As early as 1903, Gamble and Keeble described a tidal rhythm in a flatworm on the coast of Brittany (See review by 35). The worms migrate to the surface during daytime low tides, but burrow in the sand during high tide and at night. Photosynthetic algae live within the tissues of these worms and it is to the worms' advantage to get as much sunshine as possible. They burrow during high tide to prevent dislodgement. This rhythm persists in the lab when the worms are kept in constant light, but not in constant dark.
Visitors to the beach are familiar with the low tide scurrying of
fiddler crabs as they feed, court, and threaten one another. As high tide
approaches, they return to their burrows. Do these crabs simply burrow as
they notice the water coming, or do they have an endogenous rhythm
synchronized with the tides? In a study of fiddler crabs Barnwell found
that crabs showed tidal locomotor rhythms lasting almost a month in
constant light in the laboratory (See review by 35).
The graph shows the activity of a male fiddler crab in a natural light/dark cycle.
Tidal activity rhythms have also been observed in other marine invertebrates such as isopods, pink shrimp, midges, snails, and mollusks. Tidal rhythms have also been observed in fish. The shanny is a fish which lives under stones in pools during low tide and feeds during high tide. In the laboratory, the tidal rhythm persists in either constant darkness or light. In 1958, Brown observed that Australian reef herons flew from rookeries out to sea to feed on animals exposed during low tide. Because the low tide could not be seen from the nesting area, he suspected that the birds possessed a lunar clock .