Unexpected plate appearance

Well, this is a story that's fun, interesting, and also humbling.  It begins with a red-and-white barnacle and a sand dollar.

Some of you might remember that we've been seeing a lot of Megabalanus californicus — a red-and-white striped barnacle that is generally uncommon in the Bodega Bay area.  It's been noticeably more common during the last few years — e.g., see the post from 1 June 2015.  Here's an example of Megabalanus growing on a California Mussel on Bodega Head:

Recently, we've seen a few sand dollars washing up on beaches with red-and-white barnacles on them.  Because of the increased abundance of Megabalanus, we didn't think too much about it.  (Mistake #1 — we didn't stop to look closely at these barnacles).

In February 2016, Alex noticed one of these sand dollar/barnacle specimens, too, and brought one in to ask us about it:

 

Well, we made the same assumption and thought the barnacles were probably Megabalanus.  (Mistake #2 — We hadn't thought about the possibility that another species of red-and-white striped barnacle could occur in this area.)  We decided to keep this specimen for documentation and put it aside.

Then Jim Carlton came to visit in September 2016.  He's interested in barnacles, so we brought out the specimen to show him.  Almost as soon as he looked at it, he said something like, "You know, there's a southern barnacle that tends to be found on sand dollars."

We looked in the Light and Smith Manual and found a description for Paraconcavus pacificus (formerly Balanus pacificus).  Sure enough, it often occurs on sand dollars, but the Manual said the geographic range was from Monterey south.

Well, we know that some southern species have been showing up during the last two warm-water years, and sometimes El Niño conditions bring "waifs" further north beyond their typical range.  So although Bodega Bay would be out of range, could these barnacles on the sand dollar be Paraconcavus pacificus?

I raised my hand lens to take a closer look:

 

And wouldn't you know it, as soon as I looked through my magnifier, I knew they weren't Megabalanus.  The wall plates were very smooth (rather than having long, vertical ridges), and the color pattern was different, with subtle horizontal stripes, too (giving it more of a "checkered" appearance).

However, since this would be a rare record in northern California, we needed confirmation.  This is where barnacle identification gets a little more complicated.  You need to look at the opercular plates — the four plates that cover the aperture or opening at the top of the barnacle.  

For reference, here's a photo of a common acorn barnacle, Balanus glandula, showing the four opercular plates — a pair of "terga" and a pair of "scuta":

The sand dollar/barnacle specimen had washed up on the beach, so I wasn't sure there would be any opercular plates left behind.  Most of the barnacle shells were empty, but I was excited to see that one barnacle appeared to have plates inside (see arrow below)!  

 

Jim asked if we could use a microscope — and if we had fine forceps — and if there was a shallow dish available.  Yes!  Then he expertly extracted three tiny plates from the barnacle.

Here's what they looked like under the microscope. [Note: One plate is shown twice (both the interior and exterior sides) because not all four plates were present.]

Views of scuta (left) and terga (right).  Scale bar at lower right is 1 mm long. 

And for comparison, here are the opercular plates of Paraconcavus pacificus illustrated in an older barnacle identification manual.  Compare the overall shapes, the textures, the ridges, and furrows against the photos above.

 

Modified from Pilsbry, H.A.  1916.  The sessile barnacles (Cirripedia) contained in the collections of the U.S. National Museum, including a monograph of the American species.   USNM Bulletin 93.

 

It's hard to believe, but as fate would have it, two of the world's leading barnacle experts were also visiting the lab that day — Bill Newman and Bob Van Syoc!  They agreed that the plates were a perfect match.  The barnacles on the sand dollar are Paraconcavus pacificus, and represent a rare record for this barnacle north of Monterey/San Francisco.

I mentioned two localities just now because although most publications list Monterey as the northern range limit for this barnacle, there are a few scattered records (either in museums, in the literature, or online) that list more northern observations:

1912 — California Academy of Sciences specimen — Ocean Beach (San Francisco)

1916 — Pilsbry questioned a specimen from Crescent City and said that the range north of Monterey needed to be investigated

1970 — Merrill and Hobson paper on sand dollars — Bodega Bay

1990 — California Academy specimen — Ocean Beach (San Francisco)

1994 — California Academy specimen — Ocean Beach (San Francisco)

1997 — Mooi paper on sand dollars — Ocean Beach (San Francisco)

2014 — iNaturalist — Ocean Beach (San Francisco)

2016 — iNaturalist — Ocean Beach (San Francisco)

2016 — iNaturalist — Point Reyes (Abbott's Lagoon?)

2016 — this record, Salmon Creek Beach, Bodega Bay

 

You can see that Paraconcavus pacificus has been known from Ocean Beach (San Francisco) for a long time, and appears to be regular there.  So far we only know of three records north of San Francisco — the note by Merrill and Hobson, the iNaturalist record for Point Reyes, and this record in Bodega Bay.

It would be very informative to hear about any other specimens of sand dollars with red-and-white striped barnacles north of San Francisco.  So let me know if you see one (and take a photo)!

P.S.  I can't help adding one more fun fact.  I learned a new word today — "vinaceous."  Pilsbry (1916) used this term to described Paraconcavus pacificus.  It means "the color of red wine."  Here's another specimen (this one collected by Eric in Baja California, Mexico, in 1998) so you can consider the color:

 

P.P.S.  Many thanks to Alex for sharing his find and to Jim, Bill, and Bob, for identification assistance.