Sara T. Gonzalez1, Filipe Alberto2
1Department of Applied Ocean Physics & Engineering, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA
2Department of Biological Sciences, University of Wisconsin Milwaukee, 3209 N. Maryland Ave., Milwaukee, WI 53211, USA
The giant kelp, Macrocystis pyrifera, exists as distinct morphological variants—or “ecomorphs”—yet the mechanism for this variation is uncertain, and whether these morphological features are genetically fixed or malleable under different environmental conditions is unclear. The two most common ecomorphs, “pyrifera” and “integrifolia,” exhibit almost no geographic overlap across their ranges in North and South America; however, in laboratory settings they are able to interbreed. Our study directly compared the growth and morphology at all life stages of the two Macrocystis ecomorphs in a common garden experiment. We experimentally tested the influence of local environment on giant kelp morphology by rearing lab-cultured embryonic sporophytes from spores released by “pyrifera” and “integrifolia” sporophylls collected from multiple individuals at Stillwater Cove, California. The spores were cultured in three treatments: pyrifera only, integrifolia only, and mixed (50:50 pyrifera:integrifolia spores). We outplanted the resultant embryonic sporophytes to concrete blocks at 7.5-9 m depth in the ocean and monitored their development over five months. Results showed distinct differences in morphology between the pyrifera and integrifolia treatments at multiple stages of development including reproductive adults, indicating that the morphological differences between the two ecomorphs are genetically determined rather than environmentally induced. Primary stipe length and number of branches can be used as diagnostic traits for distinguishing the ecomorphs prior to the stage when adult sporophyte morphology can be definitively characterized. Additionally, no morphological hybrids were observed in the mixed treatment, and individuals resembled either integrifolia-like or pyrifera-like forms.