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It's in the water by Ron Shimek September 1997 Aquarium.Net

Ron takes a very close look at the microscopic organisms in the water of a reef tank, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

It's (in) the water...


Ronald L. Shimek

When aquarists set up a marine reef aquarium, particularly if they use "Jaubert" or "Berliner" method, they are in effect setting up small microcosms of marine ecosystems. The live sand and the live rock along with the bottom-dwelling animals represent the benthic environment, and the fish and the water are a version of the nearshore pelagic environment. As we specifically stock and design these components of our reefs, we have some idea of what organisms are present in them.

Real reef ecosystems have another component, the planktonic environment, that is seldom thought of in the design of our microcosms. Plankton is composed of those, generally small, organisms that are found in the water environment and which generally drift with the whim of the currents.

A harpacticoid copepod collected from the plankton. This animal is about 100 m (1/250 inch) long .

Planktonic organisms are generally thought of as too small to have much capability to swim against currents, but the definition is rather loose, and depends much on the situation being discussed. The large Antarctic krill, Euphausia superba , which can reach lengths of several centimeters and is quite capable of directed locomotion is

This large diatom, 200 m (1/125 inch) long was common in a system with few suspension feeding animals.

often considered planktonic, while herring of about the same size are rarely considered as being members of the plankton (Nybakken, 1997).

Nonetheless, most planktonic organisms are small; in fact, most are so small that special fine mesh nets are necessary for their capture and examination. Subsequent to their capture, plankton generally are examined microscopically. Probably because of the need for special equipment, most aquarists ignore the plankton in their tanks, if they think about it at all.

This is probably a mistake. Plankton may be rich and diverse over many coral reefs (Morales and Murillo, 1996). Many of the so-called SPS corals, as well as many other animals such as gorgonians and soft corals feed on plankton or get nutrition from plankton byproducts (Sebens, 1977; Bishop and Greenwood, 1994; Graf, 1992; Sebens et al., 1996). Some aquarists create or attempt to supplement some of their tank's plankton with additions of larval brine shrimp, rotifers, or algal cultures. However, this supplementation is generally done haphazardly with no realization of what might already be in the system.

I decided to examine the plankton of my own systems with the idea of qualitatively determining what was present. Having done a lot of plankton tows for the classes in Marine Invertebrate Zoology that I have taught, I realized some of the difficulties of sampling such organisms, and set about to construct a miniature plankton net that should sample both the plant and animal plankton. I obtained some 53 m mesh nylon cloth and built a net that I could fasten on to one of the powerheads in my tank. Most

A juvenile harpacticoid copepod crawling over the surface of a brine shrimp cyst or egg.

multicellular animals are greater than 50 m in their dimensions, so this net will capture them. Although most planktonic algae are smaller than this size, some of them will also be caught by such a net.

The net was fastened on to a power head with a hose clamp, and collected the microplankton for a half and hour. I emptied the contents of the net into a clear glass bowl and examined them with a dissecting microscope using magnifications of 8 to 30 diameters. Interesting organisms were transferred to a microscope slide and further examined using a compound microscope at 40 to 100 times. Over the course of several days, I repeated this collection procedure several times.

My three marine systems are all set up with live sand, live rock, and foam fractionators. However, the three tanks differ significantly from one another. The first is a 45 gallon rectangular tank, is set up as an inner lagoonal mini-reef with a reasonable amount of algae, a variety of corals and soft corals, and numerous other animals. Only three fish are kept in this tank, a mandarin dragonnet, a copper banded butterfly fish and one Amphiprion clarkii . The second is a 60 gallon hex tank, which is home for my carpet anemone ( Stichodactyla haddoni ) and her two clownfish consorts. This tank has rock in it, but little in the way of other macrofauna. The third is a 42 gallon rectangular tank set up for low light

This little blob is a non-feeding polychaete annelid worm larva. It is covered with cilia and swims in a spiral pattern through the water. It is about 25 m (1/1000 inch) long.

conditions and it contains many mushroom polyps, some Tubastraea coral, and numerous other animals; it has almost no algal growth of any sort and contains one firefish and two Amphiprion percula

The plankton that I found in those three systems was interesting both for what was there - and for what was absent. The plankton were surprisingly consistent from system-to-system. Most of the planktonic organisms were animal in nature; relatively few microalgae were found, and most of those were found only in one system.

The zooplankton I found was mostly from two basic animal groups: the crustacea and the annelidan, or segmented, worms. The crustaceans were represented by diverse life stages of harpacticoid copepods. I found adult, immature juveniles, and freshly hatched nauplius larvae of these copepods. These animals are often considered to be "epibenthic" zooplankton. That means they are specialized for life on the substrate, but they are capable of swimming for short distances in the plankton. Harpacticoids are the common small "bugs" that often occur in large numbers in newly set up aquaria. They are grazers on microalgae and very small particulate matter (Kozloff, 1990; Ruppert and Barnes, 1994). They are also considered to be very good fish food and are important components of some marine food chains (Nybakken, 1997). In some natural situations, they are primary foods of such commercially important species such as the Pacific salmonids, all of which graze on harpacticoids when they are young.

For a good drawing of an harpacticoid copepod, follow this link:

An illustration of a copepod nauplius larva can be seen by following this link:

My tanks have relatively low fish densities, and this may account for the abundance of the harpacticoids. Also, I maintain significant algal cover in the lagoonal reef to support the microscopic life that the dragonnet eats. This algal cover also facilitates the growth of the harpacticoids. The dragonnet, may in fact be feeding on the harpacticoids or other small crustaceans growing in the algae.

The other animal group found in the plankton was the Class Polychaeta of the Phylum Annelida or segmented worms. This was the most diverse category of the plankton. I found at least five, and possibly several more species of worms in the plankton. The worms were found in the plankton as both early developmental stages consisting of non-feeding and feeding larvae, and later stages of larger, feeding larvae.

This small feeding polychaete annelid worm larva. It is about 50 m (1/500 inch) long. Note the two bright red eyespots and the golden gut. There is a ring of propulsive cilia at about the "shoulders" and the animal moves very rapidly.

The youngest larvae were represented in my samples by two discrete types, both of which looked rather like small mobile spheres. Both of these types were not perfect spheres, but were shaped as rather lumpy and off-centered masses. The two types were opaque and lacked any visible gut structures. Such non-feeding larvae are common in the polychaetes. The early feeding larvae had only two or three segments, eyespots and a complete gut.

For an illustration of a feeding annelid larva or trochophore follow this link:

The older larvae were represented by larvae that had developed a several segments, and which actually looked quite "wormy." These animals had visible segments, lateral tufts of chaetae or bristles, and good sensory structures such as eyes and tentacles. They moved through the water by means of rows of cilia encircling their bodies. I saw several different types of these larvae, and was able identify some of them to the families Amphinomidae (the fireworms or bristle worms) and the Spionidae (a family of tube dwelling detritus- or suspension-feeding worms)

This may be a latter stage of the preceding species. This animal is about 250 m (1/100 inch) long. The eyespots and propulsive cilia can be easily seen, and the gut is well-developed.

For a good discussion of Polychaete biology and morphology at the family level, refer to Rob Toonen's series in the last few issues of Aquarium.Net. Other good supplemental information can be obtained by following this link:

I was not able to determine the identities of some of these worm larvae as they lacked sufficient adult characters that I might use to identify them. Worm larvae were found in the plankton collections from all three systems, but were most diverse in the collection from the lagoonal reef tank.

Representatives of other animal groups such as the rotifera were not seen in the collections. Some small ciliated protozoans were seen, but they were moving too fast for capture and identification and were largely ignored.

This larval worm is probably from the family - spionid Spionidae. Spionids are typically small worms that build tubes in sediments and feed on detritus or suspended particulate material such as copepod fecal pellets. This animal is about 250 m (1/100 inch) long. One of the long feeding structures called palps, characteristic of the spionids, can be seen along the side of the animal.

For illustrations of some of the typical planktonic ciliated protozoans, follow these links.

Phytoplankton were exceptionally uncommon except in my 60 gallon hex tank containing the carpet anemone. This system contains relatively little other noticeable animal life other than a few hermit crabs, and I know of no suspension-feeders in it that would be feeding on phytoplankton. Possibly as a consequence, the plankton from this tank contained a reasonable number of planktonic diatoms, but dinoflagellates and other phytoplankton were not present.

Yet another different polychaete worm larva. This animal is about 200 m (1/125 inch) long. Anterior tentacles, lateral appendages (parapodia) with chaetae (bristles) and a pair of small posterior sensory appendages are visible.

For illustrations of planktonic diatoms follow the following links:

For illustrations of some of the other marine phytoplankton, such as dinoflagellates, follow these links:

Numerically, the most abundant material collected by the plankton net was copepod fecal pellets. Copepods secrete a lining for their gut which is a thin membrane made of chitin, an odd polymer similar to cellulose. Food is enclosed in this material so that it never actually touches the tissues that compose the gut. In this way, the fragments of food, such as broken diatom frustules or shells which are made of thin silica, will not have the opportunity to lacerate the gut tissues. This chitin lining is permeable to dissolved nutrients and these diffuse outward into the tissues (Kozloff, 1990; Ruppert and Barnes, 1994). When the animal is done with the digestive process, the gut lining is expelled along with the undigested food in it, creating a "fecal pellet." Basically, this is a chitin bag containing undigested food. These pellets get rapidly colonized by bacteria which cover their in thin film.

Enclosed in a chitinous membrane which surrounds the undigested food, copepod fecal pellets such as these get colonized by bacteria and provide a food source for many animals. The large one is about 250 m (1/100 inch) long.

In nature, these pellets are an important food source for midwater plankton and for benthic suspension-feeders that encounter them. Bacterial cells have a significantly higher nitrogen to carbon ratio than do animal or plant cells, and this makes them a good food source as the nitrogen from eaten bacteria can be utilized to manufacture proteins. The chitin layer covering the pellets is generally not digestible by animals, so the pellets pass through the guts of those animals that eat them and the bacteria are digested off leaving the initial fecal pellet more-or-less intact, to be recolonized by bacteria. It has been estimated that in the open ocean, each copepod fecal pellet may get eaten as many as eight to ten times between the time of its formation and impact on the bottom (Graf, 1992; Steinberg, 1995). In our systems, such pellets probably are major food component for some of the so-called SPS corals that feed on very small materials, as well as soft-corals and some other suspension-feeding animals.

Now, how relevant are the finding of zooplankton in my systems to reef hobbyists in general? Are the animals found likely to be found in all hobbyist tanks? The answer to both of these questions is, frankly, I don't know. I know that when I look at the sand in my tank with a magnifying glass, I see a variety of tubes and almost microscopic animals. Furthermore, my tanks is lush with a variety of both algae and animals. Obviously, there has been reproduction of the organisms in these systems.

I would suspect any aquarist whose system is stable, and who is using one of the methodologies which uses sand as a substrate, should have some plankton in their system. When I initially set up my systems, I used an inoculum of live sand. The organisms present in that sand where likely the ancestors of many the present species living there. It is unlikely other aquarists will have all the same species living their systems as live in mine. Every inoculum will contain different starting species, and this probably determines, to a great degree what will be found in the sediments, and hence what is likely to be found in the plankton. Similarly, differences in live rock and the fauna found on it, and other subsequent additions of organisms will likely contribute to differences in the plankton.

There will be some features of everybody's systems which will tend to select for the same fauna in all tanks, though. The use of foam fractionators as opposed to mechanical filtration will facilitate the growth of plankton. Similarly, the lack of much in the way of phytoplankton in our systems will favor those animals with either a short feeding larval stage, or those whose larvae do not feed. Nonetheless, the plankton present in our systems may be quite different, and these differences likely account for some of the differences between hobbyists in the animals that they are able to keep successfully.

References Cited:

Bishop, J. W. and J. G. Greenwood. 1994. The contribution of excretion by demersal zooplankton, to nitrogen flux across the sediment/water interface in a coral reef lagoon: A preliminary account. Bulletin of Marine Sciences and Fisheries Kochi University. 14:15 22.

Graf, G. 1992. Benthic Pelagic Coupling: A benthic view. In: Oceanography and Marine Biology, An Annual Review. Vol. 30. (Eds: Barnes, M., A. D. Ansell, and R. N. Gibson) Aberdeen University Press, Aberdeen, 149 190.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Morales, R. A. and M. M. Murillo. 1996. Distribution, abundance and composition of coral reef zooplankton, Cahuita National Park, Limon, Costa Rica. Revista De Biologia Tropical. 44: 619 630.

Nybakken, J. W. 1997. Marine Biology, An Ecological Approach, 4th Ed. Addison Wesley Longman, Inc., Reading, Massachusetts. 481 pp.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Sebens, K. P. 1977. Autotrophic and heterotrophic nutrition of coral reef zoanthids. Proceedings of the Third International Coral Reef Symposium. 1: 397 404.

Sebens, K. P., K. S. Vandersall, L. A. Savina, and K. R. Graham. 1996. Zooplankton capture by two scleractinian corals, Madracis mirabilis and Montastrea cavernosa, in a field enclosure. Marine Biology 127, 303 317.

Steinberg, D. K. 1995. Diet of copepods (Scopalatum vorax) associated with mesopelagic detritus (giant larvacean houses) in Monterey Bay, California. Marine Biology. 122: 571 584.

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Last modified 2006-11-19 01:50