No abstract

Parapodial glandular organs (PGOs) of Spiophanes (Polychaeta: Spionidae) were studied using light and electron microscopy. These organs are found in parapodia of the mid body region, starting on chaetiger 5 and terminating with the appearance of neuropodial hooks (chaetiger 14 or 15 in adult individuals). Large PGOs in anterior chaetigers display different species-specific types of openings whereas small PGOs in posterior parapodia of the mid body region always open in a simple vertical slit. Each PGO is composed of three main complexes: (1) the glandular sac with several distinct epithelia of secretory cells and secretory cell complexes and the reservoir filled with fibrous material, (2) the gland-associated chaetal complex (including the region of chaetoblasts and follicle cells, follicular canals, two chaetal collector canals, the combined conducting canal, the chaetal spreader including the opening of the glandular organ with associated type-1 secretory cells, and the gland-associated chaetae), and (3) a bilayered musculature surrounding the gland. A considerable number of different cell types are involved in the secretory activity, in the guidance of the gland-associated chaetae, and in the final expulsion of the fibrous secretion at the opening slit. Among these different cell types the type-5 secretory cells of the proximal glandular complex with their cup-shaped microvilli emanating thick microfibrils into the lumen of the glandular sac are most conspicuous. Secretory cells with cup-shaped microvilli being involved in the production of β-chitin microfibrils have so far only been reported from some representatives of the deep-sea inhabiting Siboglinidae (Polychaeta). We suggest that the gland-associated chaetae emerging from inside the PGOs of Spiophanes are typical annelid chaetae formed by chaetoblasts and follicle cells. Functional morphology implies the crucial role of PGOs in tube construction. Furthermore, the PGOs are discussed in consideration of phylogenetic aspects.

Temporal variations in the prevalence of larval trematodes in the short-lived prosobranch mudsnail Hydrobia ventrosa (Montagu) were investigated in relation to host life history and season for 4 successive years in temperate windflats of the southern Baltic Sea. The component community of trematode larvae in H. ventrosa comprises at least 10 species; families (and species) represented include Notocotylidae (1), Echinostomatidae (1 or 2), Heterophyidae (2), Monorchidae (1), Microphallidae (3 or 4), Psilostomatidae (1), and Hemiuridae (1). The notocotylid Paramonostomum alveatum was the most prevalent species, followed by the microphallids Maritrema subdolum and Microphallus sp. Trematode prevalence in H. ventrosa fluctuated seasonally. Prevalence usually peaked in summer between July and September-October and decreased in late winter-early spring. This seasonal change is chiefly explained by the life history patterns of the semelparous snail host. Hydrobia ventrosa has a maximum life span of about 2 yr and reproduces between June and November of its second calendar year. The first trematode infections appeared annually in May when the most abundant cohort of H. ventrosa, the second-calendar-year snails, mature. The prevalence continued to increase until August-September, throughout the reproductive period of the second-calendar-year snails, Prevalence decreased during winter, when most of the second-calendar-year snails died after reproduction. On the basis of longterm laboratory experiments, it has been shown that the late autumn-winter mortality was not the result of trematode infections. Seasonal patterns of prevalence were similar among the trematode species except for the monorchid Asymphylodora demeli, the only one using fish definitive hosts. Species-specific differences in the seasonal occurrence of prepatent infections and the predominance of certain larval stages in winter are interpreted as different strategies of the trematode species to survive the harsh winter conditions, or to survive the death of the first intermediate host in autumn-winter, or both.

The impact of Maritrema subdolum Jagerskiijld, 1908 on Corophjum volutator Pallas, 1766 was examined under laboratory conditions. Adult specimens of C. volutator were placed in petri d s h e s and exposed to different numbers (3, 10, 30, 50, 100, 150, 200) of recently emerged cercariae of the microphallid trematode M. subdolum. Addition of the same number of cercariae to the dishes was repeated every second day until the crustaceans died. The experiments revealed a strong negative correlation between the density of cercariae in the dlsh and the average survival times of C. volutator. Amphipods exposed to cercariae densities of 200 cercanae per 9.6 cm2 d~e d after 1.6 d (10.5 SD) whereas specimens exposed to cercariae densties of 30 cercariae per 9.6 cm2 died after 35.8 d (17.1 SD). All dead C. volutator were inspected for larval M. subdolum. Highest infestation intensities of approxinlately 80 larval M. subdolum were found In the test group to whlch 50 cercariae were added every second day. In other test groups, where C volutator was exposed to elther higher or lower cercanae densities, the specinlens harboured fewer larval trematodes when they died. Not yet encysted metacercariae seemed to affect C. volutator in a more detrimental manner than encysted metacercariae. Changes in the behaviour of the infested crustaceans were observed during the experiments. Heavily infested specimens had problems with swimming and crawling m the normal way. All results are discussed in relation to field data. Mass mortality of C. volutator as a result of M. subdolum infestation seems to be an exceptional but conceivable occurrence in shallow water areas of the Baltic Sea.