Growth rates Growth rates are variable depending on environmental or laboratory conditions. Growth in number of polyps is exponential, the colonies doubling in polyp number every 2-4 days, although growth rate can very as much as two-fold even under standard conditions (Fulton, 1961, 1962). In addition, although old colonies could reach as much as 2000 polyps in size growth rates decreased with age (Fulton, 1962). Fulton (1961) reported that uprights grew at 0.05mm/hr while stolons extension rates vary from 0.1mm/hr (Fulton, 1961) to 2-3mm3/day (Chester et al., 2000). Fulton (1962) reported that growth rates varied with temperature, salinity, ionic composition, oxygen tension and feeding rate (see sensitivity).
Seasonal changes Cordylophora caspia dies back in late autumn and over-winters as dormant stolons and resting stages (menonts) inside the remnants of the uprights (see Roos, 1979 for figure, Arndt, 1989, Jormalainen et al. 1994). Arndt (1989) reported that colonies died back in autumn when the temperature fell to about 10 °C only to germinate in spring when the temperature exceeded 5 °C. Roos (1979) reported that colonies died back in October and new polyps budded again in early spring in the Netherlands. In the Baltic Sea growth was maximal in spring, uprights reaching maximal height at the peak of sexual reproduction in July, with a decline after sexual reproduction, and subsequent growth in August (Jormalainen et al., 1994). However, in one year, Jormalainen et al (1994) noted that the colonies regressed to the dormant condition after sexual reproduction then started growing again by mid August.
Feeding Hydroids are passive carnivores that capture prey that swim into, or are brought into contact with their tentacles by currents. Prey are then killed or stunned by the nematocysts born on the tentacles and swallowed. Diet varies but is likely to include small zooplankton (e.g. nauplii, copepods), small crustacea, chironomid larvae, detritus and oligochaetes, but may include a wide variety of other organisms such as the larvae or small adults of numerous groups (see Gili & Hughes, 1995).
Non-native status Cordylophora caspia was thought to have been introduced to British waters on foreign timber (Allman, 1871-1872). Cordylophora caspia was introduced into the Baltic Sea in ca 1803 and was reported as an alien species in the Baltic Sea and the Chesapeake Bay region, USA (Folino, 1999 (summary only); Olenin et al., 2000). Folino (1999, summary only) suggested that the distribution of Cordylophora spp. was expanding globally due to increased boat travel and ballast discharge.