Volume 11

The expression of sexual dimorphisms in the weakly electric fish Mormyrus rume proboscirostris (Teleostei, Mormyridae) following exposure of larvae to 17α-methyldihydrotestosterone

Larvae of Mormyrus rume proboscirostris were treated for 3 hours with 3.0 mg/l 17α-methyldihydrotestosterone at the age of 18, 21, 24 and 27 days. In a group of 70 surviving fish, at the age of 2 years, their electric organ discharge was recorded, the gonadal status histologically determined, and x-rays of their anal fin were taken. The genotypic sex of the fish was not known. 35 fish had developed a normal ovary, only one fish a normal testis, and 25 fish hermaphroditic (intersex) gonads. Gonads were not developed in 9 fish. Male-typical enlargements of the anal fin ray base were found in some females and in almost all animals with hermaphroditic gonads. All treated fish exhibited the elongated, male-typical electric organ discharge.

Preparatory measures and initial release as a prerequsite for the remediation of the European sturgeon, Acipenser sturio, in Germany

Sturgeon were among the first fish species that revealed a population decline in response to the increased anthropogenic pressures upon the river systems in the 19th century. Safeguarding and remediation attempts were taken only after the species was almost extinct, relying on potential populations in the historic range. Through support of the German Federal Agency for Nature Conservation, a first pilot project was initiated by the in the mid 1990s attempting to identify potential donor populations for ex – situ measures. At the same time forty juvenile Acipenser sturio originating of a controlled reproduction of wild spawners from the Gironde River were transferred from the French Cemagref to the Leibniz – Institute for Freshwater Ecology and Inland Fisheries in Berlin for experimental purposes and broodstock development, providing the nucleus of the German ex-situ stock. Habitat assessments were carried out to identify suitable river systems for remediation of the species. Genetically the French A. sturio have been proven suitable for the reintroduction in the German North Sea tributaries. Since 2007 offspring of the first ex-situ reproductions in France were used to expand the future broodstocks both in France and Germany as well as to restock the Gironde system. These transfers also were utilized to carry out the first experimental release in the Elbe River system since 2008. First insights into the migration behaviour and habitat utilization were obtained through a telemetry study between the release site at Lenzen (river – km 485) to Hamburg Harbour (river – km 614). Furthermore, an outlook on the future requirements with regard to adaptation of rearing techniques to allow the adaptation of fish designated for release as well as options for stakeholder involvement are discussed.

Reflections on the decline and management of European eel

In response to the dramatic decline of European eel, the European Commission implemented an eel recovery plan (EC 1100/2007) which obliges each member state to identify eel catchments and to apply management measures to return the eel stock to sustainable levels. This management comprises substantial uncertainties, because several aspects of eel’s life history still remain unknown, primary causes of decline are not fully clarified, and only a few of the potential causes for decline can be addressed by management measures in freshwaters. This study briefly reviewed the most recent literature and discussions on the causality of eel decline with regard to rehabilitation measures. Research needs have been identified in determining the relative contribution of freshwater grown eels to the spawner stock. Current studies provided some evidence that not all migrating silver eel perform a catadromous life cycle and that eel spawner may primarily recruit from marine or coastal habitats. If the latter becomes scientifically confirmed it seriously calls the potential into question managing eel recovery in the designated freshwater catchments and would rather support increasing the glass eel escapement instead of stocking.

Parasitengemeinschaften von ,Wirbellosen’ und Kleinfischen an sechs Standorten der SW Ostsee – ein Vergleich

[Parasite communities of ‘invertebrates’ and small-sized fish at six localities of the SW Baltic Sea – a comparison]

Die Parasitengemeinschaften von sechs Probeorten der SW-Ostsee wurden verglichen mit dem Ziel, evtl. Unterschiede und ihre Ursachen aufzudecken. Als Wirte wurden die Gilden der Schnecken, der planktischen und benthischen Krebse, Kleinfische und an der Schlei sogar einige Seevögel einbezogen. Die höchsten Artenzahlen (47-49) wurden in der Flensburg-Förde, in Dahmeshöved und im Salzhaff festgestellt, die geringsten in Blank Eck (18), während Schlei (33) und West-Fehmarn (29) eine Zwischen-Position einnehmen (Abb. 1). Unter den insgesamt 83 Parasiten-Arten dominieren deutlich die Digenea (38), Cestoda, Nematoda und Acanthocephala folgen mit 8-15 Arten. Mit 1-4 Arten waren noch Protozoa, Monogenea, Mollusca, Annelida und Copepoda vertreten. Die regionale Parasitenfauna war an keinem der Orte und in keiner Parasitengilde vollständig vertreten, am meisten noch in Dahmeshöved zu 60 %, am geringsten in Blank Eck zu 25 %. Diesem entsprach auch die Analyse der Artenidentität. Leichte Unterschiede zwischen den sechs lokalen Parasitenfaunen konnten durch eine Varianzanalyse abgesichert werden. Unter der Voraussetzung, dass die regionale Parasitenfauna eigentlich in allen ausgewählten Standorten identisch vertreten sein sollte, können folgende Ursachen für die Abweichungen verantwortlich sein: Untersuchungsmethoden, Salzgehalt und Verschmutzung. Die hohe Artenidentität zwischen der Flensburg-Förde und dem Salzhaff sprechen gegen den Einfluss des Salzgehaltes, der sich zwischen diesen am weitest entfernten Orten etwa halbiert. Da West-Fehmarn nur in einer Saison beprobt wurde, könnte die geringe Artenzahl auf der kurzen Untersuchungszeit beruhen. Eher sind die hohen Artenzahlen aus der Flensburg-Förde, von Dahmeshöved oder aus dem Salzhaff auf die intensive Eutrophierung zurückzuführen, die mit dem gesteigerten Nähstoffangebot auch das Wachstum der Parasitenpopulationen fördert. Eine mäßige Belastung wie in Blank Eck lässt daher nur eine vergleichsweise reduzierte Parasitengemeinschaft zu.

[The parasite communities of six localities of the SW Baltic Sea were compared in order to detect differences and their causes. As potential hosts were snails, mussels, benthic and planktonic Crustacea, small-sized fish and also some birds investigated. The highest numbers of parasite species (47-49) were found in the Flensburg-fjord, in Dahmeshöved and in the Salzhaff, the least in Blank Eck (18) whereas the Schlei (33) and West-Fehmarn (29) have an intermediate position (fig. 1). Among a total of 83 parasites from all six localities the Digenea dominate by 38 species followed by Cestoda, Nematoda and Acanthocephala by 8-15 species. Protozoa, Monogenea, Mollusca, Copepoda and Annelida presented only 1-4 species. This regional parasite fauna was not totally present in the investigated localities, at most in Dahmeshöved (60 %), at least in Blank Eck (25 %). These results were also confirmed by the analysis of species identity. In contrast, slight inhomogeneity between the parasite fauna of the six localities could be ensured by ANOVA. For this result and the deviation from the composition of the regional parasite fauna the following causes can be responsible in the single localities: investigation methods, salinity and grade of eutrophication. Flensburgfjord and Salzhaff present a high faunal identity which arguments against an influence of salinity which decreases on a distance of 150 km by the half. The relatively low number of parasite species in West-Fehmarn may depend on an Investigation during only one season. The high species numbers at Flensburgfjord, Dahmeshöved and Salzhaff may be correlated to the high eutrophication which promotes the prey chain and, combined with it, the growth of parasite populations. A moderate eutrophication as in Blank Eck allows only a reduced parasite community in comparison with the other localities.]

Redescription of Homaloptera ripleyi (Fowler, 1940) from Sumatra, Indonesia (Teleostei: Balitoridae)

Homaloptera ripleyi, a member of the family Balitoridae, is only known by the holotype, originally described by H.W. Fowler in 1940 from northern Sumatra as Homalopterula . The inadequately studied species is redescribed on the base of freshly collected material. The species differs from all other Homaloptera species by combination of the following characters: eigth to ten saddle-like blotches on dorsal side of body not reaching the lateral line; ventral side completely scaleless; obliquely truncate caudal fin and protruding curved shape of jaws, especially lower jaw with a pad of chondroid tissue; 72-78 scales on lateral line. H. ripleyi is endemic to Sumatra and only known from the northern provinces. 

On the structure of teeth in the viperfish Chauliodus sloani Bloch & Schneider, 1801 (Stomiidae)

We used histology (LM) and scanning electron microscopy (SEM), some data obtained by transmission electron microscopy (TEM) and x-ray microanalysis (EDX) to characterize functional oral teeth of the viperfish Chauliodus sloani. Teeth are of different length and their apices are variable to some extent. The longest are the front teeth of the lower jaw, which extend out above the upper jaw up to the eyes. The second longest are the front teeth of the premaxillae. Teeth form a kind of cage when the mouth is closed probably preventing prey items from escaping. Teeth are non-depressible and firmly attached to the jaw bone. Labially the tooth is elongated forming a tongue-like projection that covers parts of the dentigerous bone probably allowing the teeth to resist pressure from inside the mouth, e.g. by struggling prey. Teeth are not clearly divided in a pedestal (“bone of attachment” of many authors) and the dentine shaft proper, but the transition to the putative bone of attachment is characterized by a diaphragm-like structure inside the tooth, which is more conspicuous lingually than labially. Teeth possess enameloid caps and a layer of collar enameloid decreasing in thickness towards the base. The dentine appeared stratified in some teeth with alternating layers of densely and less densely arranged collagen fibrils. The pulpa contains a network of pigmented cells closely attached to the (pre)dentin, which probably represent odontoblasts. Ca-contents decreases towards the tooth base and was smallest in the jaw bone.

Do cave mollies (Poecilia mexicana) obtain information about the resource value of food patches from shoal mates?

Schwarmfische könnten Strategien entwickeln, um unterschiedlichen Nahrungs- bzw. Beutedichten effizient auszubeuten. Zunächst sollten alle Fische die beutereiche Zone bejagen, bis diese auf das Niveau der beuteärmeren Zone dezimiert ist, dann sollte sich der Schwarm gleichmäßig auf beide Zonen verteilen. Höhlenbewohnenden Poecilia mexicana wurden zwei Zonen mit unterschiedlicher Beutedichte (Tubifex tubifex) angeboten und die Fisch- sowie Beutetieranzahl im zeitlichen Verlauf protokolliert. Die Anzahl von T. tubifex nahm in der beutereichen Zone zunächst steil ab, jedoch wurden ebenfalls in der beutearmen Zone einige „Würmer“ von einigen wenigen Höhlenmollys gefressen. Eine gleichmäßige Verteilung der Höhlenmollys auf beide Zonen erfolgte mit einer deutlichen Verzögerung zur Gleichverteilung der Beutetiere, was auf einen fehlenden oder zumindest sehr geringen Informationsfluss innerhalb des Fischschwarms hinweist.