Living Rock

“Living Rock”. These two words do not sit comfortably beside each other! “Rock” conjures up visions of something inanimate and static. So how can anything justify the description of being “living rock”? Well, the pairing of the two words create a term which is acceptable to marine reef aquarium keepers, and probably with a more sound reason than might at first be realised. The base material is indeed a form of calcareous rock collected from shallow tropical seas, and it is the obvious and often extravagant covering of sealife, comprising algaes and other encrusting marine life, which justifies the “living” label. But it is rare for all that varied marine life to adapt to the entirely strange new environment of aquarium life, and much of the living rock will lose at least some of its original coating. In time the surviving elements will take over and colonise the areas left vacant by those which failed to adapt. This process is able to be witnessed by the aquarium owner who will hopefully approve of the new “born-again” image.

But there is more life within “living rock” than the purchaser may have bargained for. When the rock is collected it is kept wet or at least moist throughout the handling process with the acknowledged need to protect the eye-catching encrustations from drying out to any degree and spoiling. At the time of shipment the pieces of rock may be wrapped in damp newspaper. With or without the paper it will then be enclosed in a large polythene bag and placed inside the expanded polystyrene insulated shipping container. (It is not shipped in water in order to minimise the weight for freighting by air, as it travels perfectly well in this way). The rock remains wet or at least damp throughout, from collection to the point of eventual sale in some far off country. Once the pieces have been suitably arranged in a reef aquarium under construction, and the sea water has permeated the rock, filling the internal crevices and tiny chambers to replace the humid air, a whole Pandora’s Box is opened, and various creatures revive from a dormant state ready to invade their new “coral reef”!

It is a common experience to find some limp but still living worms and a few crab corpses in the bottom of the shipping boxes the rockwork has arrived in. But these are just the unfortunate ones that went on a “walk-about” during transit, and lost their security within the rocks as a result. But those that stayed within the internal labyrinthine structure, are now able to venture into the aquarium to establish themselves in their new environment. The aquarium owner might think at first sight that these surprise packet creatures are a bonus, and some undoubtedly are, but sorry to say most of them will outgrow their welcome all too quickly.

Fire, or Bristle worms, are the most frequently encountered stowaways in the reef aquarium, and once installed they are none too easy to remove. They have an ability to slip away from capture and back into the rockwork with amazing speed and dexterity. Uncontrolled they may reproduce and increase their numbers in the aquarium. Some species are primarily nocturnal in habit, while others may be quite active and readily seen in daylight. Whether nocturnal or diurnal these fierce worms are not welcome! They are carnivores and will attack other invertebrates, both sessile and mobile species. It is true they are scavengers and will probably be easily satisfied with what they can scrounge without having to do battle with some other creature, but if left-overs are not around they can become very aggressive and troublesome in their quest for food. The author once witnessed a long and concerted battle between a bristle worm and a eunice species of “Bobbit Worm”. Neither won the battle, but it says a lot about the power and resilience of the bristle worm, valiantly pitched against such an aggressive and superiorly well-equipped foe. The aquarium owner must take care when re-arranging any rockwork in the reef aquarium where bristle worms are established, because accidental contact with a worm will result in an armoury of sharp spiny bristles sticking painfully into fingers. Although they are not easily trapped it is possible to tempt bristle worms into a baited container at night time via a small opening, then bring them out to be destroyed. Pieces of fish or fowl flesh will usually suffice as a tempting lure.

Some crabs will emerge and be so small and delightful to watch as they go about their scavenging activities, that the marine aquarist will be happy to have them in the aquarium. Such small specimens as are able to crawl in and out of the tiny crevices in the rocks will pose little or no threat to any of the other life in the reef aquarium, and will do a grand job of picking up dropped food particles abandoned by the fish swimming above. But what starts out as a cute little baby can steadily grow, and grow, and grow still more to the point where it is no longer wanted. In their determination to reach some tempting food pieces, some of the grown up crabs will rout amongst the sessile invertebrates and disturb them, often dislodging or undermining the rocks as well as the corals they support. A frequently encountered species is the Teddy Bear Crab (Polydectus cupulifer) which has a dense coating of stubby light brown hairs. It has a way of enchanting its owner, and becoming a special attraction in view of its unconventional method of arrival, but as it continues to grow it soon reaches the point where disenchantment takes over, and it is finally expelled from the community.

Nudibranchs, or Sea Slugs as they are often known, are quite often introduced to the aquarium within living rockwork. The brightly coloured and usually foul-tasting or positively toxic species, which most aquarium keepers would dearly want to have as occupants of their reef aquarium, have such a selective dietary requirement that it is almost impossible to provide their natural diet and succeed with them in captivity. Anyway this type of nudibranch rarely travels as a concealed stow-away in living rock. But the “immigrant” nudibranchs which do travel in with the living rock often can and do survive successfully in a well stocked reef aquarium, but mostly causing havoc in the process. One notable exception is a delightful nudibranch which sometimes secretes itself within Caribbean sourced living rock. This species is sometimes known as the Lettuce Leaf Slug, and that common name is almost a fitting description of the animal To science it is known as Tridachia crispata. Being a herbivore this nudibranch is well able to settle to aquarium life, browsing the fine algal coating on various surfaces within the aquarium. It does reproduce in the reef-type aquarium from time to time, the young appearing almost like tiny flakes of cigarette ash, and rapidly maturing into handsome adults.

Unfortunately the more commonly encountered and unwittingly introduced nudibranchs are not as harmless as the Caribbean species above. It is not only in living rock that these imports arrive, but frequently they are hidden in the rockwork which arrives with soft corals attached. And it is the soft coral which forms the major food source for these unwanted lodgers. One such nudibranch is a Tritonopsilla alba, which lays its compact egg ribbon at the very base of the soft coral, so when the young hatch they have immediate access to their required food source, eating into the base of the soft coral and causing it to disintegrate. This species is largely white-ish in colour and liberally decorated with branched ciliated extensions from its body. It usually makes its first appearance at a minute size, when it is not seen to harm anything in the tank, and is shining white – the colour of purity and innocence! Another species of Tritonopsilla lives on the more finely branched gorgonians, and may either arrive in the living rock, or actually already infesting a newly purchased gorgonian either as eggs or mature animals. The eggs of this species are contained in a fine “string” which the nudibranch lays by encircling the gorgonian “branches”. When the young hatch they set about feeding parasitically from the gorgonian. Sharing the same family of Tritoniidae is a spectacular species, marionopsis. The body of this quite large nudibranch is ornamented with branched cili. Either side of a dorsal strip of lighter brown, the flanks have a marmorated chestnut brown colouring and occasional electric-blue spots. The host for this species is usually a Sarcophyton species of soft coral which it matches perfectly for colouring. Just occasionally juveniles of marionopsis will arrive with living rock, where presumably they have hidden during the time they were not feeding on the corals above. As juveniles they do not have the same dramatic appearance of the adult, but the growth rate is rapid when allowed to feast from the soft coral it requires to survive.

Another nudibranch genus, Phyllodesmium, has several species which will arrive with living rock where some wiry soft coral species is attached to it, or again might arrive within the cili-like texture which forms the body structure of certain soft coral specimens shipped for sale to the reef aquarium keeper. These nudibranchs have long cili arrayed along the length of their slender bodies, and those cili together with the rhinophores match the formation of the soft coral to perfection, rendering the nudibranchs all-but invisible. When disturbed these Phyllodesmium species are able to shed some of their cili to distract a would-be predator, leaving the animals unmolested and able to get on with living with its coral host, utilising it as its food source at the same time. So another unwelcome nuisance creature for the reef aquarium keeper who values his or her soft corals!.

Shrimps are less often found alive within living rock stock. But notable exceptions include various species of snapping or pistol shrimps. It is the natural behaviour of these shrimps to hide away, so it is no surprise they should occasionally turn up hidden within living rock. But their reclusiveness continues within the aquarium, so there is nothing much to be gained from having them there. The common name is derived from the ability they possess to “snap” their enlarged chelipeds, or front claws. This action creates a mini shock-wave which enables them to stun passing small prey, which would be mainly small crustaceans in their natural environment. The noise generated in this way is amazingly loud, and may be clearly heard even when when standing quite far from the aquarium. Small species will be no problem if they are left in the aquarium, but larger specimens might cause an upset and are best excluded. Some shellfish survive the journey from the sea to the aquarium tucked up inside the living rock where they had taken refuge by day, withdrawn tightly into their shells with the “door shut”. There are a few species which might appear from time to time, and by and large they scarcely attract much attention. But there are some fascinating species which appear from time to time. Notable among these are Stomatella species. The amazing trick these little shellfish can pull off is not apparent until they are disturbed. At first glance they are so well disguised that they are not instantly recognised as shellfish, because they have a rear half of growth which mostly conceals the shell beneath it. If attacked by a predator this rear mantle is shed, and continues to twitch and contort like an animated, living creature in its own right. This keeps the predator engaged and allows the “business end”, with its shell now properly exposed, to crawl away quite quickly and hide . The automised mantle, or tail, continues to produce movement for about 30 to 40 minutes. (We know of this distracting decoy phenomenon of a dropped body part more commonly in association with some lizard species which drop their tails when pursued, leaving them wriggling behind to take the attention of the pursuer while the lizard makes good its escape).

Reef tank keepers will all too often know the pain of having unwittingly brought in plague animals to their pristine aquarium, only to reduce it to an eyesore with a seemingly indomitable invasion of rapidly reproducing worms or anemones. A major menace is the tiny Waminoa species of Acoel flatworm. The body form is that of a flat disc, and the colour varies from olive-green to rust-red/brown. Often found on bubble coral polyps in nature, the contrasting dark colour of the worms set against the off-white or pale green bubble coral makes them very easy to spot. In the aquarium these unsightly worms reproduce at an exceedingly rapid rate, finally coating everything. When all available suitable space is infested they will begin to “double-up” by attaching to each other’s backs. Control is very difficult to achieve, in view of the rapidity of the worms to reproduce. Few fish will eat them, and not with enthusiasm. Mandarin fish and other Callionymid dragonette species will tackle the “plague flatworm” as an acceptable food, but only to a limited extent. Without stripping down and starting from scratch it is almost impossible to rid a reef aquarium of this menace by other less dramatic means.

A similarly persistent pest is a species of anemone. The Rock Anemone (Aiptasia tagites) is likely to sprout out from its dormant stage just about anywhere on the surface of living rockwork. At first sight it is an interesting addition to the growth establishing itself on the rock’s surface. When a second and third one appears they are often welcomed as still more interesting animals to grace the reef aquarium, and “special” because they were bred in the tank.. But by the time they are proliferating all over the aquarium it is too late to exercise any simple control, and the anemones spread in plague proportions to become a dreadful eyesore. Attempts to manually remove them invariably fail. They swiftly retract and form a tight ball closed down onto the foot, sticking closely and firmly to the surface they have invaded. If the decor they have attached to can be removed from the aquarium it is then possible to kill them by applying a drop of bleach to the retracted anemone, or burn it with a hot metal skewer or similar tool. Take care to ensure the bleach is entirely washed from the decor piece before it is replaced in the aquarium, and also take care that any of the area surrounding the anemone remains untouched by the bleach. Similarly, take precautions when using the hot skewer technique and be sure to take care not to burn yourself when handling the tool. As far as the author knows there are no fishes which will effectively control these pestilent creatures. Baby Platax Batfish will certainly worry them by pecking at the tentacles, which they appear to eat. This attention will eventually destroy the anemone, but the limited effect of one or even a few small Batfish will be as nothing compared to the astonishing rate of reproduction this little anemone is capable of.

The reef aquarium keeper needs to know just what it is that is creeping out of the rockwork when newly introduced “living rock” begins to release its hidden residents, and be sure to remove all but the recognised and welcome creatures. And remember the rock on which some soft corals are imported is also capable of hosting some unwelcome and damaging invertebrates too

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