Okay, I'm guessing you didn't know you'd be playing a game today where you try to identify seaweed holdfasts.
Holdfasts are basal root-like structures that hold seaweeds in place. They differ from vascular plant roots in that their primary function does not involve water or nutrient uptake. Instead they serve as all-important anchors, fastening the seaweed to the substrate (via rhizoids and adhesives).
Holdfasts vary in size and shape, but they are generally consistent within a species. Many have branching structures called haptera that grow in anastomosing patterns. Even if you aren't familiar with seaweeds, look for the differences in the haptera shape, width, number, and growth patterns.
[I'll give you a hint: these are all kelps. Most were photographed on Bodega Head on 7 May 2012; the last was taken in 2006.]
Split Blade Kelp (Laminaria setchellii)
Walking Kelp, or Woody-stemmed Kelp (Pterygophora californica)
Nerve-net Kelp (Dictyoneurum californicum)
Sea Cabbage (Saccharina sessilis)
Feather Boa Kelp (Egregia menziesii)
Winged Kelp (Alaria marginata)
Sea Palm (Postelsia palmaeformis)
P.S. There are 12 species of kelp on Bodega Head. This post featured 7 species; we'll post more about the others in the future.