Eyes Under Puget Sound’s Critter of the Month
A striped nudibranch, Armina californica, collected from Nisqually Reach, Washington, March 2019. |
July’s critter may be cute as a button, but don’t
let the squishy sea slug face fool you. This voracious hunter strikes fear into
the hearts of tiny invertebrates everywhere!
Slug of the sea
Nudibranchs, or sea slugs, are the more elegant, marine-dwelling cousins of the
slimy brown slugs you find in your garden. The striped nudibranch can get fairly large (up to 8 cm) and should be easy
to spot. You won’t find one on a beach walk or tidepooling session though. They prefer
the sandy or muddy seafloor anywhere from the low intertidal zone to 80 meters
deep. We collect them in our Puget Sound sediment samples from Nisqually Reach,
Everett, and Sinclair Inlet … often co-occurring with their favorite food, sea pens.
The pen is NOT mightier than the slug
Striped nudibranchs feed primarily on colonial animals called sea pens. In Puget Sound, their
two main prey species are the orange sea
pen, Ptilosarcus gurneyi and the slender
sea pen, Stylatula elongata.
In the southern part of their range (California to Panama), they also feed on the sea pansy, Renilla koellikeri. These species can bioluminesce, or give off light, when disturbed, so munching by sea slugs can set off a tiny underwater fireworks display!
In the southern part of their range (California to Panama), they also feed on the sea pansy, Renilla koellikeri. These species can bioluminesce, or give off light, when disturbed, so munching by sea slugs can set off a tiny underwater fireworks display!
Night terrors
Striped nudibranchs swarm and consume an orange sea pen until only the white skeletal rod remains (visible in upper right corner). Photo by Neil McDaniel.* |
Striped nudibranchs cleverly let the routine of
their prey determine their activity schedule. Instead of wasting time crawling around during the day when sea pens are
buried in the sand, they stay buried too, with only their rhinophores, or sensory structures, protruding.
The rhinophores help the nudibranchs “smell” chemicals in the water that indicate food is nearby, and they can detect changes in light with a pair of tiny eyes. When night falls they emerge, searching for fleshy victims to devour. The nudibranchs use suction to ingest pieces of the sea pen’s tender polyps, pulling the tissue off and leaving only the hard white rachis, or skeleton, behind.
The rhinophores help the nudibranchs “smell” chemicals in the water that indicate food is nearby, and they can detect changes in light with a pair of tiny eyes. When night falls they emerge, searching for fleshy victims to devour. The nudibranchs use suction to ingest pieces of the sea pen’s tender polyps, pulling the tissue off and leaving only the hard white rachis, or skeleton, behind.
Pretty with poison
You would think plenty of animals would be happy to give the striped nudibranch a taste of its own medicine in the predation department, but even the ravenous sun star, which eats pretty much everything in its path, gives this little nudibranch a wide berth. In fact, the striped nudibranch has very few natural predators. So, what makes this slow-moving nudibranch so unappetizing? It can incorporate chemicals from the tissue of its sea pen prey into its own body, making it toxic to consume.In the groove
While the striped nudibranch may be the stuff of nightmares to a sea pen, it’s a daydream for a mollusk taxonomist. Most nudibranchs are difficult to identify, but the distinct long white ridges and dark background of this species are very distinct. It has no appendages on its top surface except for the rhinophores, which project out of a notch on its head; everything else is tucked away in a groove running down the side of the animal’s body. Hidden away in these folds are flap-like gills, the anus, and the reproductive opening.Doing it all
Like other sea slugs, striped nudibranchs
are hermaphroditic (possessing both male and female sex organs). Any two individuals
can mate by connecting the reproductive “ports” on the right sides of their
bodies. Eggs are laid in brownish spirals, and larvae hatch out into the water
column to eventually settle and become tiny striped terrors in their own right!
By: Dany Burgess & Angela Eagleston, Environmental Assessment Program
Critter of the Month
Our
benthic taxonomists, Dany and Angela, are scientists who identify and count
the benthic (sediment-dwelling) organisms in our samples as part of the Marine
Sediment Monitoring Program. We track the numbers and types of species we
see in order to understand the health of Puget Sound and detect changes over time.
Dany and Angela share their discoveries by bringing us a
Benthic Critter
of the Month. These posts will give you a peek into the life of Puget
Sound’s least-known inhabitants. We’ll share details on identification,
habitat, life history, and the role each critter plays in the sediment
community. Can't get enough benthos? See photos from our Eyes
Under Puget Sound collection on Flickr.
* This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 4.0 License.
* This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 4.0 License.
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