The odd couple

Today my co-teacher, Gabe, and I took our Ecology students up the coast a bit for the first field trip of the semester. We spent the morning at the bottom of Big Basin State Park, where we did a little walking and a lot of looking and talking. In 2.5 hours we traveled maybe a mile, which is the appropriate pace for studying nature.

Much of this area burned in the CZU Lightning Complex fires of 2020. That was 2.5 years ago now, and a lot of vegetation has returned. My favorite thing to see was the lace lichen, Ramalina menziesii. It drapes over plants and has a special affinity for the coast live oak Quercus agrifolia, just like I do. I was very happy to see a lot of new growth in the lichen, all spring green and fresh.

See how pretty it is?

After lunch we crossed the highway and went down to Waddell Beach. The tide had receded and we could see that a tremendous amount of sand had been washed away by the storms, leaving exposed rocks that were usually covered. Students found all sort of interesting rocks, including fossils. We also found several sand dollars, a few of which hadn’t been broken.

This was one of my favorite finds:

It’s the test of the sand dollar Dendraster excentricus. In life, sand dollars are a purplish gray color, covered with apparent fuzz that consists of short spines. I’ve seen sand dollars described as gray shag carpets, and that’s actually not too far off. When the animal dies the spines and living tissue erode away from the test, leaving behind the white calcium carbonate.

Okay, but what’s that round black spot? That’s the super cool thing.

It just so happens that there’s a barnacle, Paraconcavus pacificus, that attaches to sand dollars. It lives on other hard surfaces, too, but the fact that it lives on sand dollars buried in the sand brings to mind all sorts of questions.

Question #1: How does the barnacle’s cyprid larva find a host? My best guess is that the cyprid locates a sand dollar by scent. Sand dollars live partially buried in sand, like tortilla chips in guacamole. Enough of the sand dollar protrudes from the sand to provide plenty of real estate for home-hunting cyprids.

Question #2: What happens to the barnacle if the sand dollar gets completely buried? Well, this I don’t know. A barnacle buried in sand cannot feed. But if the barnacle is attached to the part of the sand dollar that is always (or almost always) sticking out of the sand, then the barnacle could feed perfectly well. Which brings me to . . .

Question #3: Do the barnacles always attach to the same part of the sand dollar? The answer to this question is “Yes”, because sand dollars bury themselves with the “posterior” part sticking up. Yes, it’s hard to talk about anterior and posterior in an animal with pentaradial symmetry, but sand dollars do crawl across the sand in one direction, which defines “anterior” even though the animal doesn’t have a head. Since the sand dollars bury their anterior edge, barnacle cyprids have access only to the posterior edge.

Which in turn explains why the sand dollars with barnacles have barnacles at the bottom of the petaloid, the bit that looks like a 5-petaled flower. That’s the posterior area.

Question #4: Does the weight of the barnacle affect the sand dollar, perhaps by making it top heavy? I can’t imagine that it doesn’t have some effect on the sand dollar. A barnacle is a pretty hefty object, after all. But I can’t find any research about these barnacles and their strange habits.

Returning to that black patch on the test above, now. That’s the scar left behind by a barnacle that used to live on that sand dollar. I have no way of knowing whether the barnacle died before or after the sand dollar died, and suspect it could be either. This barnacle-on-a-sand-dollar is one of those known phenomena that are pretty much not studied at all. It’s really too bad, because I’m convinced there is a fascinating story to be told.