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  Evolution to land
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Moulting in semiterrestrial forms

  Topics regarding evolution to land in isopod crustaceans include moulting in semiterrestrial forms, considered here, and LIGIA A PROTOTYPAL LAND COLONISER, DESICCATION RESISTANCE, GAS EXCHANGE, REPRODUCTIVE MODIFICATIONS, and MODIFICATIONS IN FEEDING & DIGESTIVE PROCESSES, considered in other sections.
Research study 1


photograph of a moulting isopod Ligia pallasii
Semiterrestrial and terrestrial isopods have a unique biphasic moult, in which first the posterior half is moulted and then the anterior half, separated by a day or two depending upon species.  There are several advantages to this.  In comparison, most marine isopods moult their exuviae in one piece, as do all other crustaceans.

NOTE  lit. “that which is taken off” L., a term used in studies of insects, crustaceans, snakes, and other animals which cast off their exoskeletons or skins during growth

A male Ligia pallasii has just moulted its posterior half. Note its
larger size as compared with the anterior unmoulted half. The
posterior part is still soft, but is in the process of being
mineralised. The ventral or sternal side of this individual bears
the calcium stores to be used in this mineralisation (see Research
Study 3 below). A few days later the anterior half will be moulted 1.6X

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What are the advantages of a biphasic moult in an oniscid isopod? Consider these answers, then CLICK HERE for explanations.

It is an energy-savings strategy.

It allows for continued motility.

It reduces evaporative water loss.

It permits cycling of calcium for mineralisation.

It ensures that sensory receptors (e.g., eyes, antennae, uropods) and defensive structures (e.g., repugnatorial glands) are always fully functional on one half of the body.

Research study 2

photograph of a terrestrial isopod moulting its anterior halfIn marine isopods most or all of the calcium for mineralisation comes directly from seawater surrounding the animal.  In comparison, semiterrestrial and terrestrial isopods obtain their calcium from food, or from freshwater or possibly ingested calcareous soils, and calcium is at more of a premium than it is for marine forms. The main nutritional requirement for calcium by isopods (and other crustaceans) is to replace that lost in the cast-off moult and to provide for growth of the new exoskeleton.  In Ligia spp. about 25% of the calcium content of the exoskeleton is lost when it is moulted but, as ligiids normally eat their exuviae, the actual loss by this cause may be small. The photograph at Right shows an anterior moult or exuvia of L. pallasii, soon to be eaten. Carefoot 1984 Comp Biochem Physiol 79A: 655.

NOTE  a few terrestrial species, including Porcellio scaber are known to consume their exuviae after moulting, and it may be a common behaviour throughout the oniscideans.  Whether it photograph of a terrestrial isopod Porcellio scaberis primarily for calcium-saving, or for resorption of general nutrients, is not known

Terrestrial isopod Porcellio scaber, common under flower pots, pieces of
wood, and leaf litter in gardens; also known as woodbug or woodlouse 2X

Research study 3

photograph of the sternum of a moulting isopod Ligia pallasii to show area of calcium depositsPrior to the posterior moult, calcium and carbonate ions are resorbed from the old posterior cuticle and transported in the hemolymph across the anterior sternal epithelium into a space, known as the ecdysal gap (a space created as the old cuticle becomes hard and begins to split off), where they precipitate to form calcium-carbonate spherules. After the posterior part is moulted, the sternal calcium spherules are entirely resorbed and used for the mineralisation of the new posterior cuticle. The deposits are seen in the Left-hand photo as white areas in the first four sternites of a moulting Ligia pallasiiphotograph of calcium granules stored during moult cycle of ligiid isopod Ligia italica, courtesy Andreas Ziegler, University of Ulm, Germany

In Ligia the deposits consist of microscopic spherules (0.5um dia) of amorphous calcium carbonate precipitated within an organic matrix (see photo on Right). The large surface area and the amorphous state of the spherules facilitate their mobilisation and speed mineralisation of the new posterior cuticle.  These calcium deposits are an adaptation to the terrestrial environment and do not occur in marine isopods. Photo of spherules in Ligia italica courtesy Helge Fabritius and Andreas Ziegler, University of Ulm, Germany.

NOTE  lit. “escape or slipping out” G.  Another word for moulting is ecdysis

NOTE  non-crystalline.  Solubility of calcium carbonate in its amorphous form is 10 times greater than in its crystalline form

Research study 4

histogram showing hemolymph calcium concentrations during the moult cycleAs noted in Research Study 1 above, semiterrestrial and terrestrial isopod species have a biphasic moult involving stages of premoult, moult (posterior half), intramoult (between moult of posterior and anterior halves), moult (anterior half), postmoult, and intermoult.  During these stages the hemolymph undergoes changes in ionic composition, most notably in titre of calcium, an element essential for construction of the new cuticle (exoskeleton) and one conserved during the moulting process.  These changes are investigated for Ligia pallasii at the Bamfield Marine Sciences Centre, British Columbia by a group of German and North American researchers.  Calcium increases significantly by 18% from inter- to premoult owing to uptake from the posterior cuticle (see histogram).  This calcium is transported in the hemolymph and precipitated as calcium carbonate in storage areas under the anterior cuticle (exoskeleton; see photographs in Research Study 3 above).  Later, from pre- to intramoult the calcium content of hemolymph increases significantly by another 25% by resorption of calcium from the anterior cuticle and by mobilisation of these newly created anterior calcium-carbonate stores for formation of new cuticle.  Just after moult of the posterior cuticle there occurs a sudden increase in hemolymph photograph of isopod Ligia pallasii moultingvolume that serves to expand the new cuticle in both halves of the body (see photograph on Right).  The increased volume subsides later during intermoult.  The absence of significant change in concentration of other ions (Na+, K+, Mg++, and Cl-) during the moulting period suggests that the increase in hemolymph volume may owe to uptake of seawater rather than freshwater.  Ziegler et al. 2000 J Comp Physiol B 170: 329.

NOTE  this group, numbering 10 in total and calling themselves the International Isopod Research Group (IIRG), met at Bamfield for a 6wk period in 1998 to research physiological and biochemical aspects of semiterrestrial life in L. pallasii. The 5 major papers resulting from this collaboration can be found in various locations in this LEARN ABOUT section on isopods

Isopod Ligia pallasii prior to
moulting the anterior half 2X

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