Strange Burrow Fellows: Shrimpgobies (family Gobiidae) and Snapping Shrimp
Articles and photos © Scott W. Michael
The marine aquarium hobby can open up an incredible world to the amateur naturalist. Not only does the saltwater hobbyist have an opportunity to keep a wide array of interesting invertebrates and fishes, they can also observe some of the fascinating symbiotic relationships that occur on coral reefs. For example, we are all familiar with the Sea Anemone-Anemonefish association. This unusual partnership has been responsible for enticing many pet store browsers into setting-up a marine aquarium. But I think there is an even more fascinating fish-invertebrate association. This symbiotic relationship involves a number of Goby genera and certain Snapping Shrimps (family Alpheidae). Let’s take a closer look at these fascinating burrow fellows starting with the fish partners.
Gobies
Gobies are becoming more and more popular in the home aquarium. They are also one of the largest families of fishes represented on coral reefs. It is currently estimated that there are over 1,875 described Goby species with many more waiting to be formally named. Gobies are ideally suited for the home aquarium. One reason is because of their diminutive proportions. This family includes the smallest known fish (the Dwarf Goby, Trimmatom nanus), which attains a maximum length of less than 0.5 inches. The largest of the Gobies get about 20 inches in length; although, the majority do not exceed 5 inches.
Over half of the known Goby species live on or in habitats adjacent to coral reefs, like sand flats, sand slopes, sea grass beds or mangrove swamps. One drawback in living in open sand or mud bottoms is that they have fewer places to hide when compared to the labyrinth-like structure created by a coral reef. As a result of the lack of available hiding places, Gobies that live in these habitats have to rely on different strategies to avoid being eaten. Some are attired in cryptic colors so that they blend in with their surroundings. Others dig holes under scattered rubble or debris. But there is also a group of Gobies that form partnerships and refuge with unrelated marine organisms (a phenomenon known as interphyletic symbiosis).
Snapping Shrimps
Shrimps also reach their apex of diversity on coral reefs. One group of Shrimps, most of whose members are part of the reef’s cryptofauna (“hidden fauna”), are the Snapping or Pistol Shrimps (Family Alpheidae). These crustaceans get their name from their greatly enlarged claws. When these oversized appendages are snapped shut, it creates an audible popping sound. They use their claws to stun their prey, to ward off predators, and to warn potential rivals of their territory boundaries. These Shrimps are occasionally available to reef keepers, who use them to help keep their aquarium substrate stirred. However, most of the Snapping Shrimp that enter aquariums do so incidentally in live rock.
While many Snapping Shrimps live in hard substrate, there are some that construct burrows in the sand or mud. The size and shape of these burrows vary, depending on the species and size of the shrimp, as well as the type of substrate in which the burrow is constructed. For example, the burrows of four Snapping Shrimp species from the Red Sea are usually 32 to 47 inches in length (they can exceed 87 inches) and are situated 12 to 22 inches under the sand surface. The entrances to the burrows are typically from 1 to 2 inches in diameter, with deeper, longer burrows having larger entries. The burrows may have from one to six openings, although most have three or less. They are usually dug under hard objects, like large shells, chunks of coral rubble, or coral mounds.
After the Shrimp build their burrows, they have to maintain them. Most of the repair and maintenance occurs in the mornings. The openings cave in and erode at night when the Shrimp suspends its maintenance activities. This effectively seals the burrow inhabitants in for the night. In the morning, the Shrimp clears new openings and completely closes old ones. They push loose sediment away from the burrow and grasp pieces of rubble and shell with their claws and carefully place them around the burrow entrance. In the afternoon, the Shrimp turns its attention from burrow to metabolic maintenance, spending more of its time feeding on detritus among the bottom sediments.
While life may appear rosy for these burrow-constructing Snapping Shrimp, they have one big predicament. Because these Shrimps have poor eye-sight, they are very vulnerable to being “picked off” by predators as they take care of their burrows or as they forage around the burrow openings
Shrimp Meet Goby – Goby Meet Shrimp
So how do the Shrimp overcome their apparent visual handicap? Enter the Shrimpgobies. As we discussed above in our general Goby survey, there are a number of Gobies that live in sand or mud habitats that are looking for a way to avoid predators. It turns out there are some Gobies that avoid predators by teaming up with the Snapping Shrimp. The members of the following genera are known to live within the burrows of these poor-sighted crustaceans: Amblyeleotris, Cryptocentrus, Cryptocentoides, Ctenogobiops, Flabellogobius, Gobionellus, Lotilia, Mahidolia, Myersina, Nes, Psilogobius, Smilogobius, Stonogobiops and Vanderhorstia. These fishes are referred to collectively as Shrimp, Partner, Prawn, or Watchman Gobies.
The Shrimpgoby-Snapping Shrimp relationship is a mutualistic one – that is, it’s an association where both members benefit. The primary benefit to the Goby is obvious. The Shrimp’s burrow provides a sanctuary for the otherwise vulnerable fish. But the Goby benefits in more ways as well. When the Snapping Shrimp engages in maintenance activities, the Goby gains access to buried prey organisms. I have seen these Gobies ingest worms exposed by the Shrimp’s bull-dozing activities. I have also noticed that the Gobies take mouthfuls of newly turned sand and sift it through the gills or sort it in their mouth and then spit out the inedible materials. This newly exposed sand may have more infaunal prey items to feed on than the “old” sand.
Underwater observations have also shown that the Gobies are occasionally "cleaned" by their crustacean landlords. In captivity, Snapping Shrimp have been observed using the second pair of chelae (claws) to pick at the Goby's head, dorsal fin and tail. Snapping Shrimps have been observed picking at Gobies in the wild as well. Finally, the Shrimp’s burrow serves as a repository for the Gobies spawn. During the breeding season the female Goby deposits her fish eggs in the burrow.
Shrimp Benefits
So how does the Goby benefit its industrious partner? The Shrimpgobies act as "seeing-eye" fish for their poor-sighted crustacean partners. As the Shrimp keeps house or feeds just outside of the burrow, the Goby will position itself near the burrow’s entrance and “stand guard.” In some cases, when the Goby is reluctant to leave the burrow, the shrimp will push the Goby out with its claws or, on rare occasions, it will even pinch the Goby. The tidy little crustacean moves freely in and out of its refuge, but when it leaves the burrow it keeps in contact with the vigilant Goby. It does this by placing one of its antennae on the fish. (This antennal contact is the critical line of communication between the two animals.)
When a predatory fish approaches, the Goby will rapidly flick its tail, warning the Shrimp of impending danger. If the Goby flicks its tail once, the Shrimp may not immediately respond, but if the Goby executes a series of flicks the Shrimp will quickly back into the burrow. If the predator comes within a critical distance, the Goby will also dart (headfirst) into its hiding place. Gobies are able to discriminate between fishes that are potentially dangerous and those that are not. This is advantageous, as it would be a waste of time and energy if the Goby warned the Shrimp or retreated every time a fish moved near the burrow. The Goby is most likely to communicate (warn) the Shrimp if an intruding species is large. Smaller fish are less likely to elicit a warning from the Goby. Fishes that eat small fishes and shrimps also will elicit a warning signal.
How do the Goby and Shrimp find each other in the first place? Apparently, Gobies visually locate a Shrimp associate, while the crustaceans use chemical cues to recognize their fish partners. It’s interesting to note that certain Gobies are quite particular about the company they keep. They only occur with one or few shrimp species. For example, in the Indian Ocean, Wheeler’s Shrimpgoby (Amblyeleotris wheeleri) is only found with the Snapping Shrimp (Alpheus ochrostriatus). But, there are other Gobies that are less selective. For example, over certain parts of its range, Steinitz's Shrimpgoby (Amblyeleotris steinitzi) can be found residing with several different Alpheus spp.
Captive Care
The Shrimpgobies (family Gobiidae) are easy to keep. Their relatively small sizes and less active lifestyles mean you can keep them in aquarium as small as 30 gallons. In fact, the smaller species will be happy as a “pig in mud” in an aquarium as small as 10 gallons. One prerequisite for the Shrimpgoby aquarium is a cover of some kind. While glass covers were once popular, most hobbyists now try and stay away from them because they inhibit gas exchange at the water’s surface. Covers constructed out of egg crate material, fiberglass screen and PVC are used by many reef aquarists. A cover like this will keep the Gobies in and let the carbon dioxide out.
Be aware that these fish are able to find and jump through relatively small openings in the aquarium top. They are most likely to perform these suicidal aerial displays when they are initially introduced to the aquarium, if they are pestered by other aquarium inhabitants, or when the aquarium lights are suddenly turned off. A small night light mounted over the aquarium may prevent them from trying to bail when the lights are extinguished. These Gobies have been known to leap into overflow boxes as well.
The best way to set up your Goby-Shrimp aquarium is with an external filter and live substrate. An outside filter can be used to provide mechanical and some chemical filtration, while a layer of live sand and some live rock can be employed for biological filtration. Your Snapping Shrimp will help keep the sand stirred and detritus into suspension. In fact, I had a setup with thick aragonite substrate and two large Snapping Shrimp that kept sediment suspended in the water column almost constantly. The aqaurium had been set up for a long time and had accumulated a significant amount of sediment and detritus, which the Shrimp were effective at disturbing. Some aquarists might find turbid aquarium conditions like this annoying. But as time past, and more of the sediment was removed by mechanical filters, the water became clearer.
Snapping Shrimp and Shrimpgobies will do well in reef aquariums with a thick sand substrate. Keep in mind, it is possible for rock work to become unstable and collapse in a reef tank, or any aquarium for that matter with décor, when the Shrimp excavates underneath it. Therefore, make sure you place the rock work on the bottom panel of aquarium glass and place the sand substrate around the rock, rather than placing the sand on the aquarium bottom and putting the rock on the sand.
When it comes to selecting substrate, it is important to include a finer grade of sand. Studies have shown that Snapping Shrimp will be more reclusive, spending more time in their burrows if the burrow construction does not occasionally fall in and elicit maintenance behavior. Avoid exclusive use of oyster shell, coral rubble or other larger grades of substrate as burrow construction will be too stable. However, be sure to mix some pieces of coral rubble, small pieces of live rock and/or medium-sized shells with the sand so that burrow construction isn’t impossible. (To get a better idea of what types of material you should place on the aquarium bottom to facilitate burrow construction pay special attention to the photos accompanying this article.) A few flat pieces of live rock laying on the bottom will also provide a roof for the shrimp's burrow. I would place a layer of substrate 3 to 5 inches deep on the aquarium bottom.
If you want to enhance the color of your Shrimpgobies, try using black volcanic sand. On the darker substrate, the colors of many of the Shrimpgobies (and some other gobies as well) will be intensified. This phenomenon can also be observed in nature. For example, if you dive along the shores of Bali, Indonesia, where the sand is black, the Gobies are radiant!
Some species of the Snapping Shrimp and Shrimpgobies are particular about who they hang around with. The degree to which they maintain this species specificity in the aquarium has not been fully investigated. But I have seen species of Shrimpgobies and Snapping Shrimp associate together in the aquarium that usually do not do so in the wild. It would seem logical that if you are trying to form a Goby-Shrimp relationship in your aquarium you will have a greater chance of success with a Goby species with a more eclectic taste in Shrimp (i.e., a Goby that associates with a larger number of shrimp species in the wild). Some marine fish wholesalers actually sell Snapping Shrimp-Goby pairs. By buying a pair that was collected together, you can be almost guaranteed that you’re getting two animals that will associate together in your aquarium.
Another important thing to remember is that not all Snapping Shrimp species live with Gobies. For example, most species from the Atlantic and Caribbean are unsuitable as Goby partners. The exception is the Florida Snapping Shrimp (Alpheus floridianus), which associates with the Atlantic Orangespotted Shrimpgoby (Nes longus) and the Spotfin Goby (Gobionellus stigmalophius). This Shrimp is gray overall with some faint bands on the claws and several thin brown lines on the abdomen.
Fortunately for the aquarist, Shrimpgobies are fairly disease resistant. In fact, they are more likely to succumb to tankmate bullies than parasites. Some of the smaller Shrimpgobies may be abused by aggressive tankmates, like Dottybacks, Pygmy Angelfishes, Damselfishes, Surgeonfishes and Hawkfishes. When kept with more pugnacious fishes, these Gobies tend to hide and may starve to death.
Care must be taken when placing Shrimpgobies together, especially conspecifics (i.e., members of the same species). It is best to place them in the aquarium simultaneously and to purchase known pairs if buying individuals of the same species. In the wild, the distance between individuals varies from one species to the next. For example, in one study Wheeler's Shrimpgobies were usually found about 5 feet apart, while Steinitz's Shrimpgobies (Amblyeleotris steinitzi) were typically only 2 feet apart. In another study, it was reported that the burrows of A. steinitzi were a minimum of 14 inches apart. Not surprisingly, larger gobies prefer burrows with larger openings. This information can be used in the aquarist's favor if he or she wants to keep more than one individual in a system. Just construct different sized artificial burrows and purchase Shrimpgobies of varying sizes. The smaller Goby (or Gobies) will stick to the smaller burrows and the larger Goby (or Gobies) will occupy the larger ones. If you choose to employ this tactic, add the small Shrimpgobies to the aquarium before a larger individual(s).
Aggressive encounters between Shrimpgobies of the same species regularly occur in the wild and in the aquarium. In the wild, some species will stray up to 10 feet from their "home" burrow, which will cause neighboring conspecifics to display and try and drive them from their territories. Several different aggressive behaviors have been observed in Shrimpgobies. This includes chasing, lateral displays and circling. Chasing is when one fish swims after another to evict it from its territory. The lateral display is an impressive behavior and is used to intimidate an intruder and scare it from the area. The displaying individual will move towards its opponent with its fins raised, it mouth and gill covers spread open and its body undulating. The intruding Goby, especially if it is larger, may respond with a similar display – this may lead to circling behavior. This is when the pair circles, in head to tail orientation, each trying to bite the other's tail region. While Goby fights in the aquarium often result only in torn fins, subordinate individuals may end up hiding, not feeding or succumbing to disease.
Both the Gobies and their Shrimp associates will feed on a wide variety of fresh, frozen and flake foods. One of the best ways of meeting the Goby's long-term nutritional needs is to vary the diet. If you have a finicky Goby (these are rare), try dropping a few black worms near its hiding place. They usually cannot resist this wiggling delicacy. In the wild, these Gobies feed mainly on small crustaceans that live on the substrate near the burrow entrance and planktonic crustaceans that drift past. Unlike some of other family members (e.g., Signigobius biocellatus, Valenciennea spp.), the Shrimpgobies are not effective substrate sifters. While they will occasionally ingest sediment to feed on infaunal organisms, they do not engage in aggressive sifting like certain other Goby species.
If you are as enthusiastic about this family of fishes as I am, you may want to set up a system only for Gobies. Because most Gobies do not occupy the upper parts of the aquarium, a low aquarium with more horizontal surface area for living space is preferable to a taller one. 20 Long and 30 gallon aquariums are ideal Goby aquariums. I would recommend stocking a 30 gallon aquarium with two Shrimpgobies of the genus Amblyeleotris or a pair of Cryptocentrus spp. and several Dartfish, Ptereleotris spp. (which are placed in the family Microdesmidae by most ichthyologists).
Creating Burrows for Shrimpgobies
If you cannot provide adequate building materials for your Shrimp and Goby, or if you want to habituate them to a particular part of the aquarium, you could always construct an artificial burrow. I use PVC pipe, elbows, and caps to construct these synthetic domiciles. The pipe can be cut at varying lengths, depending on the size of the aquarium and the Goby it is meant to accommodate, but a 6 to 8 inch pipe is adequate for most species. A number of small holes (1/4 inch in diameter or larger depending on the substrate used) should be drilled in the pipe to allow fresh water to circulate through the burrow and some sand to flow in. The elbow can either be angled at 45° or 90°, but I recommend the former because it allows more debris to collect in the pipe, which will keep the Shrimp busier and permit better viewing of the pair. The other end of the tube should be capped off with the appropriate PVC fitting, but I would not glue any of the fittings in place so that you can more easily remove a Goby if necessary or clean the tube. The tube can be buried under gravel, rocks, or rubble and small pieces of coral rubble can be placed around the open end to give the burrow a more natural appearance. If you want to create an even more elaborate burrow try placing an elbow on each end, so there are two entrances, or purchase a three- or four-way PVC connector, more straight lengths of PVC and PVC elbows and construct a complex burrow system with multiple chambers and openings.
In an aquarium with thick substrate, an artificial burrow made of clear plastic tubing can be placed near the front glass so that you can view the subterranean activity of its residents. Take a piece of large diameter flexible tubing and cut it in half along the axis. Drill holes in it and cap one end. Then place the sliced side up against the glass, either the front or side, and angle it so the capped end reaches the aquarium bottom and the opening is just above the substrate. I recommend placing a piece of black acrylic in front of the burrow, which should be removed only when the room lights are extinguished; this will reduce the amount of disturbance due to activity around the aquarium, which could cause the pair to vacate their new home. If you have trouble viewing the activity of the burrow residents due to lack of light, mount a red incandescent bulb in front of the aquarium to illuminate the chamber.
The best way to introduce the pair to an artificial burrow is to put the Shrimp in a soft net or a jar and place it over the entrance. After the Shrimp enters the hole repeat this procedure with the Goby.
By the way, these artificial burrows work great for Jawfishes, Firefishes, Dart Gobies, Wrasses, Dottybacks and many other fish that refuge in burrows or small holes in the wild. And for you retailers that have substrate in your aquariums, they can make catching fish that frequently use them a snap! Just place your hand over the burrow opening, lift it to the surface and gently pour the resident fish in a bag of water.
Scott Michael
Scott W. Michael is an internationally-recognized writer, underwater photographer, and marine biology researcher specializing in reef fishes, and was the Banquet Speaker at our 2007 and 2008 Coral Conference and Frag Swap. He is a regular contributor to Aquarium Fish Magazine, Freshwater and Marine Aquarium Magazine, SeaScope, and is the author of Reef Fishes Vol 1, Vol 2, and Vol 3, Vol 4, and Vol 5., A Pocket Expert Guide Marine Fishes, A Pocket Expert Guide to Reef Aquarium Fishes, 101 Best Saltwater Fishes: How to Choose and Keep Hardy, Brilliant, Fascinating Species That Will Thrive in Your Home Aquarium, Reef Sharks & Rays of the World, and Aquarium Sharks & Rays. Having studied marine biology at the University of Nebraska, Scott has served as a scientific consultant for National Geographic Explorer, the Discovery Channel, and French educational television.