Supplementary for

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 measurable shifts in seasonal phenology, yet their temporal coordination remained intact. S. pistillata displayed a 3-month extension of its planula-release season, whereas L. lessepsianus now begins and ends reproduction approximately one month later than historically observed. Despite these shifts, larval settlement continues to coincide with the coral’s reproductive period. These findings indicate sustained phenological coordination within a tightly integrated symbiosis and underscore the importance of multi-decadal datasets in resolving ecological stability under environmental change.