Temperature resilience facilitates invasion success of the solitary ascidian Herdmania momus

The tropical ascidian Herdmania momus (Savigny 1816) has been rapidly expanding its distribution in the Eastern Mediterranean Basin. To examine the role of temperature resilience on its invasion success, we conducted monthly field surveys for two years at Achziv (Israel) Marine Protected Area, and examined the survival of adult individuals from native (Red Sea) and invasive (Mediterranean) populations under different temperature treatments. In addition, temperature effects on fertilization and larval development were examined in controlled laboratory conditions. Results indicate that temperature has a significant effect on H. momus from its early life stages through adulthood. Field surveys revealed a significant decline in H. momus abundance in the Mediterranean with the decrease in seawater temperature. Low temperature inhibited development of early life stages, and individuals from both populations demonstrated low survivability under low temperature treatment. All the above may be derived from the tropical origin of H. momus, and may further limit its dispersal into colder areas. However, adult individuals from the invasive population demonstrated significantly higher survivability to the high temperature treatment in comparison to the native population from the Red Sea. Larvae development, metamorphosis and settlement were enhanced and with higher rates of success under the high temperature conditions. The high-temperature adaptation of the Mediterranean population of H. momus from fertilization to adulthood may thus contribute to its expanding distribution in the Eastern Mediterranean.