Building a Heron Island Biotope, Part 1: Natural History of the Reef (Full Article)Author: Scott Michael
Every marine fish brings baggage with it from the reef into its new aquarium home. This behavioral baggage has evolved so the fish might better survive and reproduce in the habitat and community in which it lives. If you have been a fishkeeper for any length of time, you know that damselfishes and dottybacks are pretty aggressive, blue-green chromis do better when kept in shoals (sometimes erroneously referred to as schools), and a clown triggerfish is more than likely going to take out any shrimps you place in a tank with it. All of these attributes are related to the fish’s behavior and ecology.
While there is no doubt that living in the confines of a glass box can change these behaviors, exaggerating some and squelching others, it is a good idea to know as much as possible about a fish’s potential behavioral baggage before you place it in your fish community. This includes information about the habitat in which it lives and how it interacts with other organisms in its community. In many cases, by reproducing aspects of its natural surroundings, you can help ensure that the fish more readily acclimates to its captive home. This can also lead to natural interactions with tankmates that can be a delight to witness in the confines of their enclosed ocean.
In the current and following issue of TFH, we will be investigating the behavior and ecology of reef fishes. This is by no means an exhaustive review, as space is limited, but hopefully it will be a primer that will help you see how this information has lots of practical applications. We will begin our inquiry by first looking at the coral reef itself and the various zones where the reef fish we keep come from, and then initiate the process of setting up a fish community representing a specific region and habitat. Next month, we will look at the behavior of the species we selected and how it impacts their care.
Anatomy of a Coral Reef
Whenever I purchase a fish for my marine aquarium, the first thing I want to know is where it naturally occurs. This includes its geographical range, the reef zone it inhabits, as well as any microhabitat preferences. I prefer to keep sympatric fish species—that is, species that live together in nature. I may select a specific spot or region to begin with and then build a fish community from this area.
Let’s say, for example, I’d like to build a community of species from the Great Barrier Reef (GBR). If you know anything about this awesome animal-made structure, you know it is approximately 1,600 miles long, extending from the Torres Strait, not far from Papua New Guinea, all the way south to Lady Elliot Island in south-central Queensland. Approximately 1,300 fish have been reported from the GBR, with a distinct difference in ichthyofauna occurring from the southern to the northern end. This is not surprising, considering the tremendous north-south distance covered. For example, approximately 24 percent of the species found at the Capricorn-Bunker Group (southern Barrier Reef) do not occur at Lizard Island (northern Barrier Reef), and vice-versa.
There are also fewer fishes (860 species) reported from the southern Great Barrier Reef, which also has a small subtropical-temperate element. For example, Choat's wrasse (Macropharyngodon choati) is found only in northern New South Wales and the southern Great Barrier Reef, while the Australian dottyback (Ogilbyina novaehollandiae) is found only on the southern Barrier Reef. When I try to recreate a particular fish community, I want to select a more specific location to ensure the fishes in my aquarium are species that would regularly mix in nature. So, let’s get more specific than just the GBR and go with a fish community from Heron Island, which is in the southern portion of the GBR.
To narrow our fish community down even further, I am going to select species found in a particular habitat. Coral reefs can be broken down into a number of different zones. The environmental conditions can vary from one zone to another, and as a result, there are also some differences in the plants and animals you will find inhabiting them. The primary reef zones are the lagoon, back reef, reef flat, reef face, and reef slope.
Lagoon and Back Reef
The lagoon is an area enclosed by a barrier reef or an atoll ring. The lagoon is usually more protected from surge and features more quiescent water movements than the outer reef zones. It thus tends to support more fragile, highly branched coral species/forms. There may also be seagrass beds in the lagoon, adjacent mangrove areas, vast open sandy expanses, and patch reefs.
If a lagoon is bordered by a barrier reef, you will find a back reef zone. It too is well protected from turbulent water movement and tends to be home to tracts of beautiful, delicate stony corals and soft corals. While lagoons can vary in their depth, most are relatively shallow (less than 60 feet deep), so light levels tend to be high.
The reef flat is the area of a barrier reef that extends from the back reef to the reef face. It can be a few to several hundred yards wide. This zone is impacted by tidal flux, and at low tide (especially during spring tides), portions may be partially exposed or pools may be temporarily isolated from the lagoon and oceanic waters. The marine animals that live here have to be resilient to changes in environmental parameters (e.g., temperature, salinity)—they tend to be real survivors in the aquarium because they can deal with more radical changes in environmental conditions.
There is often more reef pavement (where there are no live corals) that provides growing substrate for algae. It is not uncommon that during the tidal influx, groups of herbivorous fishes will move on to the reef flat to feed on these algae. The coral heads and tide pools of the reef flat may serve as a nursery for some reef fishes. On some reef flats, the edge on the open ocean side may have a reef crest. This zone can be exposed to violent wave action and is home to few stony corals. Instead, it tends to be a growing surface for coralline algae. Only fishes that enjoy turbulent conditions live here. This includes a number of surgeonfishes that feed on algae that can grow here. The reef crest may be totally exposed at extreme low tides. This reef zone is also exposed to very intense sunlight.
The reef face (a.k.a. shallow fore reef) and slope (a.k.a. deep fore reef) are the two zones where divers spend most of their time. The reef face starts at the edge of the reef flat (or crest) and extends down to a depth of approximately 65 feet. It is a zone that is often exposed to strong currents and has the clearest water and greatest biodiversity you will find on most coral reefs. Many of the fishes collected for the aquarium trade come from this zone.
Depending on the reef, soft corals may be predominant in the shallow portions of the reef face, while in other locations, varied fields of stony corals cover much of the substrate. There are also pockets of sandy substrate or sandy grooves between stony spurs, where animals that prefer soft substrate can make their home.
The slope of the reef face can vary—in some cases, there is a gentle slope, while on other reefs there can be a precipitous wall that plummets into the depths. These reef walls are often honeycombed with caves, crevices, and overhangs that are not only fun to explore but are home to a very different fauna.
The reef face exhibits relatively limited variation in environmental conditions. The reef face is also home to numerous plankton-feeding animals that rely on the currents that sweep through this zone to bring them life-giving planktors. This includes sponges, soft corals, sea fans, crinoids, anthias, and fusiliers. The many stony corals also provide sanctuary and food for a myriad of fish species. The light levels in this reef zone are fairly high, but as you get to the lower reaches of the reef face, they are much more attenuated compared to the more shallow areas.
The reef slope zone begins where the reef face ends. In most cases, it consists of a sand or rubble slope, with scattered coral heads or patch reefs. The latter can be composed not only of stony and soft corals, but also sponges (some of which can be massive). These prominent structures are the focal point of fish activity. The corals on the reef slope tend to be sheet-like, a growth form that is better able to capture the diffuse light rays. As one travels deeper, coral coverage is less (there are more soft than stony species) and the fish fauna begins to change. At great depths (over 200 feet), you find fewer algae or coral eaters (because the growth of both is relatively sparse) and more zooplankton feeders, like anthias and Chromis damsels. In some locations, clouds of these planktivorous fishes hang over the slope. There are also unique sponge-feeding fishes. Large predatory fishes lurk in these deep reef areas, and some move into the shallows at night to feed. On some deep slopes, there are great escarpments, the result of past geological activity. In some cases, these form walls that provide growing substrate for sponges and other encrusting creatures and hiding places for a variety of motile animals. Light levels are reduced here, and the spectrum of light consists almost entirely of shorter wavelengths, like blue and violet.
Creating a Captive Reef Fish Community
Okay, I have decided that I want to create a reef fish community from the reef slope around Heron Island. While it may once have been more difficult to find information on what is going on at 70 feet or more around this southern GBR island, it is no longer that exacting of a task with the resources on the Internet. One of the best places to start is Google Scholar. Here you can find scientific journal articles about all kinds of topics, including those related to fish communities from all over the world. Two other great resources are reefbase.org and fishbase.org.
The hypothetical aquarium I have set up is a 180-gallon tank. I’ve placed a low-profile, live rock reef right down the middle of the tank and have created some caves and arches using some of the cements that are safe to use in the aquarium. I also plan to add some soft corals and maybe a few large-polyped stony corals. There will also be plenty of snails and algae-eating hermit crabs, some cleaner shrimp, serpent stars, and a few tuxedo sea urchins. Also, I will be adding a tiger pistol shrimp (Alpheus bellulus)—more on this later.
Because I am trying to create an aquarium that resembles a deeper reef habitat, I am going to use two 250-watt metal halides with 1400K bulbs, which will give the tank more of a bluish hue. (Of course, there are many other lighting choices, including fluorescent fixtures with more actinic than daylight bulbs or LEDs.) The reef slope is often exposed to some brisk currents, so I will also want to make sure there is significant water flow in my tank. I will mimic this feature by adding several propeller water pumps.
By using Google and Google Scholar, I have been able to locate a checklist of fishes from Heron Island and have come up with a fairly good assortment of species from both the location and the reef zone I am interested in recreating. Before I make a final choice, however, I need to research the behavior of each species to see how they will interact with one another as well as the invertebrate occupants of the aquarium (we will do some sleuthing into their behavior in the next installment of this article and do some tweaking to the list if needed based on our findings).
So here is the community of fishes in my Heron Island reef slope biotope tank. Note that I am not necessarily choosing fishes that are endemic to the GBR or even those occuring only in this specific reef zone. My first selection is the lyretail anthias (Pseudanthias squamipinnis). While this species is ubiquitous in the Indo-Pacific and often found in shallow water, the papers I have found indicate that it occurs down to 115 feet near Heron Island. If you wanted something more unique to this region, you could go with the more exotic painted anthias (Pseudanthias pictilis), which occurs in deeper water around Heron Island, but they are expensive and not as readily available. The lyretail anthias occurs in shoals in the wild, so I am going to go with one male and five females. Fortunately, anthias are easy to sex because males and females differ in color.
Another species I want to try is the oblique-lined dottyback (Cypho purpurascens). Like the lyretail anthias, C. purpurascens is regularly found in the shallows, but it too occurs on the reef slope down to 130 feet. Stark’s demoiselle (Chrysiptera starcki) is a resident of the reef slope and sports glorious chromatic attire—I want to add one of these. Another species that is characteristic of current-prone reef slopes is the Watanabe’s angelfish (Genicanthus watanabei). It too is sexually dichromatic and lives in groups, but because my aquarium is just on the edge of being comfortable for a trio of these fishes, I will acquire a male and a female. I have long been a fan of the pygmy angelfishes (genus Centropyge), so I have included a bright yellow Herald’s angelfish (C. heraldi) on my want list, another species that is found in both the shallows as well as deep reef slopes. I am going to attempt to keep a male-female pair of C. heraldi.
The chocolate surgeonfish (Acanthurus pyroferus) is a beautiful and interesting species that also occurs at depth around Heron Island, so it too has made the list. No aquarium is complete without a wrasse or two, so I am also going to add a pair of Laboute’s fairy wrasse (Cirrhilabrus laboutei)—once again the female and male differ in color—and a blackear wrasse (Halichoeres melasmapomus). Both of these species are regularly found in deeper water on the reef slope. I also want to include a Midas blenny (Ecsenius midas), an unusual member of the family that spends much of its time swimming in the water column. Finally, I never set up a tank without a shrimp goby, so I have decided to add a pair of yellow shrimp gobies (Cryptocentrus cinctus). This species is more common on shallow sand flats and slopes, but it also occurs near the upper limits of the reef slope, and because it is one of the more attractive and readily available members of this group, this is the species I selected.
Okay, the aquarium is ready and we have a tentative list of 10 fish species (19 individuals in all) for my Heron Island reef slope aquarium. In next month’s installment, we will look at the behavior of the various fishes we selected and see how this impacts how they are likely to interact in our proposed community. Until then, happy fish watching!
See the full article on TFH Digital http://www.tfhdigital.com/tfh/201301#pg89