Size Dependent Male Reproductive Tactic in the Two-Spotted Goby (Gobiusculus flavescens).

Male investment in testes and sperm duct gland in the polygamous nest breeding two-spotted goby Gobiusculus flavescens (Fabricius) was investigated in relation to time in reproductive season and individual physical parameters. This small teleost fish is most likely the most abundant species found along the rocky shores of the North East Atlantic. The two-spotted goby has a single reproductive season, during which nest-caring males can raise several clutches of offspring. According to the literature the males are on average larger than the females. Here we report for the first time a population showing a reversal of this trend, with males on average being smaller than females, a difference likely caused by a large proportion of small males. Early in the breeding season these small males have typical sneaker characters, with relatively large testes and small seminal duct glands compared to the larger dominant territorial males. The presence of these two alternative male reproductive tactics is confirmed by histological studies, which shows the presence of sperm in the sperm duct glands (SDG) of smaller males, but not in the SDG of intermediate and larger males. To our knowledge, males with typical sneaker characters have not been reported in earlier studied populations of two-spotted goby. Interestingly we found that testes investment declined significantly over the course of the breeding season, and that this reduction was significantly more pronounced in small compared to the large males. Further, a significant increase in seminal duct gland (SDG) mass was observed for the smaller males over the breeding season. We propose that this indicates a possible shift in mating tactic by smaller males from a parasitic to a nest-holding tactic over the course of the breeding season. Thus, the observed size dependent plasticity in investment in SDG over time suggests that the reproductive tactic of G. flavescens is conditional, and possibly influenced by mate availability and male--male competition.

The protection afforded by the outer layer to procercoids of Schistocephalus solidus during passage through the stomach lumen of their vertebrate host (Gasterosteus aculeatus).

Intestinal cestodes with complex life cycles have to pass through the acid stomach lumen of their vertebrate host(s) in order to reach their preferred site of development. The cestode's tegument is the only organ in constant contact with this hostile environment. Procercoids of Schistocephalus solidus (order Diphyllobothriidea) lose their outer layer on passing through the acidic stomach of their second intermediate host (Gasterosteus aculeatus). We wanted to investigate if the outer layer is an adaptation that enables passage through this hostile environment. We used fish bile to force the procercoid larvae to shed their outer layer. This allowed us to compare the survival of the normal procercoids and the transformed ones when exposed to hydrochloric acid. We observed that the presence of the outer layer significantly improved the survival and active period of the procercoid larvae. Thus we conclude that in cestodes which inhabit the digestive tracts of vertebrates, the outer layer is an adaptation which enables them to pass through the acidic stomach environment of their vertebrate host(s).

In vitro transition of Schistocephalus solidus (Cestoda) from coracidium to procercoid and from procercoid to plerocercoid.

With the present study, a culture system for successive life-cycle stages of the tapeworm Schistocephalus solidus was developed and this report documents for the first time, cultivation of the procercoid stage of S. solidus from eggs. Additionally we have transformed procercoids dissected from experimentally infected copepods and cultured procercoids into the early plerocercoid stage in vitro. Observations in the culture suggest that the coracidia can interact with their external environment and need no host specific stimuli, except for the components in the culture medium, for activation and hatching from the embryophore. Increasing the culture medium pH from 7.3 to 8.0 improved escape rates and frequencies of hook contractions, suggesting that the oncosphere may recognize and respond to environmental conditions along the host intestine. Procercoids in the culture did not stop growing indicating that conditions within the copepod may be important to limit growth and to induce transformation to plerocercoids. When procercoids are dissected from copepods and transferred to the culture, the outer tegument layers and cercomer starts to loosen. Comparison of the lectin staining of the loosened outer tegument layers and cercomer in procercoids dissected from copepods confirms that transitions of both, the oncosphere to procercoid and procercoid to plerocercoids, has taken place in the in vitro cultures.

Selection pressure towards monoxeny in Camallanus cotti (Nematoda, Camallanidac) facing an intermediate host bottleneck situation.

This paper describes the ability of the Asian fish nematode Camallanus cotti to carry out both heteroxeny, i.e. an indirect life-cycle using copepods as intermediate host, and monoxeny, i.e. direct infection and development in the definitive fish host. C. cotti occurs naturally in various freshwater teleosts in Asia. During the past decades it has been disseminated into closed or semi-closed aquaculture systems and aquaria around the world, mainly due to the ornamental fish trade. Under such conditions the species may frequently face a bottleneck situation with regard to the availability of copepods. It is known that C. cotti may reproduce and persist in copepod-free aquaria for several months. In order to investigate whether C. cotti has selected towards monoxeny in water systems lacking copepods, in contrast to the opposite selection pressure when copepods are present, 2 separate infection trials were run. It was shown that the parasite can infect the fish host both indirectly via copepods, and directly. However, C. cotti has significantly higher fitness, expressed as survival to maturity, when transmitted indirectly compared to the direct transmission mode. We suggest that the ability of aquarium populations of C. cotti to carry out a direct life-cycle is favoured by selection in order to avoid extinction whenever copepods are absent. It still remains unknown, however, whether the parasite shows the same characteristics in the wild.

The tapeworm Schistocephalus solidus alters the activity and response, but not the predation susceptibility of infected copepods.

Cyclops abyssorum and Cyclops scutifer were experimentally infected with procercoids of the cestode Schistocephalus solidus, then examined for altered behavior and susceptibility to predation, respectively. Infected C. abyssorum differed from uninfected controls in their initial response to disturbance and their degree of activity but only when they harbored procercoids that were potentially infective to the next host. To examine the possible consequence of the altered behavior for susceptibility to predation, three-spined sticklebacks (Gasterosteus aculeatus) were offered equal numbers of infected and uninfected C. scutifer. Infected copepods were not preyed upon significantly more than controls.