The surface swarming of Polybius henslowi [Brachyura: Portunidae]

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Abstract
A dense population of Polybius henslowi Leach was recorded at the surface of the sea 15 miles off the north coast of Spain. The depth of water at this point was 1740 m. Visual observations of the swimming behaviour show that Polybius tends to swim with the body facing diagonally and backwards against the current. The space between individuals appears to be maintained deliberately and was never less than 6 ft. during daylight. Measurements show that there are at least two sizegroups present. Females represent 40% of the total of 251 animals collected, and these were at approximately the same reproductive state with the ova maturing. The males are on average 2·3 mm longer than the females. No food was found in the stomach of any of the animals taken.

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