Endogenous tidal rhythms are known for many marine organisms, including isopods Excirolana chiltoni on sand beaches in southern California.  As these rhythms may persist for several cycles in a constant laboratory environment, the question arises as to the mechanism of entrainment.  Possible cueing factors are drying, water movement, sediment impact, or  even perhaps the moon’s gravitational force.  This  is investigated at the Scripps Institution of Oceanography, La Jolla, California using wave-simulating devices in sand-containing glass jars.  Counts of swimming isopods show that cyclic swimming behaviour in the EXPERIMENTAL “wave simulators” remains entrained for several days, even if the sand is removed (a possible source of mechanical stimulation). Note, however, that the cyclic swimming behaviour of the CONTROL animals (also in jars but without “wave simulators”) degenerates after about a week in the absence of water movement.  Other data show that artificially imposed light cycles or variation in oxygen levels do not disrupt these rhythms (data not shown here).  The author concludes that mechanical stimulation by water agitation is likely the dominant component in maintaining activity rhythms in E. chiltoni. Enright 1965 Science 147 (3660): 864; for review of the ecological significance of endogenous behavioral patterns in animals in general see Enright 1970 Ann Rev Ecol Syst 1: 221. Photograph courtesy Peter J. Bryant, University of California, Irvine.

NOTE  horizontal blue bars in the graph indicate times of swimming activity based on an expected resynchronisation of behaviour to the wave-simulator treatment (for experimental animals) or to natural tide cycles (for the control animals0

In a later study at the Scripps Institution of Oceanography, the same author is able to maintain isopods Excirolana chiltoni for periods of up to 2mo in constant, non-tidal conditions, and again observes persistent tidal rhythms in swimming activity.  In this case, however, the activity bursts are closely correlated with the tide cycle, but with a period of about 24h 55min, or about 5min longer than the average period of the tides.  This means that loss of synchrony with the tides is gradual, amounting to about 2d over a 2mo period.  Interestingly, superimposed on this cycle is another endogenous monthly pattern that leads to maximum activity on the days of extreme high tides.  These new results force the author to abandon the previous notion of entrainment by wave vibration, for one of combined circadian/lunar involvement.  Enright 1972 J Comp Physiol 77: 141.

NOTE  while the earth rotates once on its axis every 24h, the moon travels 1/28th of its orbital circumference around the earth; thus, the earth has to “catch up” this difference every day in order to position once again any given point directly opposite the moon.  One-28th of 24h is 51min

Another laboratory study on isopods Excirolana chitoni collected from beaches around the Scripps Institution of Oceanography confirms that  locomotory movements synchronise with natural tidal rhythms.  Tidal rhythms in the La Jolla area alternate between a single high and a single low per day (diurnal pattern), and 2 highs and 2 lows per day (semidiurnal pattern), with transition between them being gradual. 

In their beach habitat the isopods lie buried until times of high tide when they emerge to swim and feed for periods of 2-3h.  In the laboratory, swimming activity is on a 24.8h rhythm that approximates a diurnal tidal rhythm. Under semidiurnal conditions with 2 high tides per day the isopods emerge and swim on the higher of the 2 high tides. As the tides switch to diurnal conditions, the isopods appear to swim on both high tides during the day (see graph upper Left).

During diurnal patterns, the isopods continue with this pattern, with swimming activity occurings on every high tide (see graph lower Right). This natural rhythm can be disrupted or shifted at any time by  agitating the  aquarium-tank water to simulate wave action on a beach.  The isopods begin to swim and this subsequently re-jigs the natural rhythm.  One such stimulus per day initiates a diurnal activity, while 2 such stimuli per day evoke semidiurnal activity.  The changes persist for several days after cessation of treatment. The author conducts these experiments under conditions of constant light, so effects of natural alternating light-dark cycles on Excirolana’s activity rhythm are not known.  Klapow 1972 J Comp Physiol 79: 233.

Many bottom-dwelling or demersal invertebrates periodically emerge and swim freely in the water column.  This happens mostly at night, and the question arises as to whether swimming behaviour is reduced during full- or quarter-moon periods.  Researchers from University of California Santa Barbara investigate this by quantitatively sampling zooplankters as they emerge from sand-flats in the Gulf of California, by use of cone-shaped traps placed on or near the sand surface.  Collections are made at 2h intervals over 24h periods during various phases of the moon, sampling invertebrates as they emerge from and return to the sea bottom.  Results are generally varied, with some types showing no evident behavioral response to phase or timing of the moon (e.g., polychaetes, copepods, mysids, shrimps), and others (e.g., amphipods and isopods) significantly avoiding moonlight.  These last tend to emerge only after moonset, and then return to the benthic habitat at moonrise (see sample graph).   Better results are obtained for isopods when half of 12 traps are experimentally blackened with plastic during 10 daylight hours, while the other half are left exposed; 230 emerge in the blackened traps versus only 3 in the lighted ones.  The authors consider the daily vertical migrations to be most likely a means of dispersing to new habitats.  Alldredge & King 1980 J Exp Mar Biol Ecol 44: 133.

NOTE  these include many species of copepods, mysids, shrimps, amphipods, polychaetes, cumaceans, and isopods, the last of interest in this section of the ODYSSEY

NOTE  isopods sampled include Exosphaeroma sp. and Paranthura elegans

Initial darkness elicits much swimming activity, both
emerging from the sea bottom and returning. These
carry on during the night at a moderate level until
moonrise when both become significantly reduced