While it is possibly one of the best sand-sifters and also one of the most interesting gobies to observe, in most cases, I would recommend that the twinspot or signal goby (Signigobius biocellatus) be left in their natural habitat. It is hard to confuse with any other goby. It has large, black pelvic fins with tiny blue spots and a large black eye-spot (ocelli) on each of the dorsal fins. The twinspot goby occupies burrows, which they excavate. They use their mouth to carry away sand and shell fragments and will also vigorously beat their tail to waft sediment from the developing burrow. The burrow is used for shelter and reproduction. The adults of this species are usually observed in pairs. When they feed, this fish will take a mouthful of sand, with its large, scoop-like mouth and will then sift the sand through their gill-rakers. It filters out infaunal invertebrates, like worms and tiny crustaceans.
The twinspot goby spends much of its time hovering over the sea floor. When it is agitated, it will swim forward and backward in an erratic manner. The fins are held erect, exposing the large eye-spots. The lateral view of this goby, with the large ocelli, apparently mimics the “face” of a piscivorous fish and may discourage a predator from attacking. Some have suggested that the movement and appearance of this fish may serve to mimic a crab (in fact, some choose to call it the crab-eyed goby). However, this makes little sense seeing that crabs are a preferred food of many carnivorous fishes. Why would you try to resemble something that is commonly preyed upon?
Unfortunately, the twinspot goby is difficult to keep in the home aquarium. In many cases, captive individuals slowly starve to death. If you want to keep this fish, it is imperative to have a layer of live sand on the aquarium bottom (a sand bed that is at least 2 inches in depth is recommended). You should also consider adding new infaunal animals (e.g., worms, copepods, amphipods) to the tank on occasion to augment their numbers in the aquarium substrate. Unfortunately, it does not take long for this vigorous sifter to decimate micro-invertebrate populations in the sand. (I have seen these fish take as many as 30 mouthfuls of sand a minute when feeding.) You may be better able to supply a natural food source if you connect a refugium to the S. biocellatus tank. Although no studies have been published on their alimentary tract parasites, de-worming the twinspot goby may also facilitate the keeping of this fish.
Live brine shrimp and black worms can be used to induce feeding in finicky individuals, but vary the diet as much as possible once they are eating. This goby will typically not take the food from the water column. Instead, it will ingest the food when it lands on the aquarium bottom by taking a mouthful of substrate. (If food never gets to the bottom of the aquarium because of more aggressive feeders, than your S. biocellatus is likely to become malnourished.) Of course, the feeding activity of this fish can help keep the sand surface clean of detritus and algae. An adult S. biocellatus will disturb the upper 1 inch or so of sand when feeding. When constructing a burrow, it will dig even deeper into the sand bed.
The twinspot goby is usually sold in pairs and are thought to do better in captivity if they are kept together. However, a pair will require twice as much food! Pair members will often engage in tactile contact, where one individual will mouth the other. It appears as though one member of the pair engages in more mouthing than the other (I have yet to figure out if it’s the male or female doing more of the mouthing). This behavior probably helps maintain the pair-bond. If you want to keep a solitary S. biocellatus, your chances of success may increase if you acquire a juvenile.