Isopod miners in the leaves of two Western Australian Posidonia species

Abstract

Isopods of the genus Lynseia have been found burrowing (leaf mining) in meadow forming seagrasses ( Posidonia) from Western Australia. These small (1.5–1.8 mm) isopods burrow beneath the epidermis consuming the mesophyll, forming a linear mine of similar width to the isopod along the leaf lamina. At Rottnest Island in January 1991, 75% of Posidonia australis Hook. f. and 66% of Posidonia sinuosa Cambridge and Kuo leaves were burrowed by isopods. Isopods were found in leaves of all sizes, but number of burrows, number of isopods and the length of burrows increased in older leaves. In P. australis, there were 3.15 ± 0.1 burrows and 2.6 ± 0.1 isopods per burrowed leaf, in P. sinuosa there were 2.9 ± 0.1 burrows and 2.48 ± 0.2 isopods per burrowed leaf. Isopods were mobile, vacating older burrows and forming new ones. Male and female isopods were found in pairs within burrows and young isopods were then brooded within the burrows, forming new branches from the parent burrow. Within monospecific stands of P. australis and P. sinuosa there were 2950 and 2280 isopods m −2 respectively. Mean length of individual burrows was 22 mm in P. australis and 28 mm in P. sinuosa. Total burrow length per leaf (mean ± SE) was 69.6 ± 3.6 mm in P. australis and 81.7 ± 4.7 mm in P. sinuosa. The consumption of leaf tissues and the resulting lines of weakness along burrow lines may have negative effects on the seagrass. Burrows facilitate entry of water, bacteria and fungi to the blade, leading to localised tissue death. Epiphytes may proliferate along burrow lines, owing to surface roughness or solute loss associated with the burrow. Burrows also split and increase fragmentation of leaves in storm events. Loss of photosynthetic area due to epiphytic growth and fragmentation of the leaves may reduce the growth of Posidonia. These potential effects of lynseid seagrass leaf miners may be important if the seagrass meadow was already under stress.