Eelgrass is thought to play a crucial role in sheltering young fish and other sea creatures . The eelgrass found around Santa Cruz and Santa Rosa is wide-bladed eelgrass (Zostera pacifica). No eelgrass has been found around San Miguel (to my knowledge) and the eelgrass bed on Anacapa, at Frenchy's Cove, was destroyed by storm and replanted from Santa Cruz.
Coyer and collaborators have published an ambitious study of the genetics of eelgrass in the Channel Islands and along the Pacific Coast. The eelgrass at Santa Cruz and Santa Rosa is genetically fairly diverse, but genetically distinct from eelgrass beds at other islands; it's quite distinct from the eelgrass in beds off Isla Vista and Goleta Beach.
The uniformity is surprising, since these islands (and their eelgrass beds) span such a wide range of underwater climate and inhabitants: famously, the northern islands are more like Oregon than like the southern islands; which in turn are more like Mexico than like the northern ones.
The eelgrass at Isla Vista and Goleta Beach had long been thought to be invasive: perhaps Zostera asiatica from Japan. But, it turns out to be Zostera pacifica, a native; and most closely related to that at the Channel Islands.
The eelgrass around Santa Catalina and San Clemente also tells an interesting story, with lots of variation and two distinct species that have apparently hybridized. Human intervention and re-rooting of plantlets dispersed by storm may have played important roles there.
Coyer and collaborators suggest that the eelgrass populations at Santa Cruz and Santa Rosa date back to the last Ice Age, when all the islands were connected into one big one. It's an interesting thought. At the moment, at least, the eelgrass at Santa Cruz and Santa Rosa seems to be a persistent native population.