The creation of massless scalar particles in asymptotically flat spacetimes containing shell-focusing naked singularities which evolve from nonsingular initial data is studied. In the case where the singularity is marginally naked, i.e., its Cauchy horizon coincides with the event horizon, we are able to compute the spectrum of created particles by Hawking's method. The spectrum of particles is no longer thermal, but can be expressed as a quasithermal spectrum with a frequency-dependent temperature. In the highfrequency limit the effective temperature approaches a constant value greater than the Hawking temperature. In the more general case where the Cauchy horizon and event horizon do not coincide, we calculate the expectation value of the stress-energy tensor of the scalar field in the two-dimensional spacetimes obtained by suppressing the spherical coordinates. In all cases the energy flux along the Cauchy horizon diverges in a positive sense. This strongly suggests that the metric's back-reaction to the flux of created particles will prevent the formation of naked shell-focusing singularities.

Filtration efficiency of the bay scallop Argopecten irradians and the oyster Crassostrea virginica was examined in the laboratory using dilute algal suspensions, over a range of concentrations from 0.88 to 10.89 mg wet algal weight· 1-1 • Efficiency of retention was measured in flowing seawater for each of 8 size classes of particles (1.73 to 9.97 /l-m diameter) by comparing number of particles in both inhalent and exhalent water. Algal suspensions were composed of isogravimetric amounts of Dunaliella tertiolecta, lsochrysis galbana, Platymonas suecica, and Thalassiosira pseudonana. In low algal concentrations, A. irradians and C. virginica showed no change in filtration efficiency for particles larger than 7 and 3 /l-m in diameter, respectively. As algal concentration increased, A. irradians became more efficient in retaining small (2 to 4 /l-m) particles, due to increased mucus (pseudofecal) production. Conversely, as algal concentration increased, C. virginica periodically became less efficient in retaining small particles, probably due to changes in effective ostial size. A. irradians conditioned to feeding on a large (10 /l-m) alga showed the same filtration efficiency as scallops conditioned on a small (4 /l-m) alga. These results indicate that both species show adjustments of filtration efficiency in concentrations of particulate matter representative of coastal and estuarine environments, and that these changes are utilized to control the amount of food collected for ingestion.

Newly hatched oyster drills, Urosalpinx cinerea (Say), that had not eaten prey and had no prior experience in prey detection, were used to screen 25 potential attractants. Of these, odors of intact, living barnacles, Semibalanus balanoides and Balanus eburneus were most effective, optimally causing upstream migration in over 90% of the snails and retaining detectable activity after 200 fold dilution. The odor of a mixed bryozoan culture evoked a 70% and Sabellaria vulgaris a 30% response, whereas responses to Crassostrea virginica, oyster valves containing Polydora \vebsteri, and Trypetesa lampas were low but still significant. Both rheotactic and chemotactic factors were involved in the upstream migration. Behavior reminiscent of trail search was observed in homogeneous dilute stimulus solutions. It is argued that the odor stimuli may be discrete molecules.

Predatory muricid gastropods,Urosalpinx cinerea, respond to specific chemical stimuli by creeping upcurrent. Attractant substances originate from living barnacles. Newly hatched snails have no prior predatory experience but respond strongly to attractants. We report here methods for rapidly extracting and desalting attractants from seawater. Attractants from living barnacles are relatively large, at least partially proteinaceous, heat-stable molecules (> 1000 but < 10000 dallons) that adsorb onto Amberlite XAD-7, a polyacrylate water purification resin, at neutral pH. Attractants remain adsorbed to the resin during a wash with deionized water and can be eluted in a small volume with 100% methanol. Attractant substances are effective in the bioassay in μg/liter concentrations (octa- to nanomolar range). Potency is destroyed by nonspecific proteases (carboxy-peptidase and pronase) but not by trypsin. Attractant is not sequestered within barnacles.

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