1. 1.

    The mechanism of fluid absorption at ecdysis was investigated in the stenohaline marine decapod, Homarus americanus.

  2. 2.

    Sea-water uptake began approximately 1 h before ecdysis, increased rapidly during ecdysis, and was completed 2 h after ecdysis.

  3. 3.

    Increased drinking rates were measured during and just after ecdysis. The quantity of water ingested was equal to the total amount of water absorbed during moult, indicating that fluid entered the haemolymph exclusively via the lining of the digestive tract.

  4. 4.

    It was estimated that 91 % of the ingested sea water appeared in the haemolymph by 2·5 h postecdysis.

  5. 5.

    The midgut appeared to be the principal part of the digestive tract involved in the absorption of fluid into the haemolymph. X-radiography of lobsters that ingested a suspension of barium sulphate during ecdysis demonstrated that accumulation and concentration of this compound occurred within the midgut during the 2·5 h of haemolymph expansion following ecdysis. Elevated net water fluxes across perfused midgut were observed in late proecdysis and 0·5 h after ecdysis.

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