Welcome to the twilight zone
( First printed in Ultra marine Magazine Issue 9)
In previous editions of Ultra marine, we have covered various methods of filtration in use today. And whilst one method may suite a certain individual or system, it is frequently the case that it may fall into conflict in other situations. From a reef keepers point of view, one of the greatest challenges we face today is simply ‘choosing’ which method of running a reef tank we wish to employ. Most methods only fulfil part of the required cycle of nutrient control, be that chemical breakdown in the case of trickle filters, or physical waste removal in the case of skimmers, with only a few methods such as Deep sand beds and plenums completing the entire nitrogen cycle to any really advantageous degree. We are frequently faced with the dilemma of which method to choose as a main method, and what to add on as a supplemental filter to fill in the gaps as it were. The old adage that you cant have too much filtration may be a good one, but only if you can factor the various pros and cons of each method, and put together a combination that is well balanced in relation to the amount of waste broken down and to what degree, how much is assimilated (the conversion of nutrients into body mass, such as that found in algae’s and assimilation sand beds) and physical removal at a source level by way of the protein skimmer etc. There is rarely a ‘single’ method that we could bolt strait onto a system as an additional filter, without it affecting the rest of the systems nutrient cycling capabilities to a greater or lesser degree. So you’ll be pleased to note that in this article I’m going to offer you exactly that, i.e., an additional filtration method that can be added to ‘any’ existing reef system, or designed into a new system which offers nothing but benefits and wont leave you with some form of by-product that needs further attention..
Firstly I will lay total credit for this article at the feet of a well known reef keeper that many of you will no doubt be acquainted with, namely Steve Tyree who came up with the formulas and principles a few years back. And that idea is of course ‘Cryptic Zone filtration’
So to clear up the history and dynamics in one clean sweep, what exactly is a cryptic zone? And, why is it so beneficial to a reef tank? Luckily for those of us who made it to the WYMAG seminar in Leicester way back in 2004, it was here that Steve introduced us to this area of coral reef dynamics, and attempted to guide us through the methodology involved.
To understand Cryptic zone filtration, we first need to go back to the wild, and look closely at the intricate relationships between light and flow on the reef, and how depending on their nature, they can cause dramatic changes in the types of animals found in any given location. Steve looked at these interactions and combined them with an already existing system called the ‘gradient system’ which categorises organisms mainly by light saturation zones, and came up with a more unified grading system whereby we could categorise a certain environment or micro environment by the combination of ambient light, flow ‘and’ the type and nature of particulate waterborne suspended matter ‘or food’ for want of a better word. As an example, the most typical zone we encounter is the exposed and semi exposed zones. Typically these will be the reef crest or upper part of the reef where light saturation is intense. Flow ranges from strong to extremely strong and particulates of all sizes are plentiful. With even large particulates held buoyant by the strong surges. Flow rates in these areas can range between 10 to 150cm/sec dependant on the surge characteristics, and organisms found in this region are typically photosynthetic with strong and dense structures ideally suited to handling this kind of environment. Next we have the ‘Semi-exposed’ region, which can either be deeper or under the shelter of overhanging structures etc, we typically have flow velocities up to 10cm/sec and light intensities in the region of between 5 and 10% of that found at the surface. Even at these reduced flow rates, the organisms found are still quite sturdy and predominantly photosynthetic in nature. Typically these areas would be deeper reef slopes, and lagoon regions which are high in particulates. Because flow rates, are still quite high, all but the largest particulates are still held in suspension, so the organisms present need to be able to handle a range of sizes from small to large. Typically those corals present, will still be heavily reliant on light as a main source of nutrient production, but also a larger volume of suspended food to make up the deficit in lower light intensity. More delicate filter feeding organisms are still omitted due to the relatively harsh conditions.
A typical cryptic tank fed from the return pump, which overflows back to the main display tank. In this case the front cover is down for viewing
We then add three more areas that can cross over slightly dependant on the mix of environmental characteristics. The first two are the semi-cryptic and cryptic zones. The first of which we would typically call the deep reef slope, or it may be found in shallow water areas deep under overhangs or at the entrances of caves etc. Both these areas represent the lower threshold of photosynthetic survival, where light levels are just enough to allow only the hardiest of photosynthetic organisms to survive.
The ‘cryptic’ or ‘twilight’ zones are those where light no longer holds any significance as far as photosynthesis goes. These areas may be found at the deepest points of the reef slope, deep in caves, or occasionally in shallow water, deep within the structure of the reef itself. The creatures found in this area are typically very delicate in structure, lack any ability to deal with anything but the gentlest of flow rates, and lack the ability to produce any form of pigmentation that would protect them from the intense UV light ranges that are found in more exposed areas..
The final zone is defined as the ‘filter feeding’ zone which falls across the semi-exposed zone and the semi cryptic zones. Typically this area will have flow rates high enough to keep a high volume of medium sized particulates in suspension. Essential for the more dominant organisms present in these areas like gorgonians, and many of the filter feeding soft corals of the dendronepthia family.
To define these new areas, Steve had to appoint definite characteristics in relation to the governing environmental factors. To do this, it was decided that again flow and light intensity would play key roles, as well as the nature of particulates present.
The semi-cryptic zone will typically have light intensity in the ranges 1-5% of that found at the surface, with flow velocities in the ranges 0.1-1.0cm/sec carrying only small particulates in suspension, and be dominated by more ‘passive’ filter feeders that simply sit there and let particulates drift towards them in the currents. Taking things further, the ‘cryptic’ zone would have light levels less than 1% of that at the surface, and flow velocities not exceeding 0.1cm/sec a flow rate that only allows the very finest of particulates to remain suspended. In this region, the most dominant creatures are those that ‘actively’ filter feed by self generated currents within or around their bodies.
Finally the filter feeding zone which may overlap other zones, is an area that has a wider range of parameters whereby light levels can range from 1-10% of that at the surface, and currents can range from 1-10cm/sec. These higher ranges would allow a larger amount and size of particulates to be carried, and the area would be dominated by passive filter feeding organisms.
What is immediately noticeable here is that there is some cross over, and whilst at first this may seem confusing? It is important to realise that we are not trying to ‘chop’ the reef up into clearly defined and ‘single layer’ separate areas, like some huge slab patio. In nature nothing is ever that simple. In this instance we are taking something as complex and convoluted as the reef, and defining areas be they vast or small, based upon more clearly defined characteristics. These areas may on occasions overlap to the degree that we will find cryptic zone conditions hidden ‘within’ and ‘under’ the confines of a differing area such as a semi cryptic or semi-exposed zone. The largest factor in this, is simply ‘geography’ at a local level, which can be from a multi channelled large volume tunnel through the reef itself with exposed openings, to the smallest inner recesses of a single rubble mound or coral thicket that has an intricate and heavily sheltering nature.
So where does this lot come into play within our reef tanks you ask. Well in many cases we may already have some of these areas, without us ever consciously trying to create them. Every time we create a shaded area or overhang which is sheltered from our powerful lighting rigs and not subjected to direct output from a power head or circulation device, we unknowing create a multitude of semi- exposed and semi cryptic zones. Typically in these regions, we may find a few sponges and a variety of hardier filter feeders such as calcareous tube worms etc. The main aim of ‘cryptic filtration’ is to go one step further and create an environment that falls within those even gentler boundaries mentioned earlier, whilst protecting those organisms present from predation. An inevitable action, baring in mind the commonly predatory grazing habits of the many species of fish and invertebrates we keep in the main display. Why would we want these organisms present in the first place you might ask? Well simply put, in the ever demanding world of nutrient control in the reef tank, what better way than to put some of that suspended matter to good use and employ the filtering capabilities of those creatures that have spent millions of years developing by far the most efficient apparatus for dealing with it, far beyond any efforts we may have come up with. On top of this much like the DSB and refugium, we also provide the system with an invaluable and constant stream of larval planktonic food as a by-product. From a corals point of view, there is nothing on this earth, or in our freezers that compares to a ‘live’ meal.
So to surmise so far, we will usually have inadvertently created within the confines of the aquarium, a variety of habitats that encourage differing groups of organisms, From our upper main rock structures that we attach the corals to ‘the exposed zone’ to the back side of these structures where light isn’t as intense and flow is reduced ‘the semi-exposed zones’ to the undersides of our rock structures where flow and light are greatly reduced and softer bodied organisms can be found growing including a few hardy sponges etc or more accurately, semi-cryptic zones.
So to finalise this spread of environments, we now need to consider how and where to create a fully cryptic zone, where flow rates are but a fraction of those found in the main display, and nearly all light is expelled to less than 1% of that found in the main display area. We can do this in either of two ways.
Method one employs the installation of a baffle plate within the main aquarium that effectively forms a false back pane with a void behind it. This will usually be made of black or blue acrylic sheet, and will be perforated across its entire surface with small 5mm diameter holes, spaced at 1” intervals. The void can be from just a few inches wide to as large as we wish dependent on how much of the display are we are happy to loose. We will usually have supports placed at regular intervals between the back of this perforated sheet and the back pane of the tank to spread the load of any rockwork we lean against it in the main display and to prevent bowing or sagging. Seeing as this area will be directly under any lighting we have over the main display, we will also need to make up a cover to block out any light penetration. Flow through this area is simply random ‘very low level flow’ that travels through the perforated holes. This then meets our criteria where flow velocities are less than 0.1cm/sec and only the finest of particles make it through to the organisms we are trying to promote in this area. Likewise, larval spread from this area travels back out through these holes as well, to feed the rest of our tank inhabitants.
Possibly, the only drawback with this method is the progressive encroachment of coralline algae across the perforated sheet which will, over time, block up the perforations reducing flow even more or blocking it off completely from the main aquariums water body. In this instance, the only option is to regularly scrape this panel clear, and clean out the perforation holes. Whilst this may be easy in a new set-up, it will obviously represent a problem in more established tanks, especially those with a heavy hard coral and branching growth that impedes effective access without causing damage to those corals.
Method two, gets round this issue by moving this entire area to a remote ‘cryptic’ tank that is blacked out, and fed just a small amount of water from the return pump in our sump, ‘after’ our main filtration has removed the bulk of waste present. This means ‘after’ any skimmers, sand beds, algae beds or other filtration media’s. The main reason being, that we are only interested in feeding this tank the smallest and last remnants of particulates that other filtration has missed. This tank can either be mounted above the sump in the main display cabinet, or mounted away from the aquarium in a raised position so that water flowing out of it simply flows strait back to the main display tank without having to pass through a pump which may damage larval life that is trying to make its way back to the main display to become a food source.
The feed should be controllable so that just a very small amount of flow is generated through the cryptic zone.
It should be noted here, that cryptic zones are not something that will become effective as a filtration method instantly. Whilst we can import a variety of organisms into these areas on bits of live rock etc, many of them are so delicate that they will not survive the rigours of transportation from the reef, via the shop to our tanks. Commonly, what we do develop is an offshoot of chance larval colonisation. In effect, you can’t really ‘build’ a fully functional cryptic zone, you have to ‘grow’ one over the duration of the tanks life. When it comes to filling these areas out initially, we should hand pick from our shop tanks, a just few pieces of choice live rock that exhibit a higher than normal accumulation of cryptic life, to do this, we will have to delve deep into the holding tanks looking for rocks that have spent some time in these systems under the shade of other more attractive coralline encrusted surface or décor rock. The more convoluted, pitted and porous the rock, the better, as this will already house areas that have the highest proportion of cryptic life hiding in its recesses. As a growth media for further cryptic life, we may consider a nice selection of branching rock. This offers a very good surface area ratio, whilst also allowing a more even settlement and movement of water born particulates through it. The resulting growth of cryptic life will benefit from this type of habitat immensely, rather than being restricted to just a few boulders. Over some time, we will also see a growth of life on the side panes and flat surfaces within this tank as life takes advantage of any free area. This is to be encouraged as much as possible, so we shouldn’t go cleaning this area at all. In effect, we want this to be an area that has as little disturbance as possible and is allowed to do its own thing.
Typical life forms that will grow to populate these areas are a variety of encrusting sponges, tree sponges, tunicates, various types of fan worms from smaller very delicate species up to larger 10mm plus calcareous tube varieties, various sea squirts, hydrozoans, various micro anemones and predatory non photosynthetic polyps, a multitude of differing mobile scavengers such as copepods and mysid shrimp, various flat worms of the non invasive variety, The list is pretty much endless as far as these habitats go. All of these organisms over time add up to a very efficient filtration system that traps and utilises extremely fine particulate debris to grow and reproduce. Effectively this lowers the levels of dissolved nutrients in the system as a whole thereby keeping the system cleaner and more efficient.
Typical Aiptasia growth on a branch rock , stretching into the nearly still current, to catch any passing particulates.
When looking into a cryptic zone some time later on in its life, we will usually find a settled out layer of detritus on the bottom. Whilst at first we may think this isn’t a good thing ‘especially seeing as we try to minimise detritus build-up in the main tank. It should be remembered that this material is just the ‘inorganic’ remains and silt that’s left over ‘after’ the biologically available nutrients have been assimilated by the organisms above, so it represents absolutely no danger to the system in terms of nutrient load. If anything, this silty layer provides yet more habitat for a variety of scavenging worms and micro fauna that burrow through it in search of any missed morsels. We will commonly find a good population of aiptasia anemones as well. This shouldn’t cause concern on the whole, because these anemones will generally sit there and mind their own business just budding off by division when the need arises, rarely making them selves a pest in the main aquarium by way of spawning. If anything the small amount of food available actually limits this process unlike the readily available food that is encountered in the main display area.
Juvenile Sponges and tube worms live together, filtering the water as it passes.
More tube worm and sponge growth in this 6 month old cryptic tank.
It should be noted, that unless utilised on a large scale in the realms of duplicating the main tank volume, a Cryptic tank will offer only marginal filtering capacity as far as the system goes. So it should be remembered that this method is designed as an 'addition', or 'tweak' to an already well planed out and maintained filtration system. Our main aim with the cryptic zone, is to take care of what’s ‘left’ after the main filtration has taken care of the worst. For the average tank though, even a relatively small area dedicated to this task will serve a good role in the long term health of the system by regulating and limiting particulate accumulation and clogging of our main display area and the live rock that is present.
Cryptic tank zones needn’t be huge. In this case, a simple canister filter filled with rubble, which has a very slow flow directed through it, serves a valuable role on the tank of Darryl Brett previously featured in Ultra Marine. As can be seen on this more established cryptic zone, the growth of calcareous tube worms can be quite prolific.
The following images were taken of the authors cryptic tank which is still in relative infancy after just 6 months of colonisation. In an effort to obtain images, it was necessary to light the area temporarily. Under normal circumstances this tank remains blacked out at all times.
As a summary on this article, I hope this has given you a little insight into the dark and wonderful world we frequently pass strait by when looking at our tanks from a distance. What we find in the deepest darkest corners of our systems can be just as important and beneficial to the long term health of a system as any of the major equipment we bolt to it, or animals we directly add as a clean up crew or scavengers. Indeed the total amount of life hidden within these areas that grows over time, may in fact surpass in terms of filtration capacity what we commonly consider as secondary inhabitants of little or no consequence. What better way then, than to encourage this life in a safe area where it can be protected from predation, and offered the chance to flourish to even larger populations than it would otherwise reach in the confines and cut throat world of the main reef display.