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Larval recruitment of the invasive colonial ascidian Didemnum vexillum, seasonal water temperatures in New England coastal and offshore waters, and implications for spread of the species.

The invasive colonial ascidian Didemnum vexillum occurs in Japan, North America, northern Europe, and New Zealand. It forms adhering mats on living and non-living hard substrates and alters seabed habitats. We examined the relationship of the first and last occurrences of D. vexillum recruits to water temperature and suggest it is possible to identify coastal and offshore locations that are at risk of colonization by the species based on seasonal water temperature trends. Temperature loggers and settlement plates for recruits were deployed and monitored at three shallow coastal settings in New England, including an open harbor, a marine estuary, and a nearshore island. In addition, the distribution of D. vexillum at sites on Georges Bank, an offshore fishing ground (where settlement plates could not be deployed), was compared to long-term bottom temperature data. Recruits of D. vexillum are small (0.2 to 1.0 mm) but easy to identify, and photographs illustrating the developmental stages of oozooids are presented. Recruits of D. vexillum did not appear on settlement plates at the same water temperature at the three coastal sites. Recruitment occurs in the range of 14 to 20°C and apparently is dependent on local climatic conditions. At coastal sites where we have complete records, recruitment persisted for 3.5 to 5 months; and recruits continued to appear as waters cooled to below the temperature of first occurrence. Recruitment ceased in the range of 9 to 11°C. On Georges Bank, the yearly temperature range (4 to 16-17°C) is the same in areas where D. vexillum is present and in a nearby area where D. vexillum is absent. However, in the reproductive season on the bank, temperature variability is but a few degrees in areas where the species is present; whereas it is high (11°C) where the species is absent. It is known from previous studies that in extremely cold conditions in shallow water D. vexillum colonies degenerate and all but disappear, only to regenerate as waters warm. We suggest that: 1) the degree to which colonies degrade in the cool season influences the length of time they require to regenerate, reproduce sexually, and brood larvae; 2) larval recruits will be released at the end of a developmental period as water temperatures warm, not necessarily when a particular water temperature is reached; 3) larvae likely will appear at different temperatures at climatically different sites, and at approximately the same time and temperature at climatically similar sites; 4) highly variable temperatures during the warm season likely inhibit the reproductive process; 5) the time required for a colony to develop and release larvae and the length of the warm season probably affects the length of the recruiting period at a site; 6) as waters cool, larval release by healthy colonies and recruitment might be regulated chiefly by declining temperature and therefore could end at approximately the same temperature at all sites. At deeper water sites, where minimum temperatures are warmer than at shallow sites, it is possible that D. vexillum colonies are not as affected in the cool season (i.e., do not degenerate) and thus could have a longer recruiting season. This may explain the species’ successful colonization of several large areas of gravel habitat on Georges Bank.