Chapter 5 - Cnidaria

Abstract

The freshwater representatives of Cnidaria are small animals belonging to the class Hydrozoa, with relatively few species and somewhat monotonous anatomy. The five freshwater genera of Hydrazoa include: the common brown (Hydra) and green hydras (Hydra or Chlorohydra) with four species groups, the rare medusoid Craspedacusta, and polypoid Calposoma and Polypodium. Cordylophora, a mostly estuarine taxon, occasionally penetrates relatively fresh waters, where it lives in a colonial, sessile form. The cnidarian body consists of a generally thin, acellular layer of mesoglea sandwiched between two thicker cellular layers (ecto- and endoderm), all surrounding a central body cavity (the coelenteron). Because of their small size, soft bodies, and often sessile habits, freshwater Cnidaria are either not collected or not well preserved in most routine collecting procedures. However, they are widely distributed and can be found in most ponds and streams when a specific search is made. This chapter describes the general biology, ecology, and classification of the five freshwater genera of Hydrozoa, and includes taxonomic keys for freshwater cnidarians.