Abstract
Some inherently poorly dispersive marine species have surprisingly large or patchy distributions that might be explained by rafting or 'hitchhiking'. The genus Durvillaea (southern bull kelp) includes both highly buoyant and entirely non-buoyant species. Several of the non-buoyant, poorly dispersive species have puzzling distributions that are hard to explain without invoking long-distance dispersal hypotheses. We propose that these non-dispersive species of Durvillaea may be able to hitchhike with buoyant, dispersive congenerics by interspecific holdfast coalescence. Although many cases of intraspecific holdfast coalescence have been recorded, interspecific coalescence is less well documented. To determine whether interspecific holdfast coalescence occurs in Durvillaea, a rock platform on the south-east coast of New Zealand was surveyed, revealing multiple examples of naturally occurring interspecific holdfast coalescence. Samples were taken from coalesced holdfasts and genetic sequencing was performed to attempt to gauge whether tissue from both species was mixed throughout the holdfast or remained discrete. The discovery of interspecific coalescence between non-buoyant and buoyant Durvillaea raises the possibility that non-buoyant seaweeds may disperse with buoyant congenerics by rafting, and could help explain the distributions of various other non-buoyant macroalgae.