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  Evolution to land
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Reproductive modifications

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


drawing of female Ligia pallasii releasing mancas from the brood chamberReproduction is more primitive in semiterrestrial ligiids than in terrestrial forms.  In Ligia the brood chamber, or marsupium, actually resembles that in marine forms, with 5 pairs of oöstegites overlapping loosely to form a pouch in which the fertilised eggs are deposited.  To keep the eggs moist and to enable gas exhange, the gravid females dip their paired uropods or juxtaposed 6-7th legs into puddles, allowing the fluid to run up into the marsupium.  This is a so-called “amphibious” or "open" marsupium, as opposed to the"closed" marsupium of terrestrial species. Development takes place in the egg capsule and there is no larval phase.  In Ligia pallasii on the coast of British Columbia the juveniles hatch after 40d (at 15oC) from a late developmental stage known as a fusiform "larva".  Soon after hatching, the mancas start crawling about within the brood chamber. At this time any undeveloped eggs are eaten by the juveniles. A day or so later, the brooding female, photograph of a female Ligia pallasii showing eggs in marsupiumpresumably stimulated by the activity of the juveniles, flexes her abdomen upwards, thus causing the oöstegites to gape open at the back, and the juveniles crawl out (see drawing above Right).  Brood numbers average 45-50.  Carefoot 1973 Mar Biol 18: 302.

NOTE lit. "egg covers" G.

photograph of a gravid isopod Ligia pallasii with mancas nearly ready to be releasedNOTE  in semiterrestrial and terrestrial oniscids the newly hatched juveniles are called mancas

View of brood
chamber about 5d
after egg release



40d after egg release theoöstegites are gaping
open from the volume of maturing juveniles

 
DEVELOPMENTAL STAGES OF LIGIA in laboratory culture (40d duration at 15oC
photograph of "comma" stage of development of isopod Ligia pallasii
Day 5: "comma" stage with tail bud visible
photograph of early fusiform stage of development of isopod Ligia pallasii
Day 10: early fusiform with much yolk; eyes visible
photograph of mid-fusiform stage of development of isopod Ligia pallasii
Day 20: mid-fusiform with less yolk; eyes developed
photograph of late fusiform stage of development of isopod Ligia pallasii
Day 30: late fusiform stage, with an undeveloped egg
photograph of hatchlings in brood chamber of an isopod Ligia pallasii
Day 40: mancas hatched, moving in brood chamber
photograph of a nearly empty brood chamber of an isopod Ligia pallasii
Day 41: brood chamber empty but for 2 individuals
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Research study 2
 

photograph of a pillbug Armadillidium vulgareIn semiterrestrial forms such as ligiids, the brood chamber is of the "open" type; that is, the oostegites overlap loosely and there is water exchange between the inside and outside of the chamber. In contrast, all terrestrial species have a “closed” type of marsupium, that is, the oöstegites overlap tightly and the chamber is sealed front and back.  In most or all terrestrial species there are special structures within the brood chamber, called cotyledons, that extend from the thorax tissues of the female and make direct contact with many or most of the embryos. It is thought that these cotyledons conduct nutrient and energy materials from the female directly to the developing young.

 

The pillbug or "rolly-polly" Armadillidium vulgare is the most terrestrially
evolved of the several common non-indigenous west-coast species 6X

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Research study 3
 

histogram showing energy utilised during development from egg to juvenile in the isopod Ligia pallasiiAre there no maternal contributions to growth of developmental stages in ligiid and other species with "open" marsupia? Two lines of evidence suggest that some nutrients and possibly calcium (for mineralisation of the exoskeleton) are provided to the brood in Ligia pallasii.  Studies at Bamfield Marine Sciences Centre, British Columbia on embryonic development in Ligia pallasii show, first, that 60% more energy is respired during development than can be accounted for by depletion of yolk stores (see histogram on Right).  Such shortfalls between the amount of energy supplied as yolk by the parent and that actually respired has been noted in many other studies of development in marine invertebrates, and is attributed to direct uptake of dissolved organic matter (DOM) from seawater, which is then metabolised.  If DOM were also to be used as an energy source by developing embryos of Ligia, the source it could be the seawater used for irrigation of the brood chamber.  Rough calculations based on concentrations of DOM in coastal seawater and rates of amino-acid uptake measured in other studies suggest that 0.5ml of seawater per embryo of irrigation water over the 40-d developmental period would suffice to meet the energy shortfall.  Carefoot 2003 p.121 In, The Biology of Terrestrial Isopods, V (Sfenthourakis et al., Eds.) Crustaceana Monographs 2, Brill, Leiden.

NOTE  DOM consists of a wide diversity of organic molecules, much of it of bacterial origin, including neutral sugars, amino sugars, and amino acids

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Research study 4
  histogram comparing ash contents in developmental stages of the semiterrestrial isopod Ligia pallasiiA second line of evidence for the presence of maternal contribution of energy and nutrients during development in Ligia pallasii comes from measurement of "ash budgets". Note in the accompanying histogram that content of ash, consisting most likely of calcium salts used in mineralisation of the cuticle, increases 4-fold during the 40-d brooding period from egg through fusiform stage and manca.  Where does this come from? It could come from secretions from the parent, but calculations based on calcium concentrations in coastal seawater and known uptake rates from other studies suggest that irrigation of 0.1ml of seawater per embryo over the 40d period would be sufficient to "balance" the ash budget in L. pallasii. Carefoot 2003 p. 121 In, The Biology of Terrestrial Isopods, V (Sfenthourakis et al., Eds.) Crustaceana Monographs 2, Brill, Leiden.

NOTE the fusiform stage is a non-motile developmental form still in the egg membrane, prior to hatching to the manca stage

NOTE another possible source for calcium for mineralisation of the cuticle in the embryos of Ligia pallasii is parental urine.  This is released from nephridiopores on either side of the mouth. The urine flows posteriorad along lateral and vertical cuticular joinings to the pleopodal gas-exchange region of the abdomen.  Some urine presumably enters the brood chamber for general moistening of the eggs and embryos, and the calcium contained in it could be used by the embryos
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