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The California Naturalist | Meet a Worm that Builds Castles

If, like me, you’re lucky sufficient to stay near the coast in Northern
California, you get to visit the tidepools. And whenever you do, you could notice
something that seems like a pile of sand within the mid-tidal zone under the mussel
beds. Whenever you venture down and touch the sand, you’ll discover that it’s arduous — exhausting
enough to stroll on, should you step very rigorously, but in addition somewhat brittle.

It’d look one thing like this:

Mound of Phragmatopoma californica tubes at Natural Bridges. (Photograph by Allison J. Gong)

Meet Phragmatopoma californica, the sandcastle worm. Exhausting to
consider that these mounds, which might be the dimensions of a small dining room table,
are constructed by little worms, isn’t it? Phragmatopoma is likely one of the
many marine segmented worms grouped together as the Polychaeta. We now have plenty of
polychaetes on our coast, ranging in measurement from greater-than-hand-length nereids
and glycerids that can take a chew out of you and draw blood, to tiny worms
sufficiently small to swim within the layer of water between sand grains. The truth is, the
majority of our worm fauna on the coast consists of polychaetes.

Polychaetes make up a giant and really numerous taxon, comprising some 80 or so
households. Polychaete taxonomists may argue towards it, however to make things
easier, we will divide them into two subclasses, the Errantia and the
Sedentaria. As the identify implies, the Errantia includes the worms that are
errant, or free-crawling. That stated, most of them don’t truly crawl around
in plain sight; they have a tendency to burrow in sediment, shell particles, or gravel, or
wiggle their method by means of numerous benthic faunal communities. A few of them make
momentary shelters by wrapping themselves in pieces of algae sewn shut with
mucus threads. The Sedentaria, however, are pretty much all, properly,
sedentary. They reside in kind of permanent tubes made of varied supplies,
and usually can’t reside outdoors of them.

Phragmatopoma could be very a lot a sedentary worm. It lives in a tube
that it builds out of sand grains. Sure, this little worm is a mason!

Phragmatopoma californicaPhragmatopoma californica tubes within the mid intertidal at Pure Bridges. (Photograph by Allison J. Gong)

What you see in these mounds is an aggregation of tons of of individual
worms. The mounds don’t type accidentally or probability. Phragmatopoma has
a planktonic larval stage that floats around on ocean currents for some weeks
earlier than returning to the shore. The larva is interested in areas already
colonized by members of their species, which it detects by sniffing out the
chemical sign of the glue used to create the tubes (more on that under). This
phenomenon is known as gregarious settlement. In the event you contemplate the problem of
being a tiny creature looking the complete shoreline for a place to settle and
stay endlessly, one huge clue as to the suitability of a given location is the
presence of adults of the identical species. In any case, if your mother and father’ era
grew up there, likelihood is it’s a great place so that you can develop up, too.

Every of these holes within the huge sandy mound is the doorway to a worm’s tube.
Tubes may be as long as 15 centimeters, but the worm itself is far smaller:
a whopping massive one can be four centimeters long, and most are within the 2-3 centimeter
measurement range. From this pair of observations I infer that the worms can and do
move up and down the tube. They have to move to the open end of the tube to
feed, and may withdraw in the direction of the closed finish to keep away from predators, or search
safety from drying out.

Phragmatopoma californica feeding tubesProlonged feeding tentacles of Phragmatopoma californica at Pure Bridges. (Photograph by Allison J. Gong)

Phragmatopoma‘s tube isn’t
a haphazardly constructed object. It is the worm’s residence for the whole thing of its
post-larval life, and is constructed to defend its builder/occupant from the
mechanical bashing that happens twice every day because the tide floods and ebbs. As
such, it have to be robust and capable of keep its structural integrity. Let’s
take a nearer take a look at an isolated tube underneath the dissecting scope:

Phragmatopoma californica tubeTube of Phragmatopoma californica. (Photograph by Allison J. Gong)

The tube itself is manufactured from debris–sand grains, bits of shell, the
occasional tiny sea urchin spine–that the worm gathers from its surroundings.
Glandular areas on the worm’s anterior region secrete a cylinder of sticky
cement round its body that is chemically just like both spider silk and the
byssal threads that mussels use to attach themselves to rocks within the
intertidal. The inside the tube is lined with a chitin-like material. The
worm makes use of tentacles on its head area to gather and type the “stones” and
glue them to the surface of the liner. There’s a point of selection
involved; in the photograph above you possibly can see that all the sand grains are more
or much less the identical measurement, with none standing out as being conspicuously smaller or
bigger than the others. Rising worms that are actively constructing their tubes
may be geographically restricted a minimum of partly by the supply sand grains
of the appropriate measurement; if the sand is just too high-quality or too coarse, the worms either
can’t or don’t reside there.

Life inside a tube

Dwelling in a tube might provide vital protection from wave bashing and
predators, however it does present some challenges as nicely. One thing that involves
thoughts is the matter of private hygiene: What occurs to the worm’s poop? As we
know, the worm lives contained in the tube however isn’t hooked up to it, and may crawl
up and down inside it. To know how it does, we’ve to evaluate some basics
of polychaete anatomy.

The phrase “polychaete” comes from Greek (“many bristles”) and refers back to the
reality that these segmented worms have chaetae, or bristles, along the left and
proper sides of the physique. In some worms the segments, including chaetae, are
pretty much the identical from the entrance of the physique to the back. In others, the
segments and chaetae are differentiated from one body region to a different. In the
case of Phragmatopoma, all you’ll be able to see sticking out of a tube is the
head region, consisting of the slender feeding tentacles and a giant
disc-shaped construction referred to as an operculum, which made from fused cephalic chaetae
and serves as a door to close off the tube when the worm withdraws. Behind the
head is a collar area, a collection of three adjoining segments that have very
stiff chaetae that may be pushed out towards the liner of the tube to anchor
the body in place. The rest of the physique behind the collar is the trunk, which
bears smaller chaetae on each phase. The whole epidermis is ciliated, which
keeps water flowing across the body.

However what concerning the poop? As in most vermiform animals, Phragmatopoma‘s
anus is at the posterior end of the body, which is oriented in the direction of the closed
end of the tube. How, then, does it defecate with out fouling its house? The
reply is both simple and ingenious. Phragmatopoma has a long rectum,
which is curved to run anteriorly back in the direction of the top. The anus, situated at
the terminal end of the rectum, discharges fecal pellets about halfway up the
size of the body. The ciliary currents of the epidermis then flush the fecal
pellets the remainder of the best way up the tube and out the highest.

The skinny cylindrical issues in the photograph under are Phragmatopoma‘s
fecal pellets!

Phragmatopoma californica tentaclesTentacles of Phragmatopoma californica extending from tubes at Natural Bridges. (Photograph by Allison J. Gong)

Fuel trade is one other challenge for animals that stay within tubes.
Aquatic animals trade respiratory gases with the water that surrounds them,
which is straightforward for animals that reside the place the water is consistently shifting over
their bodies. But for tube-dwellers, fuel change is far more troublesome. Phragmatopoma
has paired gills on each phase of the trunk area of the body, which drastically
improve the floor space for fuel trade. Any fuel change floor is useless
until it connects with the circulatory system, so blood vessels movement into and
out of each gill. Dissolved oxygen diffuses from the water into the blood, and
is then circulated throughout the body. A specific amount of fuel change
in all probability happens throughout the surface of the tentacles, too. To make issues
simpler, the ciliated epidermis of the physique retains that small quantity of water
inside the tube shifting, minimizing stagnation. When the worm’s head is extended
out for feeding, the tube is flushed with clear water. When the worm is
withdrawn into the tube at low tide, its only oxygen provide is within the water
contained in the tube with it. Like most of its intertidal neighbors, Phragmatopoma
hunkers down and waits for the tide to return, when it will possibly feed and breathe
more easily.

And speaking of feeding, I should mention that Phragmatopoma is a
filter feeder. These purple tentacles are ciliated and create a water present
that brings small suspended particles in the direction of the mouth situated on the base of
the tentacles. In the video under the operculum is the darker object to the
left; it represents the dorsal aspect of the worm’s body. The long, filiform
tentacles are the feeding tentacles.

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As you could think about, dwelling in a tube additionally affects the best way that Phragmatopoma
reproduces. The worms by no means depart their tubes, so copulation isn’t an choice
for them. Regardless of their prevalence in giant teams they don’t seem to be clonal, and
reproduce only sexually. Each sexes of Phragmatopoma spawn gametes
into the water, where fertilization and larval improvement take place. Dwelling
in dense aggregations and spawning concurrently everyone else maximizes
the prospect that egg and sperm of the same species will discover each other. Many
marine invertebrates throughout the oceans, from corals to sea urchins, spawn
synchronously. In any case, it does a person no good to throw gametes out
into the world if it’s the solely certainly one of its sort round–all the metabolic
power that went into producing and maintaining the gametes can be totally
wasted.

Clearly, the
benefits of dwelling in a tube outweigh the costs and inconveniences. Phragmatopoma has advanced
physiological, anatomical, and behavioral variations to cope with life within the
intertidal. A type of variations is the tube, which solves one set of
problems but creates others which additionally must be solved if the animal is to
survive. Evolution comes up with options like this all the time. Every trait
has metabolic and/or health prices, and an organism’s biology is predicated on this
sort of evolutionary compromise. Life in the intertidal is a robust recreation. It is
probable that none of the numerous organic processes that hold Phragmatopoma alive work
perform quite in addition to they might if they have been remoted techniques. But inside
the our bodies of those little worms, every thing works just nicely enough for them to
be one of many extra conspicuous inhabitants of the intertidal.