The Ascidian-Amphipod Association between Phallusia mammillata (Cuvier, 1815) and Leucothoe richiardii (Lessona, 1865) in the Mar Grande of Taranto (Mediterranean Sea, Italy)

Amphipods are widespread in the marine environment, and some have adopted a symbiotic lifestyle, such as numerous species of the genus Leucothoe (Leach, 1814). Unfortunately, few examples of such a relationship are known for the Mediterranean Sea. In the present study, we report for the first time the symbiotic relationship between Leucothoe richiardii (Lessona, 1865) and the ascidian Phallusia mammillata (Cuvier, 1815) from the Mar Grande of Taranto. Two samplings were carried out in November 2021 and May 2022, and the population structure of L. richiardii was also analyzed in relation to host weight. L. richiardii was found to live its entire cycle in P. mammillata. The amphipod population was dominated by females, and in most cases, juveniles of various sizes were found together with adults, indicating prolonged parental care. The host selection by the amphipods proved to be random and was determined by the abundance/availability of ascidians rather than their weight. The ascidians showed no signs of disturbance by the inhabiting amphipods, although they were present in large numbers, and we assumed that the relationship between L. richiardii and P. mammillata was commensal.