I compare the seasonal abundance variation, population dynamics, fecundity, egg hatching mechanism and success, and apostome ciliate parasites of the euphausiids Euphausia pacifica and Thysanoessa spinifera from the Oregon coast, USA. Community structure and nearshore distributions were examined from bi-weekly oceanographic surveys (1970-1972). This region has a strong cross-shelf change in euphausiid assemblages located about 45 km from shore. Euphausia pacifica and T. spinifera have life stage-segregated distributions, suggesting active location-maintenance strategies.

Morphology and biometry of all the post-spawning embryonic stages and the hatching mechanisms of three broadcast-spawning (E. pacifica, T. spinifera and Thysanoessa inspinata) and one sac-spawning (Nematoscelis difficilis) euphausiids are described. The average embryo and chorion diameters were significantly larger for E. pacifica (0.378, 0.407 mm) than for T. spinifera (0.353, 0.363 mm) and T. inspinata (0.312, 0.333 mm). There are four hatching mechanisms. Some broadcast-spawning species have delayed hatching schedules, hatching as nauplius 2, metanauplius or calyptopis 1, rather than as the usual nauplius 1. Sac-spawning species sometimes hatch early as nauplius 2, rather than as the normal pseudometanauplius or metanauplius. Late and early hatching schedules were associated with low hatching success and small brood size.

The brood sizes (BS) of E. pacifica and T. spinifera were estimated during incubations on 23 cruises. Thysanoessa spinifera had higher BS and extended its spawning areas farther offshore during 2002 than in 1999-2001. Euphausia pacifica had a stable inshore-offshore brood-size pattern from 2001 to 2003, and had greater BS than during 1999-2000. Elevated egg production during summer 2002 was associated with cooler, high chl-a conditions resulting from anomalously high equatorward transport.

The interbrood period (IBP) was estimated for E. pacifica by (1) observation of the clutch-to-clutch interval for females maintained in the laboratory, and (2) as the inverse of the proportion of females in the field stock with a purple thoracic band. The methods produced similar frequency distributions of IBP estimates. In their first spawning season (<19 mm TL) females produce on the order of 16 eggs fem^-1 d^-1. Larger females in their second spawning season (19-30 mm TL) produce about 24 eggs fem^-1 d^-1.

I describe a parasitoid apostome ciliate that infects at least three euphausiid species: E. pacifica, T. spinifera and Thysanoessa gregaria. Each endoparasitic ciliate stage was characterized and the species was named Collinia oregonensis sp. nov. This is the seventh species described for the genus Collinia, and the second species known to infect euphausiids. Disease progression and ciliate morphology are described using modified protargol and hematoxylin stains and Scanning Electron Microscopy (SEM). All developmental stages of C. oregonensis possess between 14 and 22 kineties. The discovery in June 2001 of a mass mortality of E. pacifica caused by this parasitoid in Astoria Canyon offshore of northern Oregon, with densities up to 250 carcasses m-2, challenges the widely held notion that mortality in pelagic organisms can be primarily attributed to predation and starvation.