Unveiling symbiotic and parasitic relationships in a Mediterranean nudibranch (Mollusca, Heterobranchia) through fluorescence and molecular techniques

Nudibranchs (Gastropoda) are known for their extraordinary ability to obtain structures, cells or chemical compounds from their preys and include them in their own tissues or cells. These unique strategies co-evolved with the loss of the shell in the adult stage. In this framework, symbiotic relationships were already reported between some nudibranchs and unicellular algae but currently the pattern of distribution inside the body of the host is still lacking. To fulfil this gap of knowledge, seven Spurilla neapolitana (Delle Chiaje, 1841), a Mediterranean facelinid, and two Anemonia sulcata (Pennant, 1777), its preferred prey, were collected from Taranto (Ionian Sea), observed and photographed alive using fluorescence technique to confirm the presence of the symbiont and describe its distribution. Furthermore, molecular techniques were used to investigate on the possible presence of symbiotic and/or parasitic interspecific relationships. Fluorescence clearly confirmed the presence of the symbiont in S. neapolitana and revealed a widespread distribution throughout almost all its dorsal surface. Furthermore, while the common markers confirmed the species identity, the nuclear ITS2 showed a double band within the nudibranch sample due to the different length of ITS2 from different organisms. This result is promising since it supports the presence, in the same tissues, of both host and symbiont. This is the first study describing the distribution pattern of symbiotic algae inside the body of its nudibranch host helping to fulfil some gaps of knowledge still existing on the biology and ecology of most of the Mediterranean nudibranch species.