One trend that has gained favor with reef tank hobbyists is the use of minimal, or no, substrate on the bottom of their aquariums. This is not to say that substrate cannot be used, only that not using it makes maintenance easier. Poorly maintained substrates tend to accumulate detritus quickly. If this is not dealt with, anaerobic (oxygen-poor) areas could develop as bacteria use oxygen to additionally break down the detritus. This will eventually lead to major problems.
Detritus often becomes a source of phosphate as bacteria decompose this material, causing stored phosphates to be released. These phosphates then act as nutrients for undesirable algae growth, such as “hair algae” (Derbesia) and “slime algae” (cyanobacteria).
Because there are still some hobbyists who use undergravel filters with relatively thick substrate beds and others who would like to use a substrate in their reef aquariums for aesthetic reasons, it is important to find a way to deal with detritus. There are several strategies you can use to reduce detritus accumulation in the coarse marine aquarium substrates used with undergravel filters.
You can periodically stir the substrate and siphon out accumulated detritus when doing water changes. This is often tedious and time consuming in a large, heavily stocked aquarium.
Powerful water currents directed along the surface of the substrate can also help to prevent detritus from settling on the bottom. This keeps detritus in suspension where it can be removed by external power filters or an overflow. However, every aquarium has its dead spots and some particulates are not so easily carried along by water currents.
Neither of these methods deals adequately with detritus accumulation underneath decorations or live rock. Other factors also need to be considered as well.
If you wish to use a substrate in a reef aquarium without an undergravel filter, the grains must not be large enough to allow detritus and mulm to accumulate within and under the substrate. The commonly sold coarse crushed coral is not suitable. Fine coral gravel or coral sand (2 to 4 millimeters in diameter) would be much better. The layer can be anywhere from ½ to 4 inches thick.
Substrate in a reef aquarium with live rock and corals can play a beneficial role by supplying a refuge for small crustaceans, worms and other organisms that help to decompose waste and can act as a food source for fish. The larvae of these organisms will also contribute to the planktonic population of the aquarium and can also provide a food source for other invertebrates. Anaerobic layers may develop in the substrate and — if the organic load of the aquarium is not too great — can provide additional denitrification. If the organic load of the water is too great, hydrogen sulfide production will occur.
Many of the options listed above for keeping coarse substrates clean can also be used in reef aquariums with finer substrates. However, when using fine substrates, there are other biological options available to the aquarist. Perhaps you should employ the help of nature’s “clean-up crew” and let them facilitate your efforts in maintaining an optimal environment for your invertebrates.
Coral reefs are such diverse environments that few niches (biological roles in an ecosystem) go unused. For example, there are many organisms that use the sandy substrates that surround coral reefs for food and shelter. These include various shrimp, starfish, sea cucumbers, brittlestars, sea urchins and fish.
In Europe, the majority of reef aquariums use such organisms to help maintain healthy, calcareous substrates. Although there are hundreds of different invertebrates and fish that dwell in these communities that can be used in such a manner, we will concern ourselves with only a few fish species that are commonly found in aquarium shops.
These fish generally feed by sifting mouthfuls of fine substrate through their gills and straining out edible infauna, such as worms and small crustaceans. In so doing, they help to keep the substrate from becoming packed with detritus and overgrown with algae. As an added benefit, as they pass the sand out through their gill slits, any detritus will be resuspended and carried away by water currents into your filter or overflow.
One drawback to these fish is that they can deposit substrate onto bottom-dwelling invertebrates such as corals and anemones. Therefore, it is best not to have certain sessile organisms on the floor of the aquarium. At the same time, some of the common bottom-dwelling hard corals, such as Fungia spp., Heliofungia spp. and Herpolitha spp. (mushroom or plate and slipper corals respectively), can easily shed any substrate that may fall on them.
As an example of this philosophy, Sea World of Ohio employs a pair of yellow-headed sleeper gobies in their reef tank. These fish keep the coral sand substrate free of microalgae and maintain a sparkling clean appearance to the tank (P. Mohan, personal communication; C. Delbeek and S. Michael, personal observations).
Most of these living “vacuum cleaners” belong to the family Gobiidae. This is the largest family of marine fishes in the world, containing more than 2000 species. In the Indo-Pacific alone there are some 200 valid genera and 550 species (Myers 1989).
In this article we will cover three genera — commonly found in aquarium stores — that can be used, to varying degrees, to help maintain aquarium substrate. They are the hover gobies (Amblygobius spp.), the signal goby (Signigobius biocellatus) and the sleeper gobies (Valenciennea spp.).
Members of these genera are referred to as being “fossorial,” which means that they will make or use the burrows of other organisms as shelter. In the aquarium, the lack of suitable bottom substrate and pieces of rubble (important in the burrow infrastructure) may make it difficult for these fish to construct their own homes. The aquarist can assist by creating artificial burrows with siphon tubes.
Just cut off half of the rounded end of the tube so that when it is lying flat on the aquarium bottom, the opening is directed toward the surface. Cap the other end and bury the tube under gravel, rocks or debris. To give the burrow a more natural appearance, pieces of coral rubble or sand can be siliconed around the open end. Make sure the burrow diameter is at least twice the girth of the potential fish occupant.
Hover Gobies (genus Amblygobius)
These small fish (2 to 5 inches in length) are generally found over sand areas, but some species also occur on hard bottoms. There are at least nine species in this genus, but the species most often encountered in the aquarium trade are Hector’s hover goby, Amblygobius hectori (Smith 1956), Phalaena’s hover goby, A. phalaena (Valenciennes 1837) and Rainford’s hover goby, A. rainfordi (Whitley 1940).
All three species spend a great deal of time hovering close to the bottom, constantly picking at the substrate and spitting out quantities of sand and detritus. Amblygobius phalaena and A. hectori use the burrow of a shrimp or another fish in the substrate or under rocks, whereas A. rainfordi does not use a burrow and is often observed swimming over coral growth. In the wild, Phalaena’s hover goby is observed singly and in pairs, and often shares a burrow with sleeper gobies or juvenile surgeonfish (Paulson 1978; S. Michael, personal observation).
In the aquarium, these fish prefer peaceful surroundings with numerous hiding places. When kept in a tank with large, active fish they may hide most of the time and may not get enough to eat. Of these three species, A. phalaena is the most easily maintained and is also the best substrate sifter. It is generally a good feeder, accepting most types of foods; Myers (1989) reports that it consumes large quantities of algae.
Rainford’s hover goby is also a vigorous algae eater, browsing on filamentous red and green algae. This species and A. hectori will do poorly if kept with aggressive tankmates. Their aquarium should have a good supply of filamentous algae. Both of these fish readily accept live black worms and live brine shrimp — with perseverance they can be switched to more nutritious frozen preparations.
The members of this genus tend to be non-aggressive toward other species and will not bother any invertebrates, which makes them excellent candidates for the reef aquarium. The only problem they may have in large reef tanks that lack filamentous algae is getting enough to eat.
A. phalaena can be kept quite easily in male-female pairs, but any more than two individuals of the same species or genus will fight constantly, resulting in the death of subordinates (Achterkamp 1991; S. Michael, personal observation). This apparently holds true for the other fish in this genus as well. The little experience the authors have had with A. hectori suggests it is shy and can be difficult to feed.
Achterkamp (1991) provided details of the breeding behavior of A. phalaena and it is likely that the other species behave similarly. Adult male A. phalaena (approximately 5 inches) can be distinguished from females by several black dots on the tail fin. When spawning time approaches, the pair will search the entire aquarium for a suitable nesting hole, greatly disturbing the substrate in the process. Once the nest has been built, egg laying begins, usually at night.
Several large clumps that consist of hundreds of dirty yellow-white eggs are deposited. After egg laying, the male defends the nest vigorously and may even drive away the female. After about six days the eggs hatch out and the pair ignore the offspring as they rise to the surface.
A pair may breed several times a month or only once or twice a year. If anyone has any information on the behavior and breeding of the other species in the genus, we would like to hear from you!
Signal Gobies (genus Signigobius )
The signal goby, Signigobius biocellatus (Hoese and Allen 1977), is the only species in this genus and is easily identified by the large black “eye spots” (ocelli) on the anterior and posterior dorsal fins. Signal gobies occur in sheltered lagoons on sand and coral rubble substrates. They occupy burrows that they excavate by using their mouths to carry away sand and shell fragments and by going head first into the hole and vigorously beating their tails (Hudson 1977). The burrow is used for shelter and reproduction.
Juveniles occur singly, while adults (3 inches) are always found in pairs (Debelius 1986). They feed in a similar manner as the hover gobies, taking in mouthfuls of sand and sifting it through their gill-rakers.
These fish move by hopping along the substrate. They frequently flick the two dorsal fins forward, exposing the large ocelli present on these fins. If a predator or rival approaches, the signal goby will erect its fins, arch its back and tilt its body. The lateral view of this goby apparently mimics the “face” of a piscivorous fish and may discourage a predator from attacking. It has also been suggested that this lateral perspective, as well as the hopping behavior and the fin flicking, resemble a crab moving over the bottom (Hudson 1977).
Personal observations, discussions with other aquarists and examination of the literature all indicate that this is not an easy species to maintain. You will usually see them offered for sale as pairs, and we recommend you buy them this way. If they are separated, they usually do not live long. They should be kept in an aquarium with fine substrate, but some added coral rubble will facilitate their burrowing behavior. This species should also be provided with numerous hiding places.
Live brine shrimp and black worms can be used to induce feeding in finicky individuals, but vary the diet as much as possible once they are eating. They should be fed at least twice a day in order for them to maintain their body mass. Signal gobies should not be kept with aggressive tankmates (e.g., dottybacks, large wrasses), especially in smaller aquariums. They tend to do much better in a tank of their own (G. Schiller, personal communication).
The breeding behavior of the signal goby is very unique and has been documented in the wild and the aquarium by Hudson (1977). As mentioned previously, they form monogamous pairs and share a burrow. The gravid female nibbles the male’s body before spawning and then enters the burrow and deposits her eggs. After spawning, the male is sealed in the burrow at which time he fertilizes the eggs, guards and possibly aerates them.
For two to three days the male resides in the burrow. During this period the female will occasionally open the chamber and liberate the male, and the pair will engage in burrow maintenance. The male then reenters the burrow and is once again sealed in. Eventually the male exits the burrow one last time and both parents close the entrance and move to a new burrow or reopen an old one nearby.
The parents occasionally visit the “nursery” burrow, open it, clean it of debris and irrigate it with fresh seawater by entering the burrow tail first and slowly beating their tails. Finally, the burrow is opened one last time and a single juvenile fish emerges. It has not yet been determined how the developing young fish is nourished within the burrow, but it has been suggested that they depend on yolk reserves, feed on microorganisms or possibly one embryo develops faster than the others and feeds on its broodmates (Hudson 1977).
Sleeper Gobies (genus Valenciennea)
There are approximately 15 species of this genus in tropical waters, ranging in size from 2 to 8 inches (Debelius 1986). A number of species frequently make their way into the tanks of marine retailers, including the yellow-headed sleeper goby, Valenciennea strigata (Brousonet 1782), the maiden or diamond sleeper goby, V. puellaris (Tomiyama 1955), the banded or tiger sleeper goby, V. wardi (Playfair and Gunther 1886), the sixspot sleeper goby, V. sexguttata (Valenciennes 1837), the two-stripe sleeper goby, V. helsdingenii (Bleeker 1858) and the long-finned sleeper goby, V. longipinnis (Lay and Bennett 1839). We have included a table with information on the maximum size and the distinguishing characteristics of the above listed members of this genus.
These gobies feed primarily on tiny crustaceans (usually under 1 millimeter in length) and occasionally on minute gastropods, clams, foraminifers and polychaete worms (Hiatt and Stratsburg 1961, St. John et al. 1989). Some of the larger species, like the yellow-headed sleeper, may on rare occasion eat small fish (Hiatt and Stratsburg 1961). A study on the long-finned sleeper goby documented that this fish will take bites to a depth of 1 inch out of the bottom sediment at a rate of about six bites per minute during the day (St. John et al. 1989).
As far as aquarium cleanup is concerned, not all sleeper gobies are created equal. Some are more aggressive substrate sifters than others. For example, the authors have found that the maiden sleeper and the long-finned sleeper are more effective at displacing bottom sediment than the banded sleeper and the two-stripe sleeper.
The size of a sleeper goby burrow varies from one species to the next. In the case of V. sexguttata, the burrow is 4 to 6 inches long, with a coral rubble roof and a rectangular floor plan (Paulson 1978). In the larger yellow-headed sleeper, up to 23 gallons of resin was required to fill one burrow (Paulson 1978)!
The burrows of some sleeper gobies provide sanctuary for other fish as well as the resident gobies. For example, juvenile convict surgeonfish (Acanthurus triostegus) took refuge in 2 percent of all V. sexguttata burrows observed near Eniwetok atoll. Twenty percent of all V. puellaris burrows examined contained juvenile yellow tangs (Zebrasoma flavescens). Small groups (two to 20 individuals) of green torpedo gobies (Ptereleotris microlepis) were observed to hang above the entrances of sixspot, maiden and long-finned sleeper goby holes, and to disappear into the burrows when a predator approached (Paulson 1978; S. Michael, personal observation).
Some species of sleeper gobies have interesting defensive behaviors that the aquarist may encounter. For example, juvenile V. wardi have one spot on their dorsal fin and one on their caudal fin, and — like the signal gobies — mimic the head-on perspective of a predator when viewed from the side. When threatened, the banded sleeper presents its side toward the predator, raises its dorsal fin and spreads its tail, displaying the eye spots (S. Michael, personal observation).
Sleeper gobies usually pair for life when they are young (Debelius 1986). As a rule, the pair occupies a burrow on its own, but sometimes as many as 10 sleeper gobies, representing several different size classes, will utilize the same burrow (Paulson 1978).
Sleeper gobies are sometimes sold as pairs, and the authors recommend that you buy both individuals in such cases. If you have a single specimen you should try to find it a companion, because they apparently do better in pairs, and their pair-bonding behaviors are also fascinating to observe. Most sleeper pairs are easy to recognize because they swim very close together through the tank, with the lower jaw of one touching the upper surface of the head of the other. They look like a figure skating pair as they “waltz” through the aquarium together! Males are easily distinguished from the female — they have an elongated second dorsal spine similar to the mandarin dragonet (see Delbeek 1989).
Some sleeper gobies spend most of their time hovering above the bottom (e.g., yellow-headed sleeper goby), whereas others are most often observed prone on the substrate (e.g., maiden sleeper goby, banded sleeper goby). Some sleepers are also more secretive than others. For example, the aquarist may go without seeing a newly introduced banded sleeper for weeks and when this sleeper does show itself it is often for very brief periods of time.
When first introduced in the aquarium it is not unusual for sleepers to hide for several days as they become acclimated to their new surroundings. Once they have overcome their initial shyness they will go about the chore of building their home. This usually consists of excavating a hole under a rock that both fish will share. It is important to arrange your aquascaping in a stable configuration or you may find your delicately balanced decorations lying in a jumbled heap on the bottom!
Not only does the feeding behavior of the sleeper gobies help put detritus into suspension, but their active burrowing under aquarium decor also proves beneficial in this regard. When evening comes, the pair will retreat to their burrow and the male will create a small pile of rocks at the entrance or plug it with a ball of algae to block it off for the night. Once they have survived for several months without any problems, your sleeper gobies should live for many years (Achterkamp 1986).
Sleeper gobies will eat most foods offered but it may take a few weeks before they will accept prepared foods. They are particularly fond of small crustaceans (e.g., brine shrimp, mysid) and blackworms, but you should feed them a varied diet. It is also possible that some species may feed on larval and juvenile bristleworms, although they have been seen to ingest and then spit out adult worms (S. Michael, personal observation).
Because many of the sleepers are active fish that can grow to at least 8 inches in length, they must be kept well fed or they will slowly waste away. Therefore, it is important to have a good filtration system — preferably enhanced by the presence of a protein skimmer — to deal with the considerable amounts of waste products that they produce.
As mentioned above, male sleeper gobies are easily identified by their elongated second dorsal spine. The female generally lays about 2000 eggs on the roof of their chamber. She will tend the eggs for three weeks, during which time she will not leave the chamber. Just before hatching occurs the male will seal the entrance to the chamber, sealing in the female. After the eggs have hatched the male will reopen the chamber, at night, freeing the 2-millimeter fry (Debelius 1986). We have not read any reports of successful rearing of the fry, but it seems that it may be possible given enough time and patience.
Sleeper gobies, hover gobies and signal gobies can help the aquarist to maintain a clean and healthy substrate. However, you may still have to perform the odd stir-and-siphon routine if problems develop. Although most of these fish are active sifters, they cannot do so with a substrate that is too large, such as dolomite or crushed coral. In fact, these fish may injure themselves trying to handle substrate that is too large or coarse, and bacterial infections may result.
The best substrates to use, such as coral sand or finely crushed coral, have grain diameters less than 2 millimeters in diameter. As mentioned above, in order to aid in burrow construction, some larger sized pieces should be mixed in with the finer to provide “building-blocks” for the fish.
Now that you know about the species of gobies described here, and the benefits they can provide to your reef tank, what are you waiting for? It’s time for goby shopping.