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

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