10.8: Marine Species and the Rhythm of Tides

Tides play an important role in the life cycles of many marine species. The most obvious connection is with species that live in the intertidal zone, for which the tides expose them and their habitats to the atmosphere for variable periods during the day. These species have developed strategies to avoid dehydration and/or being eaten by birds and other predators during periods when their habitat is exposed to the air. For example, most species that live in sandy or muddy intertidal environments bury themselves in the sand when the tide is out. Many species that live on rocky shores have shells to retreat into or leathery outer skins that, in addition to deterring predators, resist loss of water through evaporation

Although we are only just beginning to learn of the scope of the many subtle influences of tidal motions on marine species, numerous well-documented connections exist between the tides and the reproductive behavior of many species, especially regarding the timing of the reproductive cycle. For example, many species that spawn eggs and larvae into the water column exhibit spawning behavior timed to benefit the species' survival. One example is that many coral species spawn en masse at a specific phase of the lunar tide, most often 1 or 2 days after a full moon in the spring months, when the tidal range is at its greatest. Spawning at this time ensures the greatest possible dispersal of the eggs and larvae to potential new colonization sites. Spawning strategies are discussed in more detail in Chapter 14.

There is one connection between tides and species’ reproductive strategies that you may be able to see for yourself. This is the use of high spring tides by some species at a certain time of year to place fertilized eggs in the sand high up on a beach where they cannot be reached by most water-dwelling predators. Perhaps the best-known examples are sea turtles that haul themselves high up the beach, usually at night when there is a high spring tide, dig a hole in the sand, lay and bury their eggs, and then leave the eggs to hatch some weeks later. Many other species use a very similar strategy. Some of these species can be seen on beaches or mudflats around North America. For example, on the Atlantic coast, horseshoe crabs (Limulus polyphemus) spawn on the beaches between the high- and low-tide lines on nights when the spring high tide occurs. Female horseshoe crabs crawl out of the water as the tide advances, dig a cavity about 15 cm deep in the sand, and deposit several thousand large, greenish eggs. The male, which is attached to the female’s back, fertilizes the eggs, and the eggs are then buried by sand moved by the advancing tide. The eggs hatch 14 days later when the next high spring tide covers the sand.

On the Pacific coast, several fishes use a similar strategy. The most famous of these fish species is the grunion (Leuresthes tenuis). This small, silvery fish comes completely out of the water onto the southern California and Baja beaches to spawn just after each maximum nighttime spring tide during March through September. The eggs are protected from wave action for the next 14 days because they are laid above the lower high-tide lines that occur at high tide between one spring tide and the next. Although they mature in about 9 days, the eggs do not hatch until disturbed by water and wave turbulence, which occur as the next spring high tide arrives 14 days after they were laid. The eggs hatch within minutes of being exposed to the water.

Although the grunion is the most famous fish that spawns on the beach at high tide, other species also use this strategy, but they do not come completely out of the water to do so. One example of such a species is the surf smelt (Hypomesus pretiosus), a small fish that spawns at high tide on the beaches of Puget Sound.