Blade Abandonment/Proliferation: A Novel Mechanism for Rapid Epiphyte Control in Marine Macrophytes

The strategy of blade abandonment and simultaneous rapid proliferation (via protoplasmic translocation) is unique to siphonaceous macroalgae owing to their lack of restrictive crosswalls. Siphonalean green algae often dominate the standing stocks and productivity of Caribbean mangrove island environs and are also abundant in virtually all calm-water reef habitats. The experimental organism Avrainvillea longicaulis is particularly abundant in the study sites west of Carrie Bow Cay and Curlew Cay, Belize. The recycling of protoplasm out of older epiphytically impaired blades of A. longicaulis, and simultaneous formation of apical siphon extensions elsewhere, leads to rapid proliferation of new blades--hypothetically, at relatively low energy cost to the plant. The rapidity of siphon extension occurs at least an order of magnitude faster than that possible by photosynthetic production alone, resulting in fully formed proliferations within 3 d. At the same time, the older epiphytized blades become empty, first at the tips, then progressively toward the holdfast, appearing translucent-brown and flaccid within 3 d, followed by senescence. We hypothesize that A. longicaulis rapidly responds to epiphyte colonization/impairment by reallocating protoplasm for new growth through protoplasmic streaming. The alternative hypothesis, that the herbivorous crab Thersandrus compressus might beneficially control epiphytes on A. longicaulis, was falsified by its strong negative effects (i.e., decreased growth and increased host mortality). The hypothesis that allelopathic secondary chemicals may play an antifouling role also was not supported by our experiments; i.e., the consistently vigorous growth rates of the five dominant epiphyte species attached to A. longicaulis were not significantly less than populations of the same species attached to carbonate rock. All of the manipulative results (i.e., experimental induction of blade abandonment followed by new proliferation), as well as the populational survey findings (i.e., greater proliferation by epiphytized plants) and physiological data (i.e., photosynthetic inhibition by epiphytes), support the blade abandonment/proliferation hypothesis, thereby documenting an effective epiphyte-control strategy.