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  Evolution to land

Of all present-day terrestrial invertebrates, only crustaceans are thought to have invaded land directly over sea beaches.  All other terrestrial invertebrates, including worms, insects, spiders, and flatworms, and all vertebrates, are believed to have evolved onto land via freshwater.  Of crustaceans, only oniscidean isopods and a few amphipods have successfully colonised land.  There are many species of semiterrestrial crabs, but only a few terrestrial ones.  This owes, in the main, to the need for females need to be within walking distance of the ocean to release their larvae for pelagic development.

NOTE  one species of crab in Jamaica eliminates the need for this by incubating its larvae in water trapped in the leaf-bases of large bromiliad plants; other tropical species either walk to the ocean to release their larvae, or release them into small streams that, within moments, carry the otherwise freshwater-intolerant larvae to the ocean

Research study 1

photograph of terrestrial isopod Porcellio scaberFew taxonomic groups of organisms provide a richer example of evolution than terrestrial isopods in the Superorder Oniscidea1.  The Oniscidea is a highly successful terrestrial group that includes over 3400 species of woodlice inhabiting seashores, forests, savannahs, and deserts.  Common semiterrestrial members of the group are known as sea slaters2 or rock lice, while terrestrial members are known as sowbugs, woodlice, woodbugs, and many other common names.  A few examples of terrestrial oniscideans are included in the ODYSSEY because: 1) they represent the largest group of crustaceans3 successfully to evolve onto land, doing so directly over sea beaches, and 2) because a prototypal form in this evolution, genus Ligia, is represented by 2 indigenous species on the west coast.  The remarkable adaptive radiation of oniscideans is exemplified by 3 European species Armadillidium vulgare, Porcellio scaber, and Oniscus asellus which, by infesting outward-bound luggage and cargo, and perhaps floating about the oceans on debris, have spread to many parts of the world including the west coast of North America.  This radiation now provides (after a few moments searching of dock areas, and near houses and associated ground clutter) for teaching and research, a selection of isopods showing a marvelous evolutionary gradation of body design, behaviour, and physiology.

NOTE1  in 1983 a group of world scientists met for the first time in London for a symposium on the biology of terrestrial isopods sponsored by the Zoological Society of London.  It was a unique gathering of researchers, united by a common interest in woodlice, and it generated many new research collaborations, a symposium volume, and a series of later meetings held in Italy (2nd: 1987), France (3rd: 1990), Israel (4th: 1997), Greece (5th: 2001), Portugal (6th: 2004), Tunisia (7th: 2007), and Slovenia (8th: 2011). 

NOTE2  a name specific to members of the genus Ligia, which are slate gray in colour.  Another name for ligiids is “boat bugs” (funemushi in Japanese), referring to their presence around docks and marinas, beached boats, and fishing equipment

NOTE3 other terrestrial crustaceans include amphipods, but their distribution is constrained, much more so than woodlice, to moist habitats

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  Ligia, a prototypal land coloniser
  Topics relating to evolution to land in isopod crustaceans include Ligia, a prototypal land coloniser, considered here, and DESICCATION RESISTANCE, GAS EXCHANGE, MOULTING IN SEMITERRESTRIAL FORMS, REPRODUCTIVE MODIFICATIONS, and MODIFICATIONS IN FEEDING & DIGESTIVE PROCESSES, considered in other sections.
Research study 1

Members of the genus Ligia are evolutionarily close to ther presumed marine ancestors and several features of morphology, physiology, and behaviour are transitional between marine and fully terrestrial forms.  In this regard, it is important to note that present-day ligiids do not represent the ancestral photograph of isopod Ligia occidentalis courtesy Jackie Soanes, Bodega Bay, California stock that gave rise to more terrestrial forms; rather, all oniscideans are descendent from a common Ligia-like ancestral stock.  Moreover, Ligia is not a group whose adaptational features stalled them at the semiterrestrial stage; rather, they are a group whose adaptational successes have enabled them to thrive in a complex habitat that straddles sea and land. Features of evolutionary interest of Ligia include desiccation resistance, gas exchange, moulting in semiterrestrial forms, reproductive modifications, and modifications in feeding and digestive processes. Carefoot & Taylor photograph of isopod Ligia pallasii1995 p. 47 In, Terrestrial Isopod Biol (Alikhan, ed.)  AA Balkema, Rott.; Carefoot 1993 Comp Biochem Physiol 106A: 413. Photograph of L. occidentalis courtesy Jackie Soanes, Bodega Marine Laboratory, California.


Ligia occidentalis 1.6X

Ligia pallasii 3X

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