One of the harshest environments anywhere is the intertidal zone. Practically nowhere else on earth is so regularly exposed to such a variety of extreme conditions within such a short period of time. Just think of how it must be for tiny animals to be comfortably basking in cool, clean sea water, then just a few hours later, be stuck practically high and dry in a small tidepool. If it is a bright, sunny day, the water will begin to evaporate, raising both the temperature and salinity, but dropping the amount of available oxygen. Yet if there happens to be a downpour of rain, marine animals can unexpectedly find themselves in a nigh-on freshwater environment.
Along the rocky shore, many creatures can survive, thanks to extensive blanketing of sea grasses and seaweeds. But in many sections of estuaries, that type of protective covering is few and far between. And without the crashing waves, the already somewhat stagnant water can get even more oxygen depleted. So the inhabitants of this area often can only make it by burying themselves below the surface of the soft mud. And for those that cannot burrow, it can come down to seeking any shelter they can find.
However, these harsh conditions do not mean that only a few animals struggle to live in this ecosystem. Au contraire, there are LOTS of inhabitants and often in staggering numbers!
Little arrow gobies (Clevelandia ios) are among these. They are only about two-inches in length when fully-grown, slender and mottled brown in color. It is usually hard, though, to get a good look at them, because they typically dart about so much when the tide is in. Moreover, when the tide ebbs, the hundreds of arrow gobies that may find themselves stranded in a single tidepool are not inclined to remain still either. As literal arrows are often kept safe within a quiver, arrow gobies will frantically seek out whatever quiver-like burrows they can, be they those of fat innkeeper worms, ghost shrimps or other animals. If these havens are unavailable, the fishes may just have to bury themselves under the soft substrate.
And these small fishes do have reason to panic when the tide goes out. The retreating water is the signal for shorebirds to come and see what sort of easy pickings they can find, and stranded arrow gobies (gulp) do make for a convenient snack indeed.
Arrow gobies range from British Columbia to Baja California and are the most numerous of Central California’s estuarine gobies. However, their abundance is especially noteworthy during the early stages of their lives. Just one female arrow goby is figured to be capable of laying up to one-thousand eggs at one time. These are laid in the burrows of other animals, then left without any parental care. When these hatch, the larvae are pelagic and can tolerate quite a wide variety in salinity. Some will even swim upstream into fairly fresh water.
As arrow gobies grow, they feed on a whole assortment of invertebrates, including copepods, ostracods and nematodes. It only takes the small fishes about a year to reach full maturity and their entire life span is probably only about three years. Their tremendous numbers do diminish, though, as they get older, since during the each stage of their life, they serve as important food sources to predatory animals.
Divers and fishermen, however, tend to disregard arrow gobies because of their small size. But when they do, they are really missing the point. What these fishes lack in size, the make up for in numbers, making arrow gobies vital links in many marine food webs. And they do all of this in one of the harshest environments on earth!