence of en-vironmental conditions on the Bosmina morpholo-gy.

References

Alhonen, P., 1970. On the significance of the planktonic/littoralratio in the cladoceran stratigraphy of lake sediments . Com-ment . biol . 35 : 1-9 .

FlOssner, D ., 1972 . Kiemen- and Blattfiif3er, Branchiopoda,Fischlause, Branchiura . In : M. Dahl & F. Peus (eds.), DieTierwelt Deutschlands, 60 : 1-499.

Frey, D . G., 1964. Remains of animals in Quaternary lake andbog sediments and their interpretation . Arch . Hydrobiol .Beih. Ergebn . Limnol . 2: 1-114 .

Hofmann, W., 1977 . Bosmina (Eubosmina) populations of theGrol3er Segeberger See during late glacial and postglacialtimes . Arch. Hydrobiol . 80: 349-359 .

Hofmann, W., 1978 . Bosmina (Eubosmina) populations ofGrol3er Ploner See and Schohsee lakes during late-glacial andpostglacial times . Pol . Arch . Hydrobiol . 25 : 167-176 .

Hofmann, W., 1984 . Postglacial morphological variation inBosmina longispina Leydig (Crustacea, Cladocera) from theGrof3er Ploner See (north Germany) and its taxonomic impli-cations . Z . zool . Syst . Evolut .-forsch . 22 : 294-301 .

Lieder, U ., 1957 . Beitrage zur Kenntnis des Genus Bosmina(Crustacea, Cladocera) . IV. Versuch einer Monographic derUntergattung Eubosmina Seligo 1900 . Diss . Berlin .

Lieder, U., 1983 . Revision of the genus Bosmina Baird, 1845(Crustacea, Cladocera) . Int . Rev . Hydrobiol . 68 : 121-139 .

Whiteside, M . C ., 1970 . Danish chydorid Cladocera : modernecology and core studies . Ecol. Monogr. 40: 79-118 .

Ziillig, H ., 1969 . Ein Klein-Rammkolbenlot zur Gewinnung un-gestorter Sedimentprofile. Schweiz . Z . Hydrol . 31 : 128-131 .

Accepted 18 March 1986.

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