Persistent phenological synchrony in a coral-bivalve symbiosis across five decades

Long-term species interactions are often sensitive to environmental change, yet some symbioses maintain coordinated phenological patterns over extended timescales. We examined a five-decade record of reproductive timing in the Red Sea coral Stylophora pistillata and its boring bivalve symbiont Leiosolenus lessepsianus at 30 m depth in the Gulf of Eilat/Aqaba. Historical data (1970s-80s) were compared with monthly observations from 2021-22 of coral and bivalve reproductive development. Both species exhibited clear shifts in seasonal phenology, yet their temporal synchrony remained intact. S. pistillata displayed a 3-month extension of its planula-release season, whereas L. lessepsianus now begins and ends reproduction approximately 1 month later than historically observed. Despite these shifts, larval settlement continues to coincide with the coral's reproductive period. These findings demonstrate sustained phenological coordination within a tightly integrated symbiosis and underscore the importance of multi-decadal datasets in resolving ecological stability under environmental change.