Bertelsen's duckbill conger
Gavialiceps bertelseni
Bertelsen's duckbill conger features a long, flattened snout and a striking coloration of dark brown with lighter speckling along its body.
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About the Bertelsen's duckbill conger
Deepwater marine conger eel from off western/southwestern Madagascar (western Indian Ocean), reported from roughly 670–1200 m depth; maximum length about 84 cm TL (reported for males). Not a typical aquarium species due to deepwater habitat.
Also known as
Quick Facts
Size
84 cm TL
Temperament
Semi-aggressive
Difficulty
Expert
Min Tank Size
300 gallons
Lifespan
unknown
Origin
Western Indian Ocean (southwestern Madagascar area)
Diet
Carnivore - predatory fish/invertebrate foods (meaty marine items)
Water Parameters
4-10°C
7.8-8.4
8-12 dGH
Need a heater for this species?
This species needs 4-10°C in a 300 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Run a covered tank with no gaps - they are sneaky escape artists and can thread through openings you would swear are too small.
- Feed after lights-out with tongs: chunks of shrimp, squid, silversides, and marine worms; start with smaller pieces because they strike fast and can miss and spit food if its too big.
- Do not keep with shrimp, small gobies, or anything that sleeps on the sand - it will get vacuumed up; pick tankmates that are too big to swallow and not bitey.
- Avoid aggressive triggers, big wrasses, and puffers that nip faces and fins - duckbills handle bullying badly and stop eating when stressed.
- Watch for sand in the mouth and snout scrapes from sharp substrate or rough rock; fine sand and smooth burrow edges prevent those annoying infections.
- Breeding in captivity is basically not a thing - treat it like a display predator and focus on getting it feeding reliably and keeping the tank calm.
Compatibility
Good Tankmates
- Rabbitfish (foxface and friends) - sturdy, not easily bullied, and they mind their own business while the duckbill conger does its hiding-and-ambush thing
- Peaceful to semi-chill wrasses that sleep in the rock/sand but are not tiny (fairy/flasher wrasses in bigger sizes, some Halichoeres) - quick enough to avoid trouble and usually not finicky roommates
- Bigger angelfish (dwarf-to-large angels that are not super tiny juveniles) - they are confident feeders and generally dont get stressed by an eel-like fish cruising at dusk
- Non-predatory, non-nippy bottom fish like larger gobies/blennies (think watchman goby size or bigger) - ok if they are not bite-sized and there are plenty of caves so everyone has a spot
- Other robust, midwater community fish like larger anthias groups - works when the anthias are established and well-fed, and the conger is not kept hungry (hungry duckbill congors get ideas)
Avoid
- Small shrimp gobies, tiny clownfish, firefish, small cardinals - basically anything bite-sized that hovers near the rocks at night, because this conger is an ambush hunter and will eventually test them
- Crustaceans you actually want to keep (cleaner shrimp, peppermint shrimp, small crabs) - most duckbill congors treat them like expensive snacks once the lights go out
- Really aggressive or territorial bruisers (big triggers, mean large wrasses, dottybacks that own the rockwork) - they can harass the conger in its burrow and turn the tank into a nightly fistfight
Where they come from
Bertelsen's duckbill conger (Gavialiceps bertelseni) is one of those deepwater oddballs that looks like it swam out of a documentary. They show up in the tropical Indo-Pacific, usually down deeper on slopes and soft bottoms. That deepwater background explains a lot: they like dim light, stable water, and they do not appreciate chaos in the tank.
This is not a typical "eel for a reef" situation. Most specimens come in stressed from collection and shipping, and they can go downhill fast if you try to treat them like a hardy moray.
Setting up their tank
Think "secure burrower" first, display fish second. Mine spent a lot of time tucked into sand with just the snout out, especially early on. If you cannot give them a place to wedge in and feel hidden, they tend to sulk and refuse food.
- Tank size: bigger footprint beats height. I would not bother under 120 gallons, and 180+ is a lot less stressful long term.
- Substrate: fine sand, 2-4 inches. They like to push into it. Coarse crushed coral can scrape them up.
- Hiding spots: PVC elbows and sections under/behind rockwork work great. Build caves that cannot collapse.
- Rockwork: stable and anchored to the bottom (or on egg crate), not sitting on sand where an eel can undermine it.
- Flow and filtration: moderate flow, strong oxygenation, and aggressive export. They are messy eaters.
- Lighting: subdued helps. If you run bright reef lighting, give them shaded zones and overhangs.
Lid, lid, lid. Any gap is a future jump point. Cover overflows, cable cutouts, and the back corners. I have seen congers find holes you would swear were too small.
Stability matters more than chasing some magic number. Keep salinity steady (no big swingy top-offs), keep nitrate reasonable, and do not let pH and temperature bounce around. These guys act "fine" right up until they are not.
What to feed them
They are predators that want meaty marine foods. The trick is getting them started and keeping them on a consistent schedule without overfeeding the tank. Mine did best with smaller, frequent offerings at first, then I stretched it out once it was bold and reliably eating.
- Best staples: pieces of shrimp, squid, scallop, clam, and marine fish flesh (not freshwater feeder fish).
- Great "get them eating" foods: live blackworms (if you can do them safely), live ghost shrimp acclimated to salt (short term), or fresh/frozen prawn waved on tongs.
- Feeding method: long tongs or a feeding stick. Keep your fingers out of the equation.
- Frequency: juveniles every 2-3 days; adults often do fine 2x per week once settled.
Train a feeding response. Offer food in the same corner, same time, with the same tool. Once they connect the dots, they come out faster and you waste less food in the sand.
Watch for "food theft" from tankmates. If a trigger, wrasse, or hawkfish is snatching everything mid-water, the conger will lose interest and you will think it is refusing food. Sometimes you have to target feed right at the burrow entrance, or feed the tankmates first, then feed the eel.
How they behave and who they get along with
They are shy at first, then gradually turn into a confident ambush hunter. Mine was mostly crepuscular (dusk/dawn) and would do slow "patrols" along the sand once it felt safe. They are not typically aggressive in a territorial way, but anything that fits in that duckbill mouth is a menu item.
- Good tankmates: larger, calm fish that will not harass it (bigger angels, larger tangs, some groupers if sized appropriately, sturdy rabbitfish).
- Risky tankmates: nippy fish (some triggers, some puffers), hyperactive wrasses that pick at everything, and anything small enough to be swallowed.
- Inverts: do not count on shrimp, small crabs, or similar. Snails sometimes survive, sometimes do not. If it crawls and fits, expect losses.
Avoid mixing with other eel-like fish unless you have a huge system and a plan. Competition for the same hiding zones and feeding lanes can turn into stress or bite damage.
If you need to move one, do not use a net. Use an eel tube (PVC) or a container. They tangle in mesh and can tear skin or damage the jaw trying to back out.
Breeding tips
Realistically, breeding Gavialiceps in home aquariums is not a thing right now. Like many marine eels, they likely have a complex life cycle with a pelagic larval stage (leptocephalus) that drifts in open water. Even public aquariums rarely report success with deepwater congers.
If you ever see a suddenly "plumper" eel and odd nighttime swimming, it can be spawning behavior in some eel species, but do not expect viable young. Keep your focus on long-term stability and nutrition.
Common problems to watch for
Most losses with these come down to shipping stress, refusal to eat, or injuries. If you take on this species, plan on a calm acclimation period and be ready to adjust your approach quickly.
- Refusing food: usually stress, too much light/traffic, or tankmates outcompeting it. Add cover, dim the area, and target feed.
- Jumping/escaping: almost always a lid issue or a spook event. Check every gap after maintenance.
- Skin damage and infections: caused by rough substrate, unstable rockwork, or netting. Fine sand and safe hides prevent a lot.
- Internal parasites: common in wild eels. Watch for weight loss despite eating, stringy feces, and fading energy.
- Copper sensitivity: many eel-like fish do poorly with copper-based meds. Quarantine with caution and research your treatment options.
Be careful with medications in general. If you need to treat, a separate hospital tank with tight cover and lots of oxygen is your friend. Many deepwater species react badly to heavy-handed dosing.
One last real-world thing: keep maintenance predictable. Big aquascape changes, loud pumps rattling, hands constantly in the tank, or chasing other fish around will keep this eel on edge. The more boring your routine is, the more you will see it out and about.
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