April 6, 2010

The True Life of an Aggressive Lobster

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The fishermen of New England sail the eastern coast from Massachusetts to Maine, utilizing natural populations of lobster to supply a multi-billion dollar industry. Sometimes, the fishermen receive a guest, and she’s not just looking for surf ‘n’ turf.

“For over 100 years, fishermen and scientists have been tagging lobsters and watching their movements,” explained Dr. Diane Cowan on Monday in the seminar, “The Social Life of Lobsters: Male Bonding and Other Evidence Homarus Americanus is not a Solitary Cannibal.” Cowan is Executive Director of The Lobster Conservancy (TLC) in Friendship, Maine.

Cowan’s research analyzes the behavior of lobsters along the coast of New England and strives to improve the local lobster industry. “I’ve had a different experience with lobsters, and I’ve seen a different side of them.”

“Lobsters get a nasty reputation as cannibals,” she explained. “The lobstermen do their best to separate them… but the lobsters seem intent on pinching each other.”

Cowan admitted, “Females are vicious, but not toward each other.”

Cowan believes this behavior in male and females relates to the reproductive strategies of lobsters, who devote time and resources to protect their young embryos.

In order to mate, a female must “molt,” or shed her hard, exo-skeletal shell.  Afterward, copulation occurs, and a pregnant female enters the shelter of a male.  This shelter is often a crevice or burrow among the rocks along the shallow coastline.  From this shelter, the male protects the female and embryos from predation.

In a natural population in which females outnumber males, the male lobsters may simultaneously mate with multiple females and protect multiple shelters.

In captivity, with fewer shelters, female lobsters stagger their patterns of molting, taking turns to mate with the dominant male and to move into his shelter.  No aggression exists because it may needlessly risk the female’s health.  In time, each female may mate with this “dominant” male, who is most likely to protect the mother and yield a fit child.

In a natural population that includes relatively more males than females, the males develop their aggressive behavior. This aggression ensures reproduction when too few females are available.

However, Cowan observed a contradiction.  In a “pound,” or isolated lobster habitat with relatively few shelters, Cowan observed groups of males living in close proximity during the cold winter months.

“There will be multiple [male] lobsters per shelter.  The lobsters will literally be piled on top of each other,” she described. “They’re probably drinking beer and watching football.”

In addition, Cowan examined the relationships between ocean temperature and the body size and movement of ovigerous lobsters.  “Ovigerous” lobster bear eggs.

Cowan spent months on fishing boats, tracking and re-capturing tagged lobsters.

“Sometimes you catch a lobster and you don’t see them for another four to six years.  It’s pretty cool.”

Analysis of the locations of the lobsters overtime indicated a pattern between body size and movement: small females remained near the coast while larger females often migrated longer distances.

In addition, larger females preferred environments with year-round temperatures with less deviation, whereas smaller females preferred environments with warmer summer temperatures with cooler winter temperatures.

“This is important to embryonic development,” she explained.  “This affects the speed of embryo development.”

Lobsters, unlike mammals, cannot regulate their internal body temperature.  Therefore, their bodies and their embryos maintain the same temperature as the external waters.

Larger lobsters, who prefer cooler waters, reproduce throughout the year because pregnancy requires nearly the same amount of time during winter and summer.  However, smaller lobsters, who prefer greater temperature fluctuation during the year, reproduce primarily during the summer, when pregnancy requires the least time.

Due to this behavior, at Lowell’s Cove, fishing pressure caused a loss of genetic variability.  Fishing during past summers removed many immature lobsters, who had yet reproduced.  At the same time, fishing reduced the entrance of variation into the population.  This reduced variability leaves the population vulnerable to disaster.

Because the lobster industry relies upon continuous repopulation of coastal waters, Cowan works with fishermen to implement change.  She hopes to adjust fishing procedures and lobster behavior to maintain lobster populations.

“This is important because if the population crashes, you need recovery, you need larvae moving in,” she related.  “We should raise the minimum legal size, and capture fewer lobsters that haven’t reproduced.”

Original Author: A. Drew Muscente