Histologische Untersuchung der Gonaden von Aalmuttern aus Nord- und Ostsee 2008
Institut für Angewandte Ökologie Forschungsgesellschaft mbH (2009), FKZ 30102029
Eelpout (Zoarces viviparus), also known as viviparous blenny has been proposed by HELCOM and OSPARCOM as a favourite sentinal species for monitoring of the impacts of marine environmental pollution. It has also been selected by the German Environmental Specimen Bank (ESB) as a bioindicator for the presence of potential hazardous substances in coastal waters.
With the intention to investigate biological effects parameters the ESB initiated an investigation in 2006 on the occurrence of gonadal disorders in eelpout which were sampled for routine chemical contaminant analysis. This report presents the results of a follow-up study performed in 2008 with the objective to confirm and expand findings on the presence of eelpout gonadal disorders, namely intersex and ovarian degenerations (atresia), which were frequently observed in the first field-survey.
The gonads originated from eelpout sampled in spring and early summer 2008 in the North Sea at the stations Borkum, Varel, Büsum and List/Sylt and in the Baltic Sea at the mouth of the Schlei fjord and off the Darß peninsula at Darßer Ort. Of this stations Varel, Büsum and Darßer Ort are regular sampling areas of the German ESB. The muscle and liver tissue of fish selected for the gonad investigation were pooled for accompanying and future chemical analysis.
The testes of male eelpout were examined by light microscopy. The analysis of the developmental stage revealed that in March testes possessed only primary and secondary spermatocytes. From late May to June spermatids and spermatozoa were predominantly present in the testicular tubules. At the North Sea stations the percentage of intersex fish was 28% at Borkum, 32% at Büsum and 40% at List/Sylt. At Varel, were samples were collected three times, the prevalence ranged from 8 to 44%. In Baltic coastal waters the intersex incidence was 20% at the station Schlei and 32% at Darßer Ort. In general, the degree of intersexuality was low with only few single primary oocytes dispersed in normally developed testis tissue. The mean severity of intersex as calculated by means of an intersex index was highest at Büsum and Borkum. At the other sampling areas the mean degree of intersex was somewhat lower and more or less at the same level.
Ovaries from female eelpout were examined by using mainly macroscopical and to a minor extent histological methods. The determination of gonadal development revealed that in March the most advanced oocytes were in an early stage of vitellogenesis. Oocytes in a late stage of vitellogenesis were present in May and June. By means of histological examination typical characteristics of different stages of follicular atresia were observed. The macroscopical analysis using a binocular magnifier revealed conspicuous forms of follicular degeneration as well as less prominent types of follicle/oocyte degeneration. The macroscopic examination was somewhat hampered by the often small size and
undeveloped stage of the ovaries. When all forms of degenerations were taken into consideration their prevalence was 90% to 100% in female eelpout from the stations Varel, Büsum and Schlei and about 45% to 65% at Sylt, Borkum and Darßer Ort.
Findings obtained from a survey of the scientific literature indicate that the very prominent type of atretic degeneration obviously represented postovulatory follicles (POF) in different stages of resorption. During gestation POF have a important function in maternal-fetal exchange of nutrients. After having given birth to the brood POF are still present during the early phase of the new gonadal development. It is emphasized that – as histology clearly demonstrated – beside POFs other forms of rather severe follicle/oocytes atresia was observed. This kind of atresia is likely the result stress factors of either natural and/or anthropogenic origin. In light of this finding it is emphasized that for a future reliable macroscopic analysis of preovulatory atresia it is important to sample ovaries in a late stage of oocytes development.
Taken together this study has documented the widespread occurrence of intersex in male eelpout from German coastal waters and thus confirmed the finding of the former ESB-investigation performed in 2006. With respect to the presence of atretic degenerations during ovarian development the results of this study are also in line with the first ESB-study. However, it has been shown that the widespread atresia observed in female eelpout was due to both atretic postovulatory follicles and to preovulatory follicle/oocytes as well. Recommendations for the future monitoring of primarily stress related atresia are presented.
Keywords: Germany, Wadden Sea, Baltic Sea, eelpout, intersex, atresia, biological effects monitoring