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  Evolution to land
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Modifications in digestive processes

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

While the role of bacterial and other gut symbionts in providing nutritional supplements to their hosts is well known in vertebrates, it is not so well known in invertebrates.  A comparison of numbers of microbial symbionts in the midgut glands (hepatopancreatic ceca) of 3 west-coast isopods at the Bamfield Marine Sciences Centre, British Columbia shows differences possibly associated with evolution to terrestrial life.  The 3 isopods examined are Idotea wosnesenskii, an intertidal species that grazes the surfaces of various macroalgae such as Fucus for food, Gnorimosphaeroma oregonense, also an intertidal species that favours under-rock habitats and subsists on plant material, and Ligia pallasii, a supra-tidal form that feeds on algae in the high intertidal region or more commonly in the strand line of the shore.  Microbial counts in homogenised hepatopancreatic glands show an apparent absence of symbionts in the 2 intertidal species, and a presence of symbionts in the semiterrestrial species Ligia.  Comparison of C:N ratios in algal foods and in feces of isopods eating those foods shows differential digestion of nutritional components, most notably phenolics and proteins, by the 3 species.  Overall, the more terrestrial species L. pallasii appears to utilise food nitrogen more effectively than the intertidal species.  The authors suggest that the acquisition of digestion-enhancing symbionts in these prototypal land-colonising isopods may have been an important component to later colonisation of land.  Zimmer et al. 2001 Mar Biol 138: 955. 

NOTE although present in Ligia, numbers are low - only 3million total in the hepatopancreatic lobes of an adult animal


photograph of isopod Idotea wosnesenskii feeding on kelp
Idotea wosnesenkii feeding on kelp 2X

photograph of isopod Gnorimosphaeroma oregonense on sponge
Gnorimosphaeroma oregonense on a sponge 5X
photograph of isopod Ligia pallasii on kelp
Large male Ligia pallasii crawling on kelp 1.3X
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Research study 2

A prerequisite to successful colonisation of land by isopod crustaceans must have been the evolution of an ability to digest cellulose and lignins, both common components of the leaf-litter food of contemporary terrestrial isopods.  A related study by the same research group as in the foregoing Resarch Study at the Bamfield Marine Sciences Centre, British Columbia investigates this ability in 2 intertidal species Idotea wosnesenskii and Gnorimosphaeroma oregonense, and in the semiterrestrial species Ligia pallasii.  Phenolics are readily oxidised by Gnorimosphaeroma and Ligia, but not by Idotea wosnesenskii, even though this species feeds on seaweeds rich in phenolics.  Similarly, Idotea is least able to digest cellulose, while Ligia is most capable of doing this.  Reduction of gut bacteria by feeding antibiotics significantly reduces cellulose digestion in Ligia, suggesting that endosymbiotic bacteria are likely involved in this activity.  These and other data lend support to the hypothesis that a cellulose-digesting ability was an important “pre-adaptation” to adoption of a fully terrestrial life in isopods.   Zimmer et al. 2002 Mar Biol 140: 1207.

NOTE   details on types and method of administration of antibiotics can be found in the authors’ previous publication (Research Study 1 above).  The possible toxic effects of antibiotics on the isopods is not discussed by the researchers, but they do note that feeding rates are not significantly affected in individuals receiving antibiotics

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Research study 3

photograph of Ligia occidentalisInformation on the pattern of distribution of hepatopancreas-inhabiting microbial symbionts in 2 species of west-coast ligiids is provided by a researcher at the University of California, Davis.  Collections of Ligia pallasii and L. occidentalis from 20 west-coast sites ranging from southern Vancouver Island to northern Baja California are used to create 16S rRNA clone-libraries of microbial symbionts.  Results show microbial map showing collecting sites for study on symbionts in isopods Ligia pallasii and L. occidentalispresence in both species, with greater diversity in L. pallasii than in L. occidentalis.  Unfortunately, the technique provides information only on presence or absence of certain bacterial types, and not on their numbers, and no attempt is made to associate a particular bacterial flora with algal food types presence in one habitat versus another.  With respect to this, some individuals are noted to lack symbionts entirely, which raises questions as to the actual role these bacteria play in their hosts’ nutrition.  Phylogenetic analysis shows, perhaps not surprisingly, that symbionts of these semiterrestrial species tend to cluster together.  Eberl 2010 Symbiosis 51: 107.

NOTE  this is not meant to be a criticism, as any such attempt would be enormously complex and unlikely to be successful

Recent observations place the northern limits of distribution of Ligia
several hundred kilometers further north than indicated;
in fact, at about the northern-most edge of the larger map shown here

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