Back to nature

The title of the article is Back to nature. In the Christmas 2003 issue, I interviewed Martyn Hulyer, an experienced and dedicated reefkeeper who has spent five years developing what has become a highly successful SPS coral farm at his home in March in the Cambridgeshire Fens.

He has extensively researched the husbandry of small polyped corals, and in doing so has broken new ground in our understanding of these complex organisms. He is a firm believer in what he calls the naturalistic approach to reefkeeping. To get the ball rolling, let’s look at the biology of these corals.

Building blocks

Scientists refer to the morphology of animals and plants when dealing with their form and structure. Detailed studies of corals can help provide a clear understanding of their needs from their environment, which in turn provide a good guide as to how they should be cared for.

The typical coral polyp is a collection of tentacles surrounding a flat oral disc with a centrally positioned mouth. With hard corals, this jelly-like mass is surrounded and protected by an exoskeleton made from calcium.

As hard corals usually live in attached colonies, the repetition of the polyp structures form a variety of shapes, making for a fantastic diversity of forms.

It’s already well established that reef-building corals thrive in nutrient-poor areas where phosphate, nitrogen, iron and other essential nutrients are barely detectable. What’s more, we know that adding such nutrients does not help as natural reefs quickly deteriorate if faced with organic pollution produced by nutrient-rich conditions.

To compensate for this lack of nutrients, corals have developed symbiotic relationships with algae, which they hold in their tissues. These algae are termed zooxanthellae and use light energy to produce carbohydrates from bicarbonate, a salt of carbonic acid. These carbohydrates benefit the coral while the zooxanthellae receive ammonia, which it considers a nutrient and which is a by-product of the coral’s metabolism.

Corals also have tentacles (complete with stinging nematocysts) which are used partially as a means of defence but mainly for catching prey – tiny zooplankton. Now while zooxanthellae offer more than 90% of their photosynthetic products to the coral in the form of carbo-hydrates, the coral gets phosphorus and nitrogen from the micro-organisms it captures and feeds on.

Martyn therefore says that an essential part of good SPS coral husbandry is to replicate the conditions where these micro-organisms are constantly available.

Unfortunately, many tanks are devoid of micro-animal populations, run as they are with clinical sterile conditions. The water is processed through high-efficiency skimmers and calcium reactors, both of which take a heavy toll on planktonic forms. There is also a severe lack of suitable habitats for micro-organisms not living in the water column.

Let the critters colonise!

Martyn said: “Reefkeepers often clean all surfaces inside the tank, when the back and side panels could become areas where a wide diversity of organisms will colonise once calcareous algae becomes established. Similarly, in many reef systems where sand-bed filtration has not been chosen, the substrate may either be non-existent or just a dusting of coral sand.

“Deep sand-beds and plenum filters provide an ideal habitat for micro-organisms and other small creatures to colonise.”

Martyn recommends furnishing the substrate with a range of grain sizes of aragonite sand to provide multiple habitats. He suggests that the top layer be sugar-sized grains as these offer the maximum possible surface area for colonisation.

“Lots of surface area combined with variations in grain size not only encourages small critters, as the Americans call them, but also as wide a diversity as possible due to the range of habitats available.”

He uses plenum filtration. Each of his five plenum tanks has full, metal halide lighting and is furnished with live rock, some covered with algal turf, to promote and encourage a diversity of micro life-forms within the substrate and water column.

In essence these become nursery refugiums where populations of tiny creatures multiply rapidly, safe from predators, until they migrate through water circulation into the frag nursery areas. All of the frag tanks are also fitted with a sand-bed topped with sugar sand.

As for fish, Martyn recommends not selecting zooplankton-feeding species because of the competition between the fish and the corals. So although water-column feeders such as chromis and anthias are popular subjects for reef tanks because they don’t damage the corals, he says herbivores such as tangs and Dwarf angels are better options.

In his coral farm he has a number of Copperband butterflyfish, Chelmon rostratus, in the nursery tanks. He has had reasonable success with these fish, which have the added benefit of not bothering the corals.

On the other hand, the Racoon butterflyfish, Chaetodon lunula, although regarded the best butterflyfish for this purpose, can’t be trusted with corals or anemones.

Comparing like with like

I wasn’t sure, however, that it was possible to create a naturalistic reef system within the confines of an enclosed aquarium system.

Martyn agreed that while it was not possible to maintain a reef ecosystem representative of all the life forms from bacteria upwards, something close to this should be the aim. The main objective should be to promote the widest diversity possible and not to neglect the lowly creatures in the food chain just because you can’t see them easily.

He said: “One of the large hidden resources in the water column are the zooxanthellae. Corals naturally expel zooxanthellae into the water every day.

“Zooxanthellae are a renewable resource on a reef and the various coral communities are constantly trading their zooxanthellae one with the other. This happens in our own tanks and is one of the ways in which corals establish themselves.

“So if you introduce a coral that has been taken from the wild into your tank, it probably doesn’t have the correct strains of zooxanthellae for the lighting or temperature. If there are no other corals in the tank, the new introduction is not likely to succeed without some initial stress.
“If, on the other hand, there are corals already adapted to the conditions, the necessary strains of zooxanthellae will be readily available to the new introduction.”

So there is a case for including a proportion of cultured corals into a new reef tank set-up that are adapted to conditions as this will help prevent any wild-collected corals from suffering this stress.

“Zooxanthellae also respond to conditions other than lighting and temperature variations,” Martyn added. “High nutrient levels can lead to the zooxanthellae taking over. It then keeps increasing to the detriment of the host, manifested in a lack of natural colours and a shift to shades of brown, the natural colour of the zooxanthellae.

“It’s known that tridacnid clams and some forms of LPS corals, such as Cataphyllia, come from high nutrient areas and can benefit from nutrient-induced increases of their zooxanthellae, but Acropora and many other SPS corals aren’t used to or able to tolerate the stress the rampant growths of zooxanthellae place upon them.”

Give us more colour!

The discussion turned to coral colour as I commented that brightly coloured wild-collected Acropora often lose their startling pink or blue colour when introduced into an tank and turn a muddy brown.

My understanding was that this was mainly due to a lack of high intensity light as shallow-water corals manufacture special colour pigments as a protection from damaging light rays, especially those from the UV end of the spectrum. Martyn agreed.

“These colours can be restored by increasing the lighting intensity,” he remarked. “There have been some interesting experiments that dispensed with the UV-protecting glass screening on metal halides, which has produced positive results.”

Colour in corals is as yet far from understood, but changes are often associated with adapting to their environment. This can include bleaching or shedding of excess zooxanthellae or changes in pigment density, which allows corals some flexibility in relation to lighting.

Unfortunately, adapting to lighting is often associated with temperature changes, and a combination of both can affect zooxanthellae as many strains are temperature sensitive. If they cannot adapt, the coral will shed its zooxanthellae – the most common form of coral bleaching – and attempt to re-populate its tissues with different zooxanthellae.

There is certainly no substitute for hands-on experience combined with daily observation. Add to this an inquisitive nature and a thirst for knowledge, and as Martyn shows, a very successful and true-to-nature reef aquarium can be achieved.

What about skimmers?

Despite aiming for a true-to-nature system, Martyn does not shun the skimmer. He has a large skimmer, but which is only used on special occasions such as to skim out the considerable mucus that is expelled when he sets a large number of frags, or if there is an emergency which could cause pollution.

Following the same principles, his calcium reactor is also zooplankton-friendly as he uses a converted plenum filter passing CO2 through the aragonite substrate which causes much less disruption to plankton populations.

This article was first published in the January 2004 issue of Practical Fishkeeping magazine.