Know your beach—this week from the beach

#2 in the 2019 series of guest blog posts by Seattle Aquarium beach naturalists Bobby Arispe and Jen Strongin.

Half of a clam shell


One of my favorite parts of exploring at low tide is discovering tiny worlds of life. Multiple creatures living, eating, competing and reproducing, all on an old clamshell or chunk of rock. I was on the beach in Des Moines for my first shift of the 2019 beach naturalist season. It was a typical Pacific Northwest morning—gloomy and chilly. A few families came down to the water's edge looking for a big sea star or trying to catch a glimpse of an octopus. We had troubling finding much beyond a few teeny-tiny shore crabs scurrying about. That's when I came upon the perfect little scene on half of a Pacific gaper clamshell.


Dorid nudibranchs


Let's start with the two brown lumps. Those are a couple of barnacle nudibranchs with their eggs. These tiny dorid nudibranchs only reach about ¾” (or 2cm) in length. Their semicircular gill plumes help us figure out which part is the front and which is the back. Barnacle nudibranchs, as their name suggests, are found around barnacles and eat small barnacles. The shell also contains their egg ribbon. It's the part in the next photo that looks like a white curvy wave or noodle.


Opalescent nudibranch


A tiny opalescent nudibranch glides across the shell through the barnacle nudibranch's eggs. Opalescent nudibranchs are little wonders of our Puget Sound and are a beach naturalist favorite—we're always excited to find one.  


Opalescent nudibranch anatomy


Opalescent nudibranchs have feather-like cerata on their backs. These structures aid in respiration and defense. These nudibranchs can ingest stinging cells (nematocysts) from their prey—such as anemones and hydroids—and move the stinging cells into the tips of their cerata. The nudibranch’s rhinophores can be seen above its two pairs of tentacles. Rhinophores help nudibranchs "smell" their food and keep it close.

Opalescent nudibranchs are voracious predators and will get into fights when they encounter each other. This one, with no competition, is happy to playfully squeeze its way through the barnacle nudibranch’s eggs.


Nudibranch eggs


Come meet us on the beach! Locations, dates and times are on our website. Spot something cool? Take a photo and post it to social media with the tag #beachnaturalist or #seattleaquarium.

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