Isopod borers in seagrass species from south-western Australia

Brief description

The presence of a number of species of isopod borers (Limnoriidae family) in seagrass species from south-western Australia were documented from sampling seagrass tissue from Rottnest Island. The abundance of isopods within seagrass meadows at Rottnest Island was estimated and the appearance of seagrass invaded by isopods and tissues consumed by these isopods are described. However seagrass from other locations were also studied. These locations included: Bremer Bay, Esperance, Fremantle, Shark Bay, Marmion, Geraldton, Peaceful Bay, Geographe Bay.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Most extensive examination of seagrass leaves for isopod borers was made at Rottnest Island. Seagrass leaves from a range of geographic localities were examined as they became available and estimates of abundance were made from randomly collected leaves at each locality. When possible a sample of 110 randomly collected Posidonia leaves was examined for isopods. - Amphibolis griffithii - In January 1991, fifteen 20 x 20 cm quadrats samples from Nancy Cove were used to determine the minimum number of quadrats required to estimate the incidence of damage. The numbers of stems, leaf clusters, damaged and destroyed leaf clusters were all recorded. Limnoria agrostisa were occasionally found within the leaf sheaths, but unlike the Lynseia species, could emerge easily from within the sheath of Amphibolis griffithii and so could be lost. Samples from A. griffithii around Rottnest were taken at Lady Edeline Beach, Nancy Cove, Porpoise Bay, City of York Bay, Thompson Bay and Little Parrakeet Bay to determine the incidence of damage, as percentage of shoots with leaf clusters damaged, % of leaf clusters damaged and proportions of leaf clusters totally destroyed by isopods. Other sites with very sparse or mixed stands of A. griffithii were also examined, but quadrat samples could not be taken. - Posidonia australis, P. sinuosa and Heterozostera tasmanica - Samples of leaf material were collected at the southern end of Thompson Bay 100 m off shore in 2-3 m depth. Two dominant species, Posidonia australis and P. sinuosa, were present in large patches. Heterozostera tasmanica was present in smaller patches or open sand areas within the seagrass meadow. Leaves were collected by divers using SCUBA equipment. Six 20 x 20 cm quadrats were haphazardly placed within monospecific patches of seagrass, at least one metre apart. All leaves within a quadrat were collected and placed in calico bags. In the laboratory, samples were held in a refrigerator and sorted within 2 days or frozen in plastic bags for later processing. The number if burrows and the number of isopods per leaf were counted. The lengths of burrows and isopods were measured from a sample of leaves collected at Thompson Bay, Rottnest Island in April 1991. Regression of burrow length and isopod size was made. A t-test for unequal sample sizes and inequality of variances was used to test for significant differences in the size of isopods from P. australis and P. sinuosa. - Cross sections of leaves containing burrows - Leaves of P. sinuosa were collected from a seagrass meadow in 3 m of water at Thompson Bay, Rottnest Island in February 1991. Leaves of P. coriacea were collected from Lal Bank, Marmion, Perth metropolitan area in March 1991. Leaves were cut into pieces (5 cm), and fixed in 2.5% gluteraldehyde with seawater. Pieces of leaf (<1.0 cm) were embedded in JB-4 embedding resin. Sections (3 um) were stained with toluidine blue (pH 4.4) for approximately 15 seconds. Photographs were taken on a Wild Photomakroskop (M400) using FP4 (ASA 100)

Notes

Credit
The study of Lynseia species was undertaken by A. Brearley as part of an Honours project in the Botany Department of the University of Western Australia. Dr G. Poore, Museum of Victoria identified specimens of Lynseia sp. and provided helpful comments. Specimens of Limnoria agrostisa were identified by Dr L Cookson, CSIRO Division of Forest Products, Victoria. Samples of seagrass were collected in January 1991 at Rottnest Island during the 5th International Marine Biological Workshop 1991 held at Rottnest Island. Anne Brearley's participation in the workshop was made possible by Dr F. E. Wells and members of the organising committee. Other seagrasses samples were collected from Esperance by C. Bryce and P. Mulvay, from Marmion by H. Kirkman and C. Manning, from Geraldton by G. A. Kendrick and T. Carruthers and from Shark Bay by B. Kipling.