An ultrastructural study was carried out on the digestive tract contents of Limnoria lignorum, frass, gallery walls and surface structures of the animals. The purpose of the study was to determine whether wood-degrading bacteria, fungi and other microbes and/or wood degraded by these microbes were present in gut regions and therefore could provide indirectly a nutritional source for Limnoria. Both bacterial (tunnelling) and fungal (soft rot) degraded wood fragments as well as lysed microbes, extracellular slime and various other microbes (actinomycetes, diatoms) were found in fore, mid and hind gut regions. The range of microbes associated with wood fragments from gut regions were similar to those recognized either colonizing or degrading surrounding gallery walls or present on the external surfaces of the organism itself. Wood materials in the gut were highly fragmented and showed evidence for either extensive bacterial or soft rot attack, or surface and sub-surface evidence for carbohydrate removal. The major wood degrading bacteria attack noted was tunnelling bacteria as well as a form capable of cell wall dissolution of pine wood. Both bacterial types carried out extensive attack of pine wood fragments present in Limnoria tunnels. Lysed fungal hyphae, and soft rot hyphae associated with residual soft rotted wood fragments were also prominent. Gut regions of Limnoria lignorum lacked a natural resident bacteria flora, although numbers of characteristic Gram-negative active vesicle producing bacteria were represented in all gut regions associated with wood fragments. In hind gut regions a prominence of wood middle lamella regions was noted suggesting that these cell wall regions were more resistant to attack. Results indicate ingestion of bacterial and fungal degraded wood and its associated microflora. It is suggested that ingestion of microbial degraded wood could provide Limnoria with increased substrate assessibility and a greater surface area over which their enzymes could act. Associated bacterial and fungal breakdown products could also provide an important supplimentary source of nitrogen.

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