Some biological patterns in marine species, particularly concerning reproduction, are related to the moon. Shallow-ocean corals, for example, undergo mass spawning events (the synchronous release of eggs and sperm into the water column to combine), the timing of which, are set to the lunar clock. Reef fishes, shallow-ocean echinoderms, mollusks and more, also time spawning events in respect to the phase of the moon.
The deep-sea, the largest biome on Earth, covering more than 326 million km2, has not been explored in terms of this lunar-synchronicity. The dearth of photosynthetically-useful sunlight below 200 meters* would appear to make such moonlight-related cycles unlikely at best.
However, in a recent paper, Annie Mercier and her colleagues have shown that this may not be the case. They demonstrate in both lab and field settings evidence of lunar periodicity in the reproduction of 6 deep-sea species, containing members from two different phyla: Cnidaria and Echinodermata.
The researchers examined preserved samples of Phormosoma placenta (a deep-sea echinoderm) collected at various stages of the moon. They found that despite being collected from between 700 – 1400 meters beneath the waves, physiological signs of recent spawning in both sexes coincided with the new moon.
Back in the lab, gamete and larval releases (reproduction events) were observed in captive specimens from 5 different species according to lunar patterns. These specimens were collected between 100-1000 meters, with most species collected below 400 meters. A minimum of 3 lunar months’ worth of data was compiled; some species actually repeated breeding periods in this timeframe.
The question remains if these animals are displaying internal rhythms that are kept in time by some sort of lunar cue, or if they are responding to something externally that follows the lunar period. But what cues, or drivers, of a lunar period could be detectable at such great depths, where even sunlight wanes or is essentially eliminated?
Organic matter from surface waters falls into the deep sea; there is the possibility that these fluxes of sustenance may show lunar patterns. Previous work has shown growth bands in some species of deep-sea corals that may correspond to monthly or lunar periods. Other hypotheses include the idea that these animals can somehow directly perceive moonlight at great depths, or that deep tidal (related to lunar phase) currents exist.
In this study, internally brooding corals released larvae during the full or during the waning phase. The 4 free-spawning species released gametes with the new moon. The authors note that while this is opposite to the mass spawning events in shallow-ocean corals, which release during the full-moon, this may be due to the very different environmental and biotic factors in shallow areas versus the deep sea.
* This is the reason that little to no primary production occurs (that is, organisms producing chemical energy) in most, but not all, ecosystems known in the deep-sea. Some deep-sea organisms are capable of undergoing chemosynthesis and can use inorganic chemicals, rather than sunlight as in photosynthesis, as an energy source. However, even with a widespread lack of primary productivity and severe food limitation in most areas, diversity in the deep sea is among the highest on the planet.
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Mercier A, Sun Z, Baillon S, & Hamel JF (2011). Lunar rhythms in the deep sea: evidence from the reproductive periodicity of several marine invertebrates. Journal of biological rhythms, 26 (1), 82-6 PMID: 21252369
Ramirez-Llodra E, et al. (2010). Deep, diverse and definitely different: unique attributes of the world’s largest ecosystem Biogeosciences, 7 (9), 2851-2899 : 10.5194/bgd-7-2361-2010