If I mention the word “worms”, most people think of
earthworms and many view them with distaste. There are several species, all looking
rather similar, and we don’t find them attractive because they are slimy and often live in decomposing organic matter. However, it is not
unknown for infants to try eating them, so our dislike is something we learned
from adults or older children.
We are less familiar with marine
worms, and there are many different forms. The white calcareous tubes you see
on rocks, or the small spiral tubes found on seaweeds like wracks, are secreted
by worms and there is every likelihood that the worm is still resident when you
see their ”home” at low tide. Marine worms of many species live in tubes and,
in addition to those made from secreted calcium salts, these may be poorly consolidated
– as with the lugworms beloved of sea anglers – or constructed of grains cemented
together to prevent abrasion by moving sand grains. However, these tubes offer
little protection against predation by wading birds in shallow water.
The mason worm (Lanice)
is widely distributed and extends from the inter-tidal down to 1900 m [1] and
may occur in very high densities (up to 20,000 individuals per square metre [2]).
Lanice makes tubes that extend above
the surface of the sandy mud in which the worms live and each tube has
extensions at its tip. Where present as dense reefs, Lanice tubes promote sedimentation of fine mineral and organic
particles and these sediments increase biodiversity [3]; the worms being
referred to as “ecosystem engineers”. Close examination shows the tube and extensions
to be made of sand grains and shell fragments cemented together by a secretion
of the worm - thus the term mason worm (see above in a wonderful image captured
by Jim Greenfield).
Lanice is in the
group of worms known as terebellids and they feed when the worm and its tube
are covered by water, so feeding can best be observed when worms are
transplanted to an aquarium tank. The tentacles at the front of the body (see
above) are extensible and very mobile and, if we look at them under a
microscope, we see many hundreds of thousands of beating hairs (cilia)
over their surface and also a covering of mucus, produced from cells within the
tissues of the tentacles [4]. Algae and detritus become attached to the
tentacles when they are spread on to the substratum and the cilia then carry
the mucus-bound “packages” to the mouth where they are ingested. Waste products
are removed by the currents of water that the worm generates by moving its body
within the tube.
Lanice also feeds
by spreading the tentacles over the “fan” of extensions constructed at the top
of its tube, collecting particles from the currents that result from wave
action. We know that the particles carried in suspension contain micro-aggregates
formed by bubbles created when waves break [5] and these, too, form part of the
food for the worms, together with anything else that becomes swept up.
How do the worms locate their habitat? The answer is that it
is largely a matter of chance. After reproduction (there are separate male and
female worms [6]), larvae become planktonic and are carried around in the water
column by currents and by their own swimming by means of the ciliated bands on
their body (a second use of cilia for larvae as they also use these organelles
to gather food). The large majority of planktonic larvae are eaten, or fail to
reach a suitable substratum, but, when they do, each larva swims down and
begins to transform into a small worm and begin their work as “masons”.
Far from feeling distaste at the sight of these worms, I
marvel at their biology and how they evolved their form and habits. The sense
of wonder is one of the pleasures of Natural History and the never-ending
fascination of looking at living creatures.
[1] R. M. S. Alves, C. Van Colen, M. Vincx, J. Vanaverbeke,
B. De Smet, J.-M. Guarini, M. Rabaut
and T.J. Bouma (2017) A case study on the growth of Lanice conchilega
(Pallas, 1766) aggregations and their ecosystem engineering impact on
sedimentary processes. Journal of
Experimental Marine Biology and Ecology 489: 15-23.
[2] A. Nicolaidou (2003) Observations on the
re-establishment and tube construction by adults of the polychaete Lanice conchilega. Journal of the
Marine Biological Association of the United Kingdom 83: 1223-1224.
[3] B. De Smet, A.-S. D’Hondt, P. Verhelst, J. Fournier, L.
Godet, N. Desroy, M. Rabaut, M. Vincx and J. Vanaverbeke (2015) Biogenic reefs
affect multiple components of intertidal soft-bottom benthic assemblages: the Lanice conchilega case study. Estuarine, Coastal and Shelf Science
152: 44-55.
[4] R. P. Dales (1955) Feeding and digestion in terebellid
polychaetes. Journal of the Marine
Biological Association of the United Kingdom 34: 55-79.
[5] R. S. Wotton (1996) Colloids, bubbles and aggregates: a
perspective on their role in suspension feeding. Journal of the North American Benthological Society 15: 127-135.
