Reconciling slow linear growth and equivocal competitive ability with rapid spread of peyssonnelid algae in the Caribbean.

Within the past decade, a functional group of encrusting red macroalgae defined as peyssonnelid algal crusts (PAC) has been rapidly spreading on shallow Caribbean reefs, frequently encrusting corals and sponges. This study focused on PAC growing on the reefs of St. John, US Virgin Islands, to (1) expand the monitoring of PAC abundance in this location, and (2) test two mechanisms, rapid linear growth and competitive dominance, by which PAC is hypothesized to enhance its capacity to spread. In August 2019, we found that the percentage coverage of PAC reached 46.6% at Tektite (3 m depth), and 63.8% at Cabritte Horn (3 m depth). Overall, the cover of PAC at these sites in 2019 was identical to, or up to 2.4-fold higher than in August 2017, and 2.1–3.7-fold higher than in November 2017, just after two category 5 storms impacted St. John. PAC was 1.8–29.1-fold more abundant at 3 m versus 9 m depth, and 1.4–22.0-fold more abundant at sites close to the tips of headlands than sites sheltered within bays. From August 2019 to January 2020, the rate of linear extension of PAC at each site and depth ranged from 26 to 43 µm d ^{− 1}, and of 80 PAC-coral interactions that were tagged in August 2019, none involved corals that had become completely overgrown by PAC by January 2020. The slow linear growth and absence of cases in which PAC completely overgrew coral colonies in 6 months are inconsistent with the high rate at which PAC has occupied space on Caribbean reefs over the past decade. One means to reconcile these observations is the possibility that recruitment of peyssonnelid spores to benthic surfaces facilitates the rapid spread of PAC. [ABSTRACT FROM AUTHOR]