Q&A - Queen Conch
by Rob Toonen
Can you tell me any information about Queen Conch? All the books I read have only a half page about them, and am very interested on their compatibility with other inverts, including other snails. Someone told me that their conch ate all their Turbo snails! The local pet shop has a bunch of captive-raised conch right now, which is great and I really want to support captive raised animals, but I don’t want to buy something until I understand completely what it is and what it will do for my reef.
I suspect that the Queen Conch (Strombus gigas) offered for sale in the hobby should all be captive raised now, because the efforts of breeders have been so successful that until demand increased, some suppliers were actually offering to release an animal into the wild for every one sold in the trade to encourage sales. These animals are not predatory, and I have never heard of one attacking a live snail (or anything else) before, so I have to wonder if the snail you heard about attacking Turbo snails is really a conch, or if (as is frequently the case, unfortunately) the snail was a misidentified predator sold to someone as a conch. Ron Shimek has a couple of detailed articles on telling the good snails from the bad ones, and if you don’t know in advance what the snail you are planning to buy actually looks like, I’d suggest a little research to avoid being duped into buying something else by mistake.
The fact is that all members of the genus Strombus are supposed to be obligate herbivores, and should therefore be reef safe. In fact, they quite simply cannot eat meat – they have neither the ability to cut it with their teeth (or more correctly, the rasping apparatus called the radula), nor the gut to digest it even if they could rip out pieces of flesh from another snail. Given that fact, the snail that attacked and ate all the Turbo snails in your friend’s tank was either sorely misidentified, or reports of its attacks might have been exaggerated. OK, so wild reports of cannibalistic conch aside, what can I tell you about the biology of the queen conch?
Well, you’re almost certainly going to be looking to purchase a juvenile snail (given the adults can reach up to a foot in length), so let me tell you about the habits and diets of the young. In the wild, juveniles typically bury themselves to escape predation, and are rarely seen on the surface until they are roughly 5cm long, but they seem to lose this behavior in captivity (whether due to the incorrect environmental cues, hunger, or some other reason remains unknown). Typically the animals would avoid any hard substrate in the wild, but in tanks this is also sometimes lost (as evidenced by the number of people who report them on the rocks and glass in their tanks). Again, it is not clear whether this is due to our keeping them in a highly unnatural habitat for them, an issue of hunger, or something to do with a shift in behavior among cative raised animals.
The juveniles do best in tanks with fine-grained sands and high detrital levels (hence they are good candidates for members of the clean-up crew in any well established reef aquarium of sufficient size), but unfortunately, the animals have very low survival rates overall. There are a number of possible factors for the low survival rate of juvenile queen conch in captivity. First, because cultured animals produce thinner shells than wild-caught, they are particularly susceptible to crab predation. I doubt crab predation is nearly as much of a factor as starvation, however. In many cases, the animals simply cannot find enough food (primarily organic detritus – see below) to sustain themselves for long in an aquarium, and unless you allow detritus to accumulate in your tank (i.e., do not vacuum or otherwise remove particulate detritus from the tank bottom), these animals are unlikely to do well for any extended period in your aquarium. Also, although these animals are cute and easily accommodated when small, they frequently reach lengths in excess of 1 ft in the wild (I think ~18" is the size record for these guys!), and although I have yet to hear of anyone raising one of these animals to maturity, the amount of food and cruising space it will require as it grows will obviously be difficult for anyone without a very large reef tank to accommodate.
Queen conch are found almost exclusively within seagrass beds in the wild, where they consume seagrass bits, epiphytes (tiny algae that grow on the sea grass blades) and detritus. The primary source of nutrition for these animals, however, is detritus, and more specifically, primarily detritus from decaying seagrass blades. In experiments which compared the amount of live seagrass, epiphytes and macroalgae in cages with or without conch at natural densities (1.2 animals per square meter), researchers found that none of these were significantly affected by the presence of the conch (the relative amount of each within the cages remained unchanged from that outside where the conch had free access), but the amount of detritus in cages without conch was as much as 97 times higher than the amount of detritus in cages with conch. Obviously, the conch eat an enormous amount of organic detritus, and can make a big difference to the amount of detritus that will be present in any enclosure (whether a tank or a cage) in which they are kept.
The massive reduction of detritus is nice in some respects, but a closer examination of the sand fauna associated with seagrasses in these cages found that the reduction of detritus by conch led to a decline in the sand fauna as well -- there were 3.8 times as many sand fauna (primarily the microcrustaceans: tanaids, amphipods, copepods and ostracods) in the cages from which conch were excluded as in the areas with conch included. This is not due to predation directly (i.e., the conch were not eating the sand fauna intentionally), but rather apparently due to the sand fauna having nothing to feed upon after the conch had eaten all the detritus in the system. This means that you face a trade-off with adding a juvenile queen conch to your tank: the snail will be a great detritivore and help keep detritus from building up in your tank, but that lack of detritus feeds much of the sand fauna so important in feeding certain fishes such as the popular dragonets known as Mandarin ‘gobies’ (Synchiropus splendidus) and Scooter ‘blennies’ (Neosynchiropus ocellatus). That is a decision that I must leave to you – if the reduction of detritus in your tank is of paramount importance, then a queen conch may be a suitable addition to your tank. If you are interested in maintaining a large population of microcrustaceans in the hopes of supporting dragonets in your reef tank, then adding a queen conch may not be the best way to accomplish that goal.
Another option that you may want to consider is one of the Hawaiian conchs Strombus maculatus or S. dentatus that have recently shown up in the aquarium trade. These smaller cousins of the Caribbean queen conch (S. gigas) has reproduced in captivity for an ever-growing number of aquarists, and stay small enough that they never cause concern for any reef aquarium. S. maculatus reaches a maximum size of only roughly ½ inch in length, while the toothed conch, S. dentatus grows to a maximum length of about 2 inches. Both of these snails provide an excellent grazing function and have a lower direct impact on the reef ecosystem than even a moderately-sized queen conch.
So, to make a short question into a long answer, there should be no direct problem in adding a queen conch to your aquarium, or to keeping these snails with other invertebrates, but their voracious appetites (they are the Pot-bellied pig of seagrass beds) can have a dramatic impact on the amount of food available to other organisms that also feed on detritus. On the plus side, they should do a great job of moving your substrate and keeping your sandbed clear of cyanobacterial mats and detrital build-up. On the minus side, they may be so efficient that they depress the population of other detritivores in the aquarium, and worse, may ultimately starve themselves out as well, leaving you without any detritus feeders in the end. Whether the addition of a queen conch ends up being ultimately a good or a poor decision for your tank, however, depends on your aquarium size, the amount of detritus in your aquarium and your preference for a large population for other detritivores (such as the microcrustaceans I mentioned above).