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Molting and the Full Moon

Are there more soft crabs during a full moon?

This is a question that is asked quite frequently and yet no one seems to have a definitive answer. There appears to be a general consensus among watermen and some scientists that a full moon (and, to a lesser extent, a new moon) does indeed play a role in a crab's molting cycle, but this is based purely on anecdotal evidence.

"While I do not have hard data to support the contention that there is increased shedding on or around a full moon, from personal observations of the commercial soft crab production industry, there is an increase in peelers and soft crabs several days before and after a full moon" says Mike Oesterling of the Virginia Institute of Marine Science.

There are several possible explanations for this phenomenon. One explanation might be that during a full (and new moon) the tides are unusually high or low (called "spring" tides). This is because twice a month the moon and sun are in line with the Earth and therefore, their combined gravitational pull is greater. During a spring tide, more shoreline grasses are submerged (or exposed) which would provide a larger area of cover and protection for crabs to molt. Or, perhaps it's because crabs are able to exploit expanding feeding grounds or have more time for feeding in the shallows. Mr. Oesterling notes that "in the summer months, food availability has a major effect on shedding activity. If a crab does not satisfy the physiological need to shed (increased muscle tissue, body cavity 'cramping', etc.), it will not enter the molting cycle." In other words, if it doesn't get enough to eat then it's not going to outgrow its shell.

Jennifer Martin of the University of Connecticut's Department of Ecology and Evolution says "In the lab I had two thousand crabs that I was rearing and a full moon definitely meant most of them were molting. Why? I am not sure." This would seem to indicate that the increased gravitational pull during a full moon somehow directly influences the crab since food availability and cover would not be factors in a controlled laboratory environment.

From a skeptical point of view there is a more realistic explanation. The lunar cycle (month) is fixed at 29.53 days. This means that the oceans and other water bodies which are affected by the earth-moon system experience a new tidal cycle roughly every 30 days. The Atlantic blue crab molts about 18 to 23 times, not including larval molts, during its life. The instar, or period between molts, grows progressively longer as the crab gets older, ranging from 10-15 days for one-inchers to 30-50 days for legal size "keepers" and beyond.

Because the lunar cycle occurs at a fixed 29.53 day interval and because the molting cycle is variable (as low as every 10 days to as high as every 50 days), any correlation between the two is purely coincidental. Perhaps it's because the 30-day lunar cycle roughly correlates to the 30-day molting interval of legal-size keepers that people tend take special notice. For example:

  • Small, immature crabs have a short 10-20 day molting cycle which does not correlate to the lunar cycle. Since these crabs are of no commercial interest their molting behavior is not closely monitored.

  • Legal-size hard crabs have an approximate 30 day molting cycle. This very closely matches that of the lunar cycle. Because of their high commercial value, crabs of this size are placed in holding tanks and are monitored very closely until they molt. Special notice is taken when, totally by coincidence, crabs are seen molting during a full moon. This is called subjective validation or selective memory. Subjective validation is a psychological phenomenon in which people tend to remember significant or coincidental events and forget the others.

  • Large hard crabs (those with a molting cycle in the 40-50 day range) are scarce and therefore are not monitored as closely.

So, are peelers are more prevalent during a full moon? Probably not. While it's difficult to argue against the fact that some commercial harvesters claim to see a correlation, we should take this with a grain of salt. We know that a crab's molting cycle is variable and the lunar cycle is fixed. It is also a fact of human nature that we try to attach special meanings and significance to things that occur in regular cycles, especially when they seem to coincide with celestial events (full moons, astrology, etc.). It's fun to say that crabs molt only during a full moon. It has an eerie, supernatural feel to it and makes for great crab lore.

Does the "peeler run" start during the first full moon in May?

Along the same subject line, most people claim that the first full moon in May signifies the start of the season's peeler run (some believe that the first run will occur when the first dogwood blossoms begin to open.) The peeler run is simply the time that crabs awaken from their winter dormancy and head to shallow waters to molt. Especially prevalent are mate-seeking females which did not reach their terminal molt the previous year.

Mike Oesterling states that the first sheddings are more water temperature controlled than by lunar phase. Once water temperatures are above about 59 F (15 C) crabs of all sizes begin to molt. Crabs begin shedding in April, generally in the shallower creeks and marshes, where sunny days can significantly elevate the shallow water temperature. These are also generally smaller-sized animals. The "first run" that people talk about is really the "doubler run" of mating females. There have been some years when this "run" did correspond to the May full moon, but over the past 8 to 10 years the "first run" in the lower York River and Mobjack Bay has occurred around May 15-19, regardless of moon phase. Additionally, this "run" occurs at different times in different sections of the Bay, again suggesting a water temperature impact.


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