Effects of environmental factors on regeneration and molting in fiddler crabs.

Abstract

Brachyuran crabs can autotomize limbs at a preformed breakage plane in the basi-ischium, and subsequently regenerate them. The limbs grow in a folded posi tion and at ecdysis they unfold and become functional. The progress of limb re generation has been divided into stages by Bliss (1956) . After an initial lag period, basal growth occurs which establishes the primary organization of the limb (Hodge, 1956) . This may be followed by a plateau of anecdysis if environmental conditions are not suitable. The next stage, one of intensive growth and further development of existing tissues, is the proecdysial stage, culminating in ecdysis and unfolding of the new limb. A terminal plateau often occurs just prior to ecdysis. Regenerating limb buds of crabs are generally described in terms of the regeneration index, or “?�R-value” (Bliss, 1956), which is (limb bud length/carapace width) X 100. Use of the R-value facilitates comparisons between crabs of different sizes. Growth of the regenerating limb is related to the molt cycle of the crab. Factors that influence the molt cycle can influence the rate of regeneration, and vice versa. The molt cycle of Brachyura has been divided into five major stages by Drach ( 1939) . Regenerating limbshave been used asanindexto thestage of proecdysis for crabs, since the growth of limb buds is closely correlated to the progress of proecdysis. The neurosecretory and hormonal systems exert important controls over the molt cycle. For example, in some species of crabs, ecdysone will accelerate proecdysial regenerative growth, leading to molt (Passano and Jyssum, 1963) . Basal growth was found to be independent of the hormone. Removal of eyestalks, which contain molt-inhibiting hormones produced by the X-organ, will lead to accelerated re generation and early ecdysis. Removal of many limbs (multiple autotomy) will also accelerate regeneration and molting in many species of crabs ( Skinner and Graham, 1972) . Fingerman and Fingerman ( 1974) have shown that removal of as few as two limbs will accelerate ecdysis in fiddler crabs. The neurosecretory system, which controls the niolt cycle, is itself affected by the external environment. Bliss and Boyer ( 1964) found that in the land crab, Gecarcin us lateralis, darkness, moderate temperature, and solitude were necessary environmental factors to permit proecdysial growth. These factors are likewise those which would increase the crab's chances of surviving ecdysis. Light, high temperature, and the presence of another crab inhibited proecdysial growth. The presence of dry sand delayed but did not stop regeneration. Rao ( 1965) found that light did not inhibit proecdysial growth in the ghost crab, Ocypode, if crabs were on a light background, and that privacy was a critical factor only for larger crabs, smaller individuals completing regeneration and niolting in the presence of