Size-selective attack by parasitic lampreys: consideration of alternate null hypotheses

Summary The hypothesis that parasitic lampreys actively select larger host fish was tested with previously published laboratory data for Petromyzon marinus and new data for Ichthyomyzon castaneus. Whereas in previous work, expected attack frequencies were proportional to the number of potential hosts in each size group (numbers-dependent null hypothesis), the present study employed a more conservative area-dependent null hypothesis by which expected attack frequencies were adjusted to reflect the surface area of potential hosts in each size group. The balance of the data suggests that lampreys are actively size-selective. It is more difficult to demonstrate size-selectivity in the field because of sampling considerations and the need to account for size-related factors (e.g., swimming speed) that affect host availability. It is unknown if handling times are greater for larger hosts or if the quality of host blood changes with host size, and it is uncertain if lampreys achieve greater energy intake rates on larger hosts, but a larger host survives longer and assures an attacking lamprey of a longer period of feeding.