Filter-Feeding Food, Featherdusters, And Phytoplankton
By Ronald L. Shimek, Ph. D.
Introduction:
Figure 1. A temperate feather duster, Pseudopotimilla. These
are about the size and shape of Sabellaste maganifica.
One of the most sadly misunderstood characteristics about coral reefs
is summed up in the phrase, “The water over the reefs is nutrient-poor.” This
is taken by most aquarists to mean that the water over reefs is low in
food and, consequently, that they should not feed their animals any appreciable
amounts of food. What that statement means, however, is that coral reefs
are in water that has little dissolved nutrient. This is in contrast to
many coastal situations where terrestrial runoff brings a significant amount
of dissolved mineral nutrients into the water column.
The water that bathes most coral reefs is exceptionally rich in all sorts
of plankton. It has only been recently determined just how rich and abundant
this plankton is. Most of the earlier work done on reefs, just about everything
done prior to about 1985 as a matter of fact, used methodologies that were
not suited for sampling or enumerating the majority of plankton types found
on coral reefs.
Much of this plankton bathing a reef is very small, often less than 10 µm
(or 0.0004 inch) in diameter. A variety of different terms have been used
to describe this material, all of being some sort metric derivitives, such
as micro-, nano-, and pico- plankton. I would love to be able to give you
a good description of each of these names, but I can’t. The study
of this planktonic component is so new that the terms for describing it
are simply not consistent enough to use across the board.
Figure 2. A Caribbean feather-duster worm. Compare with the temperate
one in Figure 1; these animals have a soft tube made of mixture of protein
and filtered particles. These worms will often do best in aquaria if
placed in the sediment in a natural position.
The smallest plankton normally consists of bacteria; small cells generally
on the order of about 0.1 µm or 1/250,000 inch in diameter or smaller.
There are larger bacterial aggregates, of course, but the basic bacterium
is truly a “wee beast.” Only slightly larger than this, are
some types of algal or “phyto”- plankton. At the risk of becoming
more technical than I want to, there are really two only types of life:
those composed of cells with internal structures such as nuclei enclosed
by fine membranes and those composed of cells without such structures.
The first type of cells are referred to as “eukaryotic” cells
and are found in animals, plants, fungi, and the various types of algae.
The second type of cell is called “prokaryotic” cells, and
these are bacterial cells. The bacteria are by far and away the most diverse
and abundant living organisms, but as we can’t see the individuals
with the unaided eye, we aquarists – and most scientists - tend to
ignore them. The roles bacterioplankton play in the nutritional dynamics
of natural ecosystems are still unclear. Their sheer abundance argues for
their being very important, however, and they are likely as important in
our artificial systems.
Figure 3. A colony of small feather dusters that is about 10 inches
long. Such aggregations are common in the tropical reefs and are formed
due to differential settlement (where larvae aggregate around adult worms)
or by asexual means, where one worm divides and both offspring stay together.
Phytoplankton organisms, on the other hand, are classically known as the
base of many, if not most, marine food chains. The term “phytoplankton” is
somewhat unfortunate, it literally means “plant” plankton,
but we know now from studies of the DNA of both plants and the planktonic
organisms that many of the various types of algae are no more closely related
to plants than they are to fungi, animals, or each other. In fact, plants
and animals are far more closely related to each other than they are to
red algae, diatoms, or dinoflagellates. Each of these types of organisms
may legitimately be put into separate “Kingdoms of Life” (as
in the Animal or Plant Kingdoms) all their own. About the only type of
phytoplanktonic organisms that are closely related to plants are the green
algae, or Chlorophyta, and this is because some ancient green alga was
probably the ancestor of all plants.
Nonetheless, most phytoplankton share a number of basic characteristics:
They are unicellular, although some of them form “floating chains” or “colonies,”
All of them are small, between 1µm and about 50 µm in diameter,
Most of them are mobile, using one or more flagella,
Most have a “shell” made of cellulose, or silica, or calcium
carbonate, and
Most are photosynthetic, but most also require some nutrients other than
the basic plant nutrients of carbon dioxide and water,
Most have approximately the same density, just a bit more than sea water.
It is worth noting that all of these characters are basic and superficial
and do not imply any degree of relationship. Even within single groups,
however, there are often some very significant differences; as an example,
most green algae are quite motile and have one or two flagella. Some even
have “eyespots” which they seem to use to orient relative to
incident light. That having been noted, individuals of one of the most
common green phytoplankton species used as food in the aquarium hobby,
Nannochloropsis oculata, have no flagella and are non-motile.
Figure 4. Christmas tree worms are serpulids and have a hard calcareous tube (inside the coral head), but they feed in essentially the same was as the soft-tubed sabellids.
Feeding on Plankton
Photosynthetic organisms everywhere are at risk of being eaten. They are the base or the start of almost all food webs because they can make their own food. In the most basic form, they take carbon dioxide and water, and use light energy to covert this into sugars giving off oxygen as the waste product. This process is called photosynthesis. The reverse of this, respiration, is what drives all metabolic processes. In respiration, these sugars are chemically broken back down to carbon dioxide and water with the addition of oxygen. In effect, the sugar is burned and the energy given off from this burning is used as the power source for other cellular chemical reactions. Because all the chemistry critical for life occurs in water, this combustion of sugars doesn’t liberate its energy as visible light and flame; nevertheless, the sugars are still burning. This slow, wet, fire creates heat, and the yield of energy per molecule is exactly the same as if the molecule had been burnt in air.
Intracellular “combustion,” or respiration, is done by a series
of well-coordinated and coupled chemical reactions that carefully “harvest” useable
energy from the breakdown of the sugars. Almost all of the energy transferring
in cells is done by the use of small molecules of that are complexes of
phosphate- and nitrogen-containing compounds. In other words, phosphates
and nitrates are absolutely essential for living organisms, and must be
available in quite high amounts. As aquarists, we are quite aware of the
need to remove excess amounts of both of these materials, but we often
forget why they are there in the first place.
In all ecosystems, it often pays to go directly to the source for food,
and that means that a lot of marine animals feed on phytoplankton. Some
very small animals will feed on this stuff directly, eating it one cell
at a time, but most of the animals collect it by feeding on quantities
of it. These animals are often called suspension-feeders, as they feed
on minute plankton “suspended” in the water, or simply filter-feeders
as they filter food out of the water.
Figure 5. The feeding crown of the Caribbean Split-Crown Feather Duster, Anamobaea
orstedii. The arrows show food grooves and the mouth is indicated.
Filter-Feeding Feather-Dusters
Many kinds of filter-feeding animals occur in reef aquaria, but one group
that depends specifically on phytoplankton are the feather-duster worms,
particularly the large feather duster worms in the genus Sabellastarte.
There are three species that more-or-less commonly are found in the aquarium
hobby, the Caribbean Sabellastarte magnifica; and two species from the
Indo-Pacific, Sabellastarte indica and Sabellastarte sanctijosephi.
These three large species are quite similar, and differ primarily as to
the ocean of origin and the pattern of tentacles in the feather duster
crown. The S. magnifica and S. santcijosephi are characterized by the presence
of a doubled row of tentacles in the crown, while the tentacle crown is
horse-shoe shaped in S. indica.
All feather dusters are characterized by being a type of worm that belongs
to the group that marine scientists call polychaete annelids. This is more-or-less
the same group that aquarists call “bristle worms.” Annelid
worms are worms whose body is divided in many small segments which are
visible externally as looking like rings of tissue separated by grooves.
Earthworms are probably the best example of annelids as most people are
familiar with them. The visible rings or “annuli” representing
each segment give the “Annelids” their name. Earthworms don’t
have any appendages, but bristle worms do.
Bristle worms, or polychaete worms, often have tentacles or other protrusions
from their bodies or heads. Additionally, each segment generally bears
a pair of flaps, one on each side; these are called parapodia. Each parapodium,
in turn, bears the bristles or chaetae that both distinguish and characterize
this group. The very name that scientists use, “Polychaete” means “many
bristles,” so both hobbyists and scientists use the major character
to define this group. Aquarists, however, seem to think that there are
only one or two species of this bristle worms, while scientists have named
over 10,000 species, and this number is almost certainly an underestimate
of the vast total number of species.
Figure 6. A generalized feather duster worm in its tube showing basic
body structures. The mucous strand is the mixture of inedible particles
and mucus that is used to build the tube in which the animal lives.
The common bristle worms found in aquaria are “fire worms” and
they are a good example of a mobile polychaete. The parapodia are large
and the bristles are exceptionally evident. A similar type of worm, or
a worm with a similar body shape and form, is generally thought to be the
ancestor of all the bristle worms, and to earthworms and leeches, for that
matter. Fossil evidence shows these worms were thriving over 500,000,000
years ago, and over the aeons evolution has highly modified the basic worm
resulting in some very bizarre body shapes. Without a hard skeleton to
constrain the growth and shape changes, these worms have developed all
sorts of “bells and whistles.” Many of these modifications
have resulted from selection to make a feeding mode more efficient. One
of the most extremely modified polychaete bodies is found in the three
or four groups that aquarists collectively refer to as “feather dusters.” Scientists
distinguish these groups, the Sabellidae, the Sabellaridae, the Serpulidae,
and the Spirorbidae, primarily on the structure of their tubes, although
there are minor differences in the body structures as well (see Figures
1-4).
These animals are no longer crawling mobile worms that search for food
where they can find it. Rather they live in a tube that they build out
of a combination of bodily secretions and collected materials. The little
appendages on the sides of each segment have become reduced to ridges,
elongated bumps really, and the bristles have changed to be either hooks
to hang on to the tube internally or to small straight bristles which may
be useful in movement in the tube.
Probably the most extreme modifications, though, are seen in the head
region. Mobile polychaetes often have a good head with many sensory structures,
such as tentacles, antennae, palps, and complex eyes. These are animals
that need to know where they are going and what they are encountering.
A worm that lives in a tube, and whose repertoire of locomotion is limited
to going up and down in that tube needs no such structures, and they are
by and large replaced or reduced in tube dwelling worms.
These worms don’t have to pursue food. It comes to them. However,
they do have to collect it. The more efficient the collection apparatus,
the more food the animal gets. The more food it collects, the more offspring
it will produce. The more offspring it produces, the more its species may
occupy space and win at competitive struggles. It should be obvious that
this is a positive feedback loop; any beneficial modification in behavior
or prey capture that is heritable will be favored and come to replace the
less efficient one. In this way, the feather dusters have had their head
region modified into a fantastic and exceptionally efficient phytoplankton
collection apparatus. Their ancestors probably had a simple feeding apparatus
which utilized one or two head tentacles. Over time, the simple apparatus
has become complex and highly specialized.
Normally, sabellids live buried in sediments with about 1 cm of their
leather or parchment-like tube projecting above the surface. They would
probably do best in aquaria in the same situations (see Figures 1 and 2).
Large numbers of Sabellastarte species are commonly imported for the aquarium
hobby. Unfortunately, their long-term survival is generally poor due to
the lack of their food in standard reef tanks. Regular additions of phytoplankton
or bacterioplankton are necessary to maintain these animals. They may survive,
albeit poorly, in tanks with a good sand bed, but will do best with, and
benefit from, the addition of phytoplankton.
Feather dusters need a lot of food, and if the food is not present, will
starve for a while and then shed the tentacle crown. It costs a lot of
energy to maintain the tentacle crown, and the animal, by shedding it,
is cutting its losses. The worm also shrinks and gets thinner. A new, smaller,
crown is regenerated within a couple of weeks, and the animal attempts
to feed. Generally food is still insufficient, and the tentacle crown is
shed a second and maybe even a third time. Eventually the animal dies of
starvation, often after having been in the aquarium for six months or so.
Giant feather duster worm survival is dependent upon being given large
amounts of the appropriate phytoplankton food.
The Feeding of Feather Dusters
A man-made feather duster was a device meant to sweep away tiny dust particles.
A marine worm’s feather duster is a device designed to capture small
plankton, phytoplankton mostly of about dust size. Small hairlike microscopic
structures called cilia are found on the edges and surfaces of the feathers.
These move water through the crown of feather-like tentacles from the bottom
to the top. This gentle water flow creates vortices on the upper surface
of the tentacles and these carry any small particulate material in the
water to the upper surface of the tentacle crown.
The upper surface of the tentacles is bathed in mucus which is continually
secreted all over the surface of the feathers. This mucus is quite liquid
and quite sticky. Any small particles that hit the mucus stick to it. Mucus
has one other important property. It is expensive to make; it is a mixture
of sugars and proteins, so most animals try not to throw it away. This
mucus flows in gutters that take it to the mouth where it is eaten to be
digested and recycled. The mucus is moved in this array of channels and
gutters by cilia, which move it along much as other cilia move the water
past the surface of the tentacles; see Figure 5.
The mucus gutters start in the tiniest parts of the feathers, and connect
to larger and ever larger channels as they near the mouth. As these channels
also convey the particles stuck in the mucus, and as these particles are
the animal’s food, the channels are called “food grooves.” One
would think that these worms would eat all the particles that they can
catch, but if one thought that, one would be wrong. It simply isn’t
the case. The food grooves as they near the mouth have an ingenious way
of sorting the particles by their size. The grooves are shaped like a deep,
narrow “V” with stepped edges; see Figure 7. Large particles,
about the size of a brine shrimp cyst and larger can’t be moved into
the channel at all and are moved along the upper edges. These particles
are moved into special “discard channels” and are shunted off
the tentacle crown and discarded.
Figure 7. A cross-section of feather-duster worm food groove, showing
how the different particle sizes would be sorted by size.
Medium-sized particles, mostly bacterial aggregates and similar-sized
particles, are moved on the internal “step” in the groove.
They are moved toward the mouth but are diverted at the last moment toward
the bottom side of the animal where the mucus and particles are mixed with
some additional secretions from the worm. They are then added to the tube,
increasing its length or repairing it
(see Figure 6).
Only the finest particles, phytoplankton and fine, very small, bacterial
aggregates, make it to the bottom of the food groove. These particles are
conveyed directly to the mouth where they are eaten. It is important to
realize that the worm eats food based only on two things. First, the particles
must have the correct density so that they will flow in the water currents
in such a way as to impact the surface and stick there. Second, the particles
must be the correct size to allow them sink to the bottom of the food groove.
If they are too small, they likely will not have enough mass to even impact
the mucus with enough force to stick. If they are too large, they are either
simply rejected, or used to reinforce the tube. Only if their sizes are,
as Goldilocks, said, “Just right,” will they be eaten.
The animal makes not overt choices in what it feeds upon. It neither tastes
nor sees its food. However, it uses a sophisticated piece of organic machinery
to catch and sort small particulate materials that are about the same size
as an average piece of phytoplankton. In fact, the crown of a feather duster
worm is well designed to be a phytoplankton trap. To thrive, these animals
need plenty of phytoplankton in their water to feed upon, for that is all
they normally feed on, and indeed, it is all they normally have as food.
Links
Picoplankton and sizes
http://www.sb-roscoff.fr/Phyto/picogallery/picogallery.html
Useful and Cited References:
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structure and productivity in the Great Astrolabe Lagoon, Fiji. Coral Reefs.
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Amsterdam. pp. 401-410.
Sorokin, Y. I. 1990. Phosphorus metabolism in coral reef communities:
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Sorokin, Y. I. 1990. Plankton in the reef ecosystems. In: Dubinsky, Z.
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Yahel, G., A. F. Post, K. Fabricius, D. Marie, D. Vaulot and A. Genin. 1998. Phytoplankton distribution and grazing near coral reefs. Limnology and Oceanography. 43:551-563.
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Phytoplankton provides a great benefit to most reef animals. Phytoplankton form the basis of marine food webs in general, and are an essential component of the diet for many reef inhabitants (such as feather duster worms, soft corals, clams, tunicates, and zooplankton), they are probably the least common element included in feeding an aquarium
