Zoological Studies

Vol. 59, 2020

(update: 2020.10.21)
 

Seasonality and Longevity of the Functional Chloroplasts Retained by the Sacoglossan Sea Slug Plakobranchus ocellatus van Hasselt, 1824 Inhabiting a Subtropical Back Reef Off Okinawa-jima Island, Japan

Shu Chihara, Takashi Nakamura, and Euichi Hirose* 

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Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan. *Correspondence: E-mail: euichi@sci.u-ryukyu.ac.jp
E-mail: ringoamemh@icloud.com (Chihara); takasuke@sci.u-ryukyu.ac.jp (Nakamura)

Received 18 July 2020 / Accepted 12 October 2020
Communicated by Yoko Nozawa

Plakobranchus ocellatus is a sacoglossan sea slug that feeds on multiple algal species and retains chloroplasts as kleptoplasts for several months. The seasonal difference in photosynthetic properties of kleptoplasts was examined in sacoglossans collected from a subtropical back reef off Okinawa-jima (2621′55″ N 12744′10″ E) in 2017–2018. The effective quantum yield of photosystem II of kleptoplasts indicated greater stress on kleptoplasts under stronger ambient light. The maximum quantum yield (QY) at 20C, 30C, and 40C indicated that kleptoplasts were more functional in photosynthesis in winter than in spring and summer, whereas kleptoplasts in summer may have a greater tolerance to high temperature. In the long-starvation experiment (LSE), the relative ratio of body weight (relW) linearly decreased and the sacoglossans died within 2 months in the total dark condition, whereas in the LSE with illumination, the animals survived up to 5 months. The time course for the decrease in the relative ratio of the QY (relQY) in the LSE indicated that the photosynthetic function was almost normal for 2 months regardless of the presence or absence of illumination and then relQY gradually decreased to zero. In the field, P. ocellatus continuously uptakes new kleptoplasts that have suitable properties of photosynthetic ability for each season. In a subtropical environment, where water temperatures vary from below 20C to above 30C, seasonal changes could cause a temporary shortage of algal food and affect the photosynthetic activity of P. ocellatus kleptoplast. Our results, however, indicated the kleptoplasts of P. ocellatus functioned normally for several months and maintained the occurrence of this sacoglossan in a subtropical environment throughout the year.

Key words: Kleptoplasty, Long-starvation experiment, Long-term retention, Pulse-amplitude-modulated fluorescence, Total darkness.

Citation: Chihara S, Nakamura T, Hirose E. 2020. Seasonality and longevity of the functional chloroplasts retained by the sacoglossan sea slug Plakobranchus ocellatus van Hasselt, 1824 inhabiting a subtropical back reef off Okinawa-jima Island, Japan. Zool Stud 59:0ww. doi:-.