Impact of short-term elevated temperature stress on winter-acclimated individuals of the marine gastropod Crepidula fornicata.

The intertidal zone is an especially stressful thermal habitat, typically exposing residents to air temperatures for up to 6 h at a time, twice daily. Tolerance to elevated temperatures has been particularly well-studied for a variety of intertidal species, especially with regard to upper thermal limits during summers. However, in recent years, as climates have been changing around the world, temperate zone intertidal organisms have sometimes been exposed to periods of unusually high air temperatures during the winter. The present study sought to examine the impact of elevated temperatures on survival and clearance rates of winter-acclimated intertidal individuals of the sedentary marine suspension-feeding gastropod Crepidula fornicata. Individuals were collected intertidally from Nahant, Massachusetts from late January to early April each year for 5 years, maintained in the laboratory at the acclimation temperature of 6 °C, and exposed in the laboratory for 3 h to temperatures as high as 37 °C in seawater either once or twice, 24 h apart. Although mean clearance rates were substantially reduced for at least the next 12-24 h after individuals were returned to the 6 °C control condition following exposures to elevated temperatures as low as 21-26 °C, we saw little mortality even following two 3 h exposures to 35 °C, or single exposures to 37 °C. Mortality was substantial, however, in one experiment following a double exposure to 37 °C. Smaller individuals (~5-12 mm in shell length) were somewhat more sensitive to the thermal stress than adults were. Intertidal members of C. fornicata in Massachusetts seem well-prepared to deal with the increasing range of winter air temperatures associated with the global climate confusion predicted for future years. Additional studies will be required to understand the physiological and biochemical mechanisms used by winter-acclimated individuals of this species to tolerate such periodic substantial temperature increases of 29-31 °C.

Neutral point testing of color vision in the domestic cat.

Despite extensive study, the basic nature of feline spectral sensitivity is still unresolved. Most electrophysiological studies have demonstrated two photopic receptors within the cat's retina, one most sensitive to longer wavelengths near 560 nm and the other most sensitive to shorter wavelengths near 460 nm, providing the neuroretinal basis for dichromatic vision. A few studies, however, have detected a third photopic receptor most sensitive to medium wavelengths between 500 and 520 nm, overlapping in spectrally sensitivity with the feline scotopic receptor, that potentially could allow trichromatic vision. Indeed, one behavioral study has demonstrated trichromatic vision in cats, but a flaw within its experimental design raises the possibility that achromatic intensity cues might have allowed the accurate identification of medium wavelength targets. This study tested for a spectral neutral point in the domestic cat using a two-choice discrimination task. The positive targets were created using monochromatic light from various single wavelength light emitting diodes (LEDs) combined with a white light of variable intensity, while the negative targets were created using white light of variable intensity. Trials were performed with varying intensities of positive and negative targets, from brighter positive targets to brighter negative targets, to eliminate achromatic intensity cues. Two cats with prior experience with two-choice discrimination tasks, one male and one female, successfully discriminated monochromatic light from 456 nm to 497 nm and from 510 nm to 524 nm, but both failed to discriminate monochromatic light at 505 nm over multiple trials. These results provide strong evidence that cats are dichromatic with a neutral point near 505 nm. This neutral point is nearly identical to the neutral point of the human deuteuranope, making feline vision a more accurate a model for red-green colorblind individuals than normal trichromats.

The effects of time, luminance, and high contrast targets: revisiting grating acuity in the domestic cat.

Based on optical clarity and retinal cone density, the cat has a potential acuity of 20-30 cycles per degree (cpd), yet most behavioral studies estimate feline acuity between 3 and 9 cpd. Those studies, however, were limited by restrictive experimental conditions that may have inadvertently lowered the estimated grating acuity. Two domestic cats previously trained on a two-choice visual discrimination task were retrained on a grating detection/discrimination task with unlimited time, high luminance, high contrast targets, and adequate space to prevent poor accommodation from affecting the results. Initially, vertical gratings of increasing cpd were tested until failure. Then, horizontal gratings of increasing cpd were tested until failure. Finally, the finest horizontal grating resolved was confirmed with a third test requiring 24 correct out of 36 consecutive trials, yielding a binomial probability less than 0.02 of non-random occurrence. M1, a 7-year-old male gray tabby with +2.00 OU refraction, tested for a grating detection acuity of 15 cpd for both vertical and horizontal gratings (binomial probability = 0.009). F1, a 2-year-old female gray tabby with +0.25 OU refraction, tested for a grating orientation discrimination acuity of 20 cpd for both vertical and horizontal gratings (binomial probability = 0.004). These results demonstrate that a young cat with good focus is capable of discriminating 20 cpd, in close agreement with the physiologic maximum. Uncorrected focusing errors appear to degrade visual performance. Optimum experimental conditions resulted in better grating acuity measurements than previously reported, emphasizing the importance of environmental factors in feline behavioral testing.