ORDER
Polypodiozoa
FAMILY
Polypodiidae
TAXONOMY
Polypodium hydriforme Ussov, 1885, Black Sea.
OTHER COMMON NAMES
None known.
PHYSICAL CHARACTERISTICS
Polypodium lifecycle as a succession of a free-living stage and of a stage parasitizing the eggs of some Acipenseridae and Polyodontidae fishes. The earliest known stage is a binucleate cell, parasitizing previtellogenetic fish oocytes. Further development may last several years, leading to a convoluted didermic stolonal structure, with inverted germ layers, forming numerous inverted buds. Before fish spawning, eversion takes place and the germ layers take their normal position (ectoderm outside, endoderm inside). The stolon exits the egg and becomes fragmented into individual buds, each giving rise to a free-creeping globular stage that multiplies by longitudinal fission. Globular stages can move and feed, having an oral mouth-cone and either 24, 12, or six tentacles, according to season. Germ cells are endodermal. So-called females have two kinds of gonads, each with a gonoduct opening in the gastral cavity; so-called males deprived of gonoducts, their gonads forming gametophores carrying cnidocysts. It is not known how the parasites get into young previtellogenic fish oocytes. The free-living stages are presumably homologous to sexual medusae, the parasitic stages being considered as polypoid. By their stolonal parasitic budding stage and their cnidome, the Polypodiozoa seem to present some affinities with the Narcomedusae, to which they were previously associated. P. hydriforme was, until recently, the only known metazoan adapted to an intracellular parasitic life.
DISTRIBUTION
Freshwater basins of Russia, Romania, Iran, and North America. (Specific distribution map not available.)
HABITAT
Fish gonads.
BEHAVIOR
Parasitic.
FEEDING ECOLOGY AND DIET
Parasitic.
REPRODUCTIVE BIOLOGY
Early developmental stages intracellular parasites of the eggs of Acipenseridae and Polyodontidae fishes; free-living stages a small medusae.
CONSERVATION STATUS
Not listed by the IUCN.
SIGNIFICANCE TO HUMANS
Polypodium parasitize and destroy the eggs of the fishes producing caviar and, consequently, has a great economical impact.
