Look carefully as you cross a patch of sand or rubble between the reefs and you might spot two of the ocean's most interesting roommates. Look for a small fish hovering just above a hole in the seabed. Look closer and you might notice the slender antenna of a shrimp that's keeping close company to the fish. Congratulations, you've found a goby and its close companion, an alpheus shrimp.
These co-dependent bottom dwellers have what biologists call a symbiotic relationship — in other words, they help each other out. You've probably read about a similar arrangement between clownfish and anemones, with the colorful little fish providing housekeeping services in exchange for living within the protective tentacles of the anemone.
The shrimp-and-goby relationship pairs an industrious but near-blind crustacean with a nervous and keen-eyed little fish. Spend a few minutes watching the pair and you'll notice that the shrimp keeps pretty busy clearing rubble from the mouth of their shared underground burrow, while the goby doesn't lift a fin to help. Television fans might equate this to the sitcom The Odd Couple, where neat-freak Felix Unger and slovenly Oscar Madison are unlikely roommates. But the shrimp's frenetic activity isn't a case of OCD. Without the constant cleaning, the tunnel walls of their shared habitat would soon collapse.
Take a closer look and you'll see that the shrimp uses its middle legs to sweep sand and rubble while employing its large front claws to move larger objects out of the way or shore up burrow walls with shell and coral fragments. If you could peer into the shrimp's excavations, you might be surprised at the size of the chamber within. These underground lairs can be quite spacious and are sometimes home to more than one pair of shrimp and goby. The interior walls and roof of a burrow are usually shored up with shells and coral fragments to create a fairly permanent structure. In contrast, the entrance tunnels are given far less attention, and often collapse overnight, sealing the occupants inside to create an added layer of protection. Come morning, the shrimp will dig its way out and create a new entrance.
You might wonder what the shrimp gets in exchange for all its labor. The answer is protection. While the shrimp digs, one of its antennas maintains near-constant contact with the fish. Should danger loom on the horizon, the goby will give a warning tail flick to alert its companion as both then dart back into the safety of their borrow. Once the coast is clear, the goby will emerge cautiously from the borrow to scan the horizon before giving an 'all clear' tail- flick signal to the shrimp.
This bottom-dwelling 'odd couple' will sometimes venture from the security of their burrow in search of a meal. The shrimp maintains antenna contact as the goby leads the way, looking like a pet being led on a leash. Once out on the open sea floor, the goby will nosh on mouthfuls of sand, while air filters through its gills to remove small bits of food. While this is going on, the shrimp is using its claws to root through the seabed, stirring up debris and overturning small rocks in search of worms and larvae.
Sometimes, the goby will make a make a solo excursion to bring food back to the stay-at-home shrimp, which in turn provides grooming services to keep the little fish free of parasites. Some varieties of shrimp are also gardeners who cultivate algae patches close to the borrow mouth.
When you are witnessing a shrimp and goby pair, there's a good chance more will be around. These cohabiting bottom dwellers will often settle into neighborhoods, creating a seafloor that's pockmarked with borrow openings. This proves convenient during mating season, as both members of the partnership are too co-dependent for long-distance romantic relationships.
When a male goby feels the urge to merge it will make brief forays in the neighborhood in search of a mate. This search for love may require the goby to compete with other equally male amours for the attention of a mate, and when push comes to shove, the big fish with an attitude usually wins the girl.
Once a male goby hooks up with a partner, they head back to his place for a few days of cohabitation. After business is finished the female will gift her partner with up to 20,000 eggs before she heads home, leaving him to tend to the brood.
Male alpheus shrimp have it a bit easier in the child-rearing department, as it is the female who goes home with a clutch of several thousand eggs. When her tiny crustacean offspring hatch they will immediately dig into the safety of the seafloor where they will forage for subterranean morsels. Only when reaching adult size will the shrimp emerge from the protection of the sea bed to search for a goby of their own.