Feeding behavior of the kleptoplastic sacoglossan sea slug <i>Plakobranchus ocellatus</i> type black

Plakobranchus ocellatus is a sacoglossan sea slug distributed in shallow coral reefs.In 1941, Kawaguti reported the presence of photosynthetically active "unicellular green algae" in the epithelium of the gill chamber of P. ocellatus.Subsequent ultrastructural studies on sacoglossans, including Elysia atroviridis, demonstrated that these "unicellular green algae" were in fact chloroplasts derived from algal food (Kawaguti and Yamasu 1965).This phenomenon-later conceptualized as a "stolen plastid" relationship by Trench in 1969(LaJeunesse et al. 2021)-is now referred to as kleptoplasty, defined as the retention of functional chloroplasts acquired from algal prey within animal cells.In this historical context, P. ocellatus represents one of the earliest species known to exhibit exceptionally long-term plastid retention.Recent phylogenetic analyses of kleptoplasts ("stolen" chloroplasts) based on rbcL sequences have revealed that P. ocellatus feeds on multiple species of siphonous green algae (Maeda et al. 2012).However, direct observations of its feeding behavior on algae have remained limited.Using video recordings, we directly observed the feeding behavior of P. ocellatus on the siphonous green alga Rhipidosiphon javensis, which is advantageous for visualizing cytoplasm removal.Specimens of P. ocellatus and R. javensis were collected from Okinawa Island, Japan in 2025.After approximately 2 months of starvation under controlled illumination, feeding events were recorded from below using a digital camera (Olympus TG-4; Fig. 1A,B; Supplementary Video 1) and a Leica THUNDER imaging system (Fig. 1C, D; Supplementary Videos 2, 3).Similar feeding behavior was observed in seven individuals, and representative images