Daily, tidal and lunar rhythms
Daily (circadian) rhythms
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A circadian rhythm has a period of about 24 hours, matching the day–night cycle.
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Organisms are classified by when in that cycle they are active:
- Diurnal — active during the day.
- Nocturnal — active during the night.
- Crepuscular — active at dawn and dusk.
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The zeitgeber is light, and the clock is entrained at dawn and dusk because those are the sharpest, most reliable transitions in the cycle.
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Examples in New Zealand species:
- Kiwi, wētā and ruru (morepork) are nocturnal.
- Tuatara are largely nocturnal as adults but bask by day, so their activity is split between thermoregulation and foraging.
- Many forest birds are crepuscular in their dawn chorus, singing in the low light before feeding begins.
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Plants show circadian rhythms too:
- Stomatal opening and closing follows a daily cycle that continues in constant conditions.
- Nyctinastic leaf folding is driven by the same clock.
- Rates of photosynthesis and of enzyme production are raised before dawn, so the plant is ready when light arrives.
Tidal rhythms
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A tidal rhythm has a period of about 12.4 hours — the interval between successive high tides.
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Because the tidal cycle is 12.4 hours rather than 12, high tide occurs about 50 minutes later each day. A rhythm tracking the tide therefore cannot be a circadian rhythm running slow; it is a separate clock.
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The zeitgebers for tidal rhythms are mechanical rather than light-based, because a shore organism is submerged and often buried:
- Hydrostatic pressure changes as water depth changes.
- Wave action and turbulence.
- Temperature and salinity changes on immersion.
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Typical tidal responses on a New Zealand rocky shore:
- Limpets and chitons move to graze algae when submerged, and clamp down onto the rock when the tide falls.
- Bivalves in mudflats open their shells and filter feed when covered, and close when exposed.
- Crabs emerge to forage on the falling tide and shelter as it rises.
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These rhythms are endogenous: shore animals brought into a laboratory tank with constant water level and constant light continue to show peaks of activity at the times of high tide on their home shore, for several days, before the rhythm gradually drifts.
Lunar and semilunar rhythms
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A lunar rhythm has a period of about 29.5 days — one cycle of moon phases.
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A semilunar rhythm has a period of about 14.8 days, matching the interval between spring tides.
- Spring tides — the largest tidal range — occur at full and new moon, when the sun and moon pull in line.
- Neap tides — the smallest range — occur at the quarter moons.
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The commonest use of a lunar rhythm is synchronising reproduction.
- Many marine invertebrates and some fish spawn on a particular tide, so that all individuals release gametes at the same time.
- The zeitgeber is usually moonlight intensity or the tidal cycle itself.
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New Zealand examples:
- Some shore fish spawn at the highest spring tides, placing eggs high in the intertidal where they develop out of reach of aquatic predators and are re-wetted only by the next spring tide, when the larvae hatch and are washed out.
- Several marine invertebrates synchronise gamete release to the lunar cycle, and traditional Māori fishing calendars (maramataka) record exactly these predictable, moon-linked patterns of species behaviour.
Selective advantage of tidal and lunar rhythms
- Tidal rhythms — feeding at the right time.
- A filter feeder can only feed when submerged. Opening at any other time achieves nothing and risks desiccation and predation.
- Anticipating the tide means the animal is open and feeding the moment water arrives, maximising feeding time within a window that lasts only hours.
- Clamping down before exposure prevents water loss and prevents shore birds prising it off — a response made after exposure would already be too late.
- Tidal rhythms — avoiding predators that switch with the tide.
- Intertidal organisms face fish when submerged and birds when exposed. Timing activity to the safest part of the cycle reduces mortality from both.
- Lunar rhythms — reproductive synchrony.
- For a broadcast spawner releasing gametes into water, fertilisation success depends entirely on sperm and eggs meeting while both are viable — a matter of minutes.
- If individuals spawned independently, gametes would be too dilute and most would fail. Synchronising to a shared, predictable cue means all individuals in the area spawn together, raising gamete concentration by orders of magnitude and so raising fertilisation rate.
- Spawning at spring tides additionally carries larvae far offshore, reducing predation by adult filter feeders on the same shore and reducing competition with the parent generation.
- Predator swamping also results: releasing all gametes at once means predators are satiated and a greater proportion survives than if the same total were released gradually.
Worked Example
Worked Example
A shore snail is collected from a rocky beach and kept in a laboratory tank.
- The tank has constant water depth, constant temperature and constant dim light.
- Activity is recorded for 6 days. Peaks of movement occur about every 12.4 hours.
- Each day, the peaks occur about 50 minutes later than the day before.
- On the shore, high tide had also been occurring about 50 minutes later each day.
- After 6 days the peaks become weaker and less regular.
Explain what these results show about the control of this rhythm, and why the rhythm provides a selective advantage.
Answer:
Identifying the rhythm. The period is about 12.4 hours, which matches the interval between successive high tides. This is a tidal rhythm, not a circadian one — a daily rhythm would have a period of about 24 hours.
The rhythm is endogenous. In the tank, water depth, temperature and light were all held constant, so there was no tidal cue of any kind available. Yet activity peaks continued for six days. A rhythm that persists when every external cue has been removed must be generated by an internal biological clock.
The 50-minute daily shift confirms the clock is tidal rather than circadian. The peaks drifted 50 minutes later each day — exactly the amount by which high tide is later each day on the shore. So the internal clock has a period matched to the tidal cycle, not the solar day. Note that this is not a circadian clock running slow: a circadian clock drifting 50 minutes a day would drift relative to the tide as well, whereas this rhythm stayed in step with what the tide was doing.
The weakening after 6 days shows entrainment is normally required. With no zeitgeber available, the internal clock gradually loses accuracy and individuals fall out of step with one another, so the population's combined rhythm becomes weaker and less regular. On the shore this would not happen, because cues such as hydrostatic pressure changes and wave action re-entrain the clock on every tide. This shows the natural rhythm depends on both an endogenous clock and exogenous entrainment.
Why the rhythm provides a selective advantage.
The snail grazes algae on rock surfaces, which it can only do while submerged — out of water it must clamp down to avoid drying out.
- Maximising feeding time. The submerged window lasts only a few hours. Anticipating the tide means the snail is already moving and grazing as the water arrives, instead of losing part of a short window detecting the change and becoming active. More time grazing means more energy assimilated.
- Avoiding desiccation. The snail has a large surface area in contact with air when active. Clamping down before the water retreats prevents evaporative water loss, which for a marine invertebrate risks lethal changes in body fluid concentration. A response made only once exposure was detected would come too late — the damage occurs during the delay.
- Avoiding predators on both halves of the cycle. Predatory fish hunt on the shore when it is submerged and oystercatchers and gulls feed when it is exposed. Timing activity precisely lets the snail feed during the part of the cycle when it is least vulnerable, and be sealed to the rock when birds arrive.
The energy gained and the water and tissue conserved mean the snail can grow faster and allocate more resources to gamete production, and it is more likely to survive to reproduce. Individuals with an accurate, well-entrained tidal clock therefore leave more offspring, so the alleles for it increase in frequency.