SEASONAL MIGRATIONS OF THE WOOD-BORER LIMNORIA LIGNORUM (RATHKE) AT FRIDAY HARBOR, WASHINGTON

¹ From the Scripps Institution of Oceanography of the University of California, La Jolla, California

1. Wooden test pieces were exposed to settlement of Limnoria lignorum in successive two and three-month periods from September 5, 1933 to September 8, 1934.

2. Monthly collections of wharf-pile populations were examined to determine the seasonal fluctuations in age groups, and in the occurrence of gravid females.

3. A marked seasonal variation in the numbers of Limnoria settling on the test blocks was found, and since the blocks could be reached only by swimming animals, it is believed to represent a definite seasonal migration which is associated with the main breeding season.

4. On the two-month test pieces, 93.60 per cent of the total yearly settlement of Limnoria occurred over a six-month period from January 5 to July 7; and 48.80 per cent settled from March 7 to May. 9. The three-month pieces showed essentially the same distribution.

5. The most active migration took place during a temperature range of about 7.71° C. to 9.42° C.

6. There was a predominance of young Limnoria throughout the year, but the greatest percentage of these was evident during the spring months.

7. Gravid females doubtless occurred throughout the year (no data for August), with a maximum in April and May and a minimum in January and February, the two months with the lowest temperatures.

8. As an aid to better protection against Limnoria attack on submerged timbers, it is suggested that the most effective time to apply protective paints etc. is at the beginning of the main migratory season.