An Introduction to Foods Acceptable to Suspension-Feeding Animals
By Ronald L. Shimek, Ph. D.
Introduction:
This is first in what will be a several-part series on plankton, plankton-feeding,
and the role of such feeding in our aquarium systems. This article discusses
some of the peculiarities of suspension-feeding as well as briefly discussing
the types of foods available for hobbyists. Subsequent articles may investigate
in greater detail some other aspects of the use of these foods. However,
without some understanding of the feeding process, the subsequent discussions
may be largely useless. So, I think this is an obvious place to begin.
I wish I had a nickel for each time some aquarist has asked me a question
about suspension-feeding in marine invertebrates. I would have a fair amount
of cash, maybe not enough to pay Bill Gates’ daily green’s
fees, but at least enough to buy his morning latté... and mine,
too. The mechanics and the factors important in feeding in suspension feeding
are about as well known to the average reef aquarist as the names of the
craters on the far side of the moon. No, that’s not right. Those
crater names are more well known. Nonetheless, suspension-feeding is probably
the fundamental way of feeding for all animals, other types had to evolve
later. Suspension-feeding, in some form, is still probably the most widespread
feeding mode amongst marine and fresh-water animals. A few terrestrial
animals, such as orb building spiders, are also suspension-feeders, but
they are another story.
This being said, on a basic level, suspension-feeding is really quite
easy to understand. It is simply a feeding mode that involves removing
particulate food from the water medium that it is suspended within. Probably
the simplest way to visualize feeding on suspended food is consider the
action of a net in water. The water and the suspended foods are forced
through a net or alternatively the net is pulled through water. Food is
trapped on or in the net and removed. This works pretty well for large
particles, for example, particles the size of sardine or bigger. It is
much less efficient for smaller particulates.
Animals removing the suspended particulate by means of a net or mesh trap
are relatively uncommon. For most animals, the use of other means to separate
food from water is probably an obligatory methodology; it is very difficult
to use a mesh type net to separate very fine particles from the water surrounding
them. Very small particles have a density similar to that of water and,
in effect, often behave as if they were simply large water particles. Suspended
in moving fluids, such particles move with the fluids and move and "flow" around
obstructions such as nets.
If a net has a mesh small enough to catch these little particles, the
water pressure necessary to force fluid through that mesh is relatively
high, and strong enough to disrupt or destroy most biological meshes. So...
most organisms must use other means to collect foods.
By far the two most common ways that suspension feeding organisms use
to collect food are:
1. Capturing food by direct impact.
2. By the use of some sort of ciliary collection method.
Food Capture by Direct Impact
Each of these methods are used in by some aquarium animals. Capturing
by direct impact is how corals and soft corals generally feed. The food
particle is moved with the water and bumps into a tentacle, when it does
so, chemicals on the surface of the tentacles cause nerve cells in the
skin where it hit to trigger a reflex arc, causing nematocysts to fire,
impaling the food item and, if necessary, killing it. This reflex is very
rapid, it typically takes less than one one-hundredth of a second to occur.
For this reflex to occur, the item that hits the tentacle must have a couple
of appropriate properties, primarily density and chemical signature or "taste." After
the nematocysts discharge, the threads they fire into the food item hold
the item in place until the tentacle can contract and bring the food to
the mouth. In this feeding method, the prey eaten and the nematocyst type
are intimately intertwined, to eat vigorous prey the organism needs nematocysts
which have potent venoms and powerful discharges. The nematocysts on the
animals eat fish are potent and venomous. The converse is also true; those
cnidarians such as the soft coral, Dendronephthya, that capture only weakly
moving prey such as phytoplankton have fewer, smaller, and less venomous
nematocysts.
The tiny tentacles of this Acropora (here magnified well over 100 times)
are designed to catch particulate material by direct impact capture using
nematocysts. Although the surface of the coral is highly ciliated, those
cilia are not used to capture food.
Ciliary Feeding
Capturing food with the use of some sort of ciliary collection method
is used by almost all other animals that are suspension-feeders. Presumably,
if the animals can secure their food by the use of nematocysts they don’t
need ciliary capturing methods, and vice versa.
Ciliary capturing methods take advantage of the properties of water, but
properties that are unfamiliar to most aquarists. On the scale of a small
phytoplanktonic algal cell, water behaves very differently than if it we
work on the scale of a human swimming through it. At these sizes and particle
densities, water is very viscous and really doesn’t flow very easily.
The unicellular algae that constitute the majority of the food in the ocean
are often flagellated and this provides their method of locomotion. Each
cell has one or more long slender processes called flagella. For example,
dinoflagellates typically have two of these, and most unicellular green
algae have one.
Flagella have been called "hair-like" which is an apt description
of their shape, but they are almost unimaginatively small. If you can picture
a structure the shape of a long human hair, but only 1/2500th of an inch
long, you have a have a good vision of a flagellum. Cilia, used in the
collection of food, differ from flagella primarily in their length. Cilia
are much shorter than flagella, often less than one tenth as long. Both
of these structures work in much the same manner. Internally, they have
stiff, but bendable, core and they are surrounded by contractile strands
arranged in a helix. From the surface, the contractile strands would appear
to be made like a barber pole, except for two things. First, the helically
arranged colored bands curl around the pole only slightly, they are almost
parallel to its axis. Second, there typically are nine of them surrounding
the pole. If the cilium or flagellum is given the appropriate amount of
food or metabolic energy, the outside helically arranged strands move against
one another. This has the effect of causing the whole structure to bend
and flex, in a rather precise pattern. When the energy is used up, the
structure extends, and springs back to its original shape. This contraction
and re-extension occurs pretty rapidly, and as the whole structure is immersed
in water, it has the effect of exerting force against the water, much like
an oar or propellor.
Phytoplankton move by using their flagella to "scull" through
the water, much in the manner of how a person with a single oar can move
a boat along. In other words, neither fast, nor in a straight line, but
with a net motion nonetheless. Given that the medium surrounds the cell,
the small cells typically move in a spiral pattern.
Flagella are generally found in groups of one or two, while their shorter
ciliary cousins are often found by the hundreds arranged in rows or large
aggregations. Cilia can either move animals, such as when they are on the
bottom surface of flatworms; the gliding motion of flatworms is due to
cilia, or, if the animal they are on is stationary, they may move water
past it. It is this last process that is important in filter feeding.
Arranged in rows, thousands of beating cilia may move a lot of water past
a surface. In doing so, they bring particulate material toward the surface.
Depending on their arrangement they may entrap or move small planktonic
particles along rather precise pathways. These pathways are often referred
to as food grooves and are common features of the feeding apparatus of
many worms, clams, and echinoderms.
Small suspension-feeding tube worms are common in aquaria. Each tentacle
has a pair of ciliated tracts and a ciliated food groove between them.
The ciliated tracts beat toward one another and trap tiny particles such
as phytoplankton in the food groove. Cilia also move the food in the
food groove toward the mouth.
Ciliary feeding is a remarkably precise means of feeding. It allows for
the sorting of food by sizes and by density depending on how far apart
the ciliary tracts are and the specific mechanics of how these little hair-like
processes beat. Often, the particles entrained in the minute water currents
of small ciliary feeding animals are sorted as they move along current.
Particles that are too large to eat efficiently are often moved away from
the mouth and discarded, while those that too small may be move along a
different groove and be incorporated, for example, into the animal’s
burrow walls or tube. Only those particles of the appropriate size and
density will be actually fed upon.
In effect, ciliary feeding is the means by which many marine benthic animals,
such as feather-duster worms and many other creatures living in sand beds,
collect their food from the water column over them, and at the same time
collect the raw material for the construction of the small tubes or "houses" that
they live in. Other animals such as clams, and suspension-feeding brittle
stars feed in much the same way, but don’t use the odd-sized particles
for anything.
The tube of this large feather duster worm is made of small inedible
particles caught in the feather fan of feeding tentacles. These particles
are glued together to form a tube with a leathery consistency.
In nature, the particles that are fed upon by this type of feeding include
small invertebrate larvae and particulate material comprised of animal
debris such as feces or mucus. However, the primary foods that eaten this
way are phytoplankton and bacterioplankton, some times also called microplankton.
These two foods are both exceptionally important, in both aquaria and the
natural world, as foods for ciliary suspension-feeding animals.
Aquarium Specifics
In aquaria, primarily five different types of food are fed upon using
ciliary mechanisms. These are live phytoplankton, dead phytoplankton, bacterial
aggregates, cellular debris, and invertebrate reproductive materials (Eggs,
Sperm, and Larvae). As might be expected, not all of these foods are of
equivalent value or utility as foods.
Live Phytoplankton
As the name implies, these are living organisms, generally unicellular
algae, ultimately produced either by culture or by growth within the tank.
Depending on the alga, they may or may not form aggregations in the system.
There are a lot of potential candidates for phytoplankton if an aquarist
wishes to culture them. Scientific researchers primarily use cultures of
species of Dunaliella, Nannochloropsis, Isochrysis, Phaeodactylum, or Tetrastemma,
but really there are a lot more that could be cultured. For the hobbyist
that does not wish to culture them, commercially available preparations
of the green unicellular alga, Nannochloropsis are available.
In general, these are great foods for most suspension-feeding invertebrates
living in aquaria. They ought to be... they are the same foods those animals
would have been feeding on in the real world. They provide many of the
essential nutrients and the presence of such algae is often the key to
successful culture of many difficult to keep invertebrates. For example,
Goniopora has been shown in nature to have as its diet a significant amount
of phytoplankton in addition to the more normal crustacean prey. It is
likely that such algal foods are more important to many stony corals than
has been previously suspected. Similarly, the soft-coral species in the
genus Dendronephthya, which typically are almost impossible for aquarists
to keep, appear to feed extensively on phytoplankton. With the use of the
appropriate phytoplankton food and appropriate currents, these beautiful
animals may soon be able to be kept with some reasonable degree of success.
Advantages:
Natural foods, stays in water until fed or removed.
Disadvantages:
Cost and lack of available variety.
Dead Phytoplankton
Dead phytoplankton is available in two vastly different forms of significantly
different utility. Cryogenically preserved algae are available from several
sources. Typically these are cultured algae that are concentrated, preserved,
and frozen at temperatures in excess of -40 degrees F. (Or degrees C, the
two thermometers coincide at this temperature). These algae are used as
food by mixing a small amount of the product with sea water and adding
that to the tank. These algae may be highly nutritious, and provide many
essential nutrients, however, they are prone to significant degradation
at warmer temperatures, even the temperatures found in standard home freezers.
Bottled preserved phytoplankton are available from several sources. Here
the algal cultures are concentrated, preserved and stored in bottles. Often
relatively little care is given to ensuring cellular integrity, and much
cellular occurs. This cellular destruction results in few cells being of
the right size and density for effective ciliary suspension feeding. Additionally,
when feeding these cultures, one adds small amounts of the preservative
to the tank.
Advantages:
Ease of use, low cost.
Disadvantages:
Decomposition byproducts can rapidly become problems and the material
settles out of suspension very rapidly. The cellular density and shapes
may be shifted sufficiently that they are not acceptable to many ciliary
feeders. Frozen products need to be maintained at very cold temperatures
to maintain their nutrient capability.
Cellular Debris
As the name implies these are remnants of cells, primarily phytoplankton,
but they can be formed from other sources as well. These are the primary
initial byproducts of the feeding of preserved algae. They circulate in
tanks to varying degrees, but smaller ones do settle out very slowly. They
may or may not be relatively nutritious.
Advantages:
Automatically generated, no special addition necessary, although some
commercial products will add only these. They may be nutritious, depending
on amounts in solution. Very good food for benthic bacteria, as they settle
out of the plankton.
Disadvantages:
These are bacterial food and much of the nutrient value may be lost to
bacterial decomposers without ever have gone to fuel the animals in a system.
Relatively small amounts of these may increase the biological oxygen demand
significantly. Rotting and decomposition can lead to fouling problems if
created in excess.
Bacterial Aggregates
These are formed from two main sources, either by the growth of bacteria
on planktonic debris or by the action of sediment animals, which either
defecate them up into the water column, or by the movements of small animals
as they burrow through the sediment which will tends to move small bacterial
particles into the water. They are small particles, and circulate freely
in the tank as long as water current move them. They are small enough that
they settle out very slowly.
Advantages:
Bacteria have a high Nitrogen to Carbon ratio. As nitrogenous food sources
are necessary for protein production, these are a good food for growing
animals. They are produced automatically without any special effort on
the part of the hobbyist.
Disadvantages:
Bacteria are alive and largely non-photosynthetic, thus large numbers
of these in the water (indicated by a rather glassy gray look to the tank
water) can cause significant and fatal anoxia, particularly at night when
the lights are off, and photosynthesis has ceased.
Invertebrate Reproductive Materials (Eggs, Sperm, and Larvae)
These are produced by the spawning efforts of animals primarily living
in the sediments, basically this the way that many of the other food sources
get recycled back up into the water. Bacterial aggregates, invertebrate
larvae, and cellular debris are the primary food sources of many small-mouthed
stony corals such as Acropora, Montipora and Pocillopora. In a real sense,
much of the food added by other means will be recycled up into the water
by the animals living in the sediments.
Advantages:
Natural food, natural animal plankton. Appropriate size range for many
animals.
Disadvantages:
Large quantities of eggs, particularly, may foul the tank rapidly.
Conclusions
For small suspension-feeding animals in aquaria, there are a wide variety
of food sources available to the hobbyist, but they are of varying utility
in the tank. Some such as phytoplankton and invertebrate reproductive products
are very useful at providing an almost perfect mimic to natural foods.
Other foods have a variety of problems, and some are relatively useless
in directly feeding the organisms that the hobbyist may think is getting
the food.
There are also various ecological pathways, or food webs, which recycle much of the food from the initial use in the tank water, though some benthic organism, and back up into the water. In this regard, the more acceptable foods, such as phytoplankton, actually may go to feed the tank more than once. Such foods will feed suspension-feeding animals which may then periodically spawn or defecate particulate material back up into the water column which may be fed upon by some other organism.
