Combined effect of temperature and salinity on the development of the holothurianEupentacta fraudatrix

Temperature (25, 22, 16, and 12°C) and salinity (32–14‰) effects on the development of the low-boreal holothurianEupentacta fraudatrix were investigated. By studying the desalination resistance of adult holothurians,E. fraudatrix was shown to be a stenohaline species. The lower salinity limit at which both the larvae and adults survived was 22‰. Fertilization and development to the stage of free-swimming blastula occurred at the bottom. Embryogenesis, a critical stage of development, was successfully completed at a salinity of 32–26‰ and temperature of 22–16°C. The fertilization and development of a free-swimming blastula proved to be most resistant to temperature changes. The blastulae that developed at lowered temperatures (16–12°C) were capable of further development and settling at the same temperatures, which is likely associated with the peculiarities of the species range. If the early development proceeded at a higher temperature (22°C), the larvae failed to adapt to and perished from sharp temperature decreases at later stages of development. Thus, the lecithotrophic larva and a short period of larval development in the pelagial larvae (3–3.5 days from fertilization to settlement) ofE. fraudatrix are caused by the stenohalinity and environmental conditions of the species and, in turn, contribute to the fact that the young animals settle in the vicinity of their parents.