The gentle glow of moonlight on water has moved musicians, poets and painters — and, it turns out, molluscs. Researchers have discovered the opening and shutting of oysters’ shells appears to be tied to the lunar cycle.
Biological clocks have intrigued scientists for centuries, and researchers in the field won the 2017 Nobel prize for studies into the 24-hour body clock. However, organisms do not necessarily have biological processes linked only to the rhythm of day and night, the circadian clock. Other patterns that include links to the tides have been found for species including the horseshoe crab, and to the phases of the moon for creatures including the bristle worm. Some have suggested the latter may also affect humans.
Now experts say they have found evidence that oysters not only have a circadian clock and a tidal clock, but are also attuned to lunar rhythms. “It was a surprise to see that there is such an effect of the moonlight,” said Laura Payton, co-author of the research from the University of Oldenburg. Writing in the Biology Letters journal, Payton and her colleague Damien Tran from the University of Bordeaux describe how they tracked the behavior of 12 Pacific oysters submerged off the French coast over three-and-a-half lunar cycles from the end of 2014.
Photo: AFP
照片：法新社
The team used electrodes to track the molluscs’ degree of opening every 1.6 seconds and looked at astronomical data to assess how much of the moon was illuminated. The results reveal the oysters were most open in the buildup to — and presence of — a new moon, and less open as the moon entered first quarter and full phases. The team says that suggests oysters can sense moonlight — even though it is far less intense than the sun’s rays.
However, Payton said the situation was complex, noting the creatures appeared to be able to tell if the moon was waxing or waning: the oysters were generally more open during the third quarter than the first quarter. Payton said one possibility was that the benthic bivalves may have evolved an internal lunar clock, rather than passively relying on direct cues. In that case, she added, the moonlight sensed by the oyster would help keep this clock in sync with the environment rather than directly triggering the opening and shutting of the shell — similar to how humans use daylight to keep their internal 24-hour clock on track.
The team suggests the increased opening of the oysters when moonlight levels are lower might be linked to the possibility that more food is available at low light levels: previous studies have suggested the movement of plankton also appears to be influenced by light. “We know that oysters open their valves when there is food,” Payton said.
However, the study did not look at the impact of the moon on oysters’ behavior in all seasons, or take into account cloud cover — and hence the actual level of moonlight the molluscs experienced. David Wilcockson, a senior lecturer in aquatic biology at Aberystwyth University, said there were still many mysteries in the field. “We know that, for example, tidal, lunar and circadian clocks appear to have separate mechanisms, but they are to some extent linked — and we don’t know quite how and to what level,” he said.
Wilcockson said human activity could cause unexpected problems in marine environments — an issue research like the latest study could help examine. “If you have coastal lighting, for example, or lighting on marine structures, then of course we don’t really know what the impacts of those might be,” he said.
(The Guardian)
自古以來，月光倒映在水面的溫潤光澤深深觸動音樂家、詩人和畫家──以及水下的軟體動物。學者日前發現，牡蠣殼的開闔似乎和月週期有關。
好幾世紀以來，生物時鐘強烈吸引著科學家興趣，該領域學者最近才以二十四小時生理時鐘研究獲頒二○一七年的諾貝爾獎。不過，生物體內並非只有跟日夜轉換節奏連動的生物程序，或稱日變時鐘。科學家已發現其他模式存在，例如潮汐和包括鱟等物種的行為相關，或是月相與海中多毛綱目等生物作息有關。有些科學家則認為，月相也會影響人類。
最近更有專家發現，證據顯示牡蠣不只具有日變時鐘和潮汐時鐘，還會調適於月周律動。該研究來自德國奧爾登堡大學，共同作者蘿拉‧培頓表示：「看到月光竟有這樣的效果，實在讓人訝異。」在這篇刊登於《生物學報》期刊的研究中，培頓和波爾多大學的同儕戴米恩‧特蘭，描述他們如何從二○一四年底開始，在三個半的月亮週期間，追蹤十二隻浸在法國近海的太平洋牡蠣行為模式。
該團隊運用電極，每隔一點六秒追蹤這類軟體動物打開殼的角度，並參考天文資料，評估月球被照亮的程度。結果顯示，牡蠣在新月逐漸發生時──以及新月當天──打開的程度最大，而當月球進入上弦月和滿月階段時打開程度較小。團隊表示，這代表牡蠣可以感知月光，就算亮度遠不及太陽光強烈。
不過，培頓也表示實際情況其實更為複雜。她指出，這類生物似乎能夠辨識月球正在轉盈或是漸虧：牡蠣在月虧期普遍比月盈期還要更開一點。培頓認為，可能性之一是這類底棲性雙殼貝也許已經演化出身體內的月相時鐘，而不是被動地倚賴直接光感受。在這種情況下，培頓補充說，牡蠣感知到的月光會幫助體內的月相時鐘和環境同步，而不是直接觸發殼的開闔──就像是人類利用日光，讓體內的二十四小時生理時鐘維持正常運作的方式。
研究團隊認為，牡蠣在月光亮度較低時增加殼打開的角度，或許跟低亮度時食物更多的可能性有關：前行研究曾指出浮游生物的活動似乎也會受光影響。培頓說：「我們知道的是，牡蠣會在有食物的時候把殼打開。」
不過，這份研究並未納入月球在不同季節對牡蠣造成的影響，也未將雲層遮蓋程度──直接影響這類軟體動物實際感受到的月光亮度──納入考量範圍。英國亞伯立斯威大學水族生物學的高級講師大衛‧威考森認為，此領域仍有許多未解之謎。他表示：「舉例來說，我們知道潮汐、月相和日變時鐘似乎是不同的機制，又在某種程度上互相關聯──但我們並不知道它們如何互相牽動，彼此間的關聯程度又為何。」
威考森指出，人類活動可能對海洋環境造成預期外的問題，這是像這份最新研究的學術調查可以協助探討的。威考森說：「舉例而言，如果海岸上有照明設施、或是海中的建築結構有燈光，我們當然就無法確知可能會有什麼影響。」
（台北時報章厚明譯）
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