Entrainment of the Circadian Rhythm in Egg Hatching of the Crab Dyspanopeus sayi by Chemical Cues from Ovigerous Females.

The subtidal crab Dyspanopeus sayi has a circadian rhythm in larval release with a free-running period of 24.1 h. Under constant conditions, eggs hatch primarily in the 4-h interval after the time of sunset. The study tested the new model for entrainment in subtidal crabs, which proposes that the female perceives the environmental cycles and entrains the endogenous rhythm in the embryos. Results verified the model for D. sayi. Hatching by embryos collected from the field when they had not yet developed eye pigments, and were kept in constant conditions attached to their mother, exhibited the circadian hatching rhythm. Attached embryos could also be entrained to a new photoperiod in the laboratory before they developed eye pigments. Further, mature embryos removed from the female hatched rhythmically, indicating that a circadian rhythm resides in the embryos. However, if mature embryos with eye pigments were removed from the female and exposed to a new light-dark cycle, they could not be entrained to the new cycle; rather, they hatched according to the timing of the original light-dark cycle. Nevertheless, detached, mature embryos would entrain to a new light-dark cycle if they were in chemical, but not physical, contact with the female. Thus, the female perceives the light-dark cycle, and uses chemical cues to entrain the circadian rhythm of hatching by the embryos.

Circadian rhythm in larval release by the crab Rhithropanopeus harrisii: entrainment model.

The subtidal crab Rhithropanopeus harrisii has a circadian rhythm in larval release; under constant conditions eggs hatch in the 2-3-h interval after the time of sunset in nontidal estuaries. Eggs that are removed from the female hatch rhythmically, indicating that the circadian rhythm resides in the embryos. The model for entrainment is that mature embryos have functional sensory systems that detect and entrain to environmental cycles. This model was reexamined by confirming that the visual system of advanced embryos responds to light and thus could mediate entrainment to the light/dark cycle. We then determined whether the hatching rhythm of mature embryos that are removed from the female can be entrained to new light/dark cycles. Contrary to expectations, these embryos did not entrain to new cycles. Instead, they remained entrained to the light/dark cycle to which they were exposed when still attached to the female, suggesting that the female entrains the rhythm. Indeed, hatching by embryos collected from the field when they had not yet developed eye pigments, kept in constant conditions attached to their mother, exhibited the circadian hatching rhythm. They could also be entrained to a new photoperiod in the laboratory. The role of the female is further supported by experiments showing that the hatching rhythm in embryos carried by females lacking one but not both eyes can be entrained to a new cycle in the laboratory. Thus, the revised model is that the female perceives the light/dark cycle and entrains the circadian rhythm in the embryos.

The physiological ecology of the supratidal amphipod Talorchestia longicornis.

Physiology, behavior, habitat, and morphology are used to determine the degree of adaptation to life on land for amphipod species and systemization within the four functional groups of the family talitridae. Talorchestia longicornis is a semi-terrestrial amphipod found in the supratidal zone of estuaries. The present study investigates the physiological adaptations of this species to life on land through measurements of osmoregulation and respiration. Over the salinity range of 1-40‰, T. longicornis regulated its hemolymph hyperosmotically at low salinities and hypoosmotically at high salinities. The isosmotic point was about 27‰. Analogously, hemolymph chloride levels were well regulated being hyperionic at low salinities and hypoionic at high salinities. This species is capable of respiration in both air and water. Slopes (b values) of the relationship between weight and oxygen uptake rates ranged from 0.316 to 0.590. Oxygen uptake rates were higher in air than water and at night versus day. Q(10) values were slightly below 2.0 for respiration in air for amphipods, irrespective of weight. These physiological adaptations, along with its behaviors, habitat, and morphology, place T. longicornis within the Group III sandhoppers of the Talitridae.

Proximate control of diel vertical migration in Phyllosoma larvae of the Caribbean spiny lobster Panulirus argus.

Phyllosoma larvae of the spiny lobster Panulirus argus undergo diel vertical migration (DVM), in which they are at depth during the day and nearer the surface at night. This study determined the visual spectral sensitivity of Stage I larvae and investigated whether light plays a proximate role in DVM as an exogenous cue and as an entrainment cue for an endogenous rhythm in vertical migration. Under constant conditions, larvae have a circadian rhythm (24.5-h period) in vertical swimming that resulted in a twilight DVM pattern. The behavioral response spectrum and electroretinogram recording indicated two photoreceptor spectral classes with maxima at 360 and 486 nm. When stimulated in an apparatus that simulated the underwater angular light distribution, dark-adapted larvae showed only positive phototaxis, with a threshold intensity of 1.8 × 10(13) photons m(-2) s(-1) (3.0 × 10(-5) µmoles photons m(-2) s(-1)). They have an avoidance response to predator shadows in which they descend upon sudden decreases in light intensity of more than 69%. When stimulated with relative rates of decrease in light intensity as occur at sunset they ascended, whereas they descended upon relative rates of light intensity increase as occur at sunrise. Thus, the DVM pattern is controlled by both an endogenous circadian rhythm in swimming and behavioral responses to light at sunrise and sunset.

Visual physiology underlying orientation and diel behavior in the sand beach amphipod Talorchestia longicornis.

Talitrid amphipods employ vision for zonal recovery behaviors on sand beaches and for entraining circadian activity rhythms. Using a hierarchy of methods, we examined visual spectral and response-intensity functions in Talorchestia longicornis, a species in which orientation and rhythm entrainment are wavelength-specific behaviors. Microspectrophotometry, electroretinogram recording and behavioral assays were used to determine visual pigments, retinal spectral sensitivity and whole-animal spectral responsivity, respectively. Diel changes in absolute sensitivity were also investigated at retinal and whole-animal levels. Two receptor spectral classes were identified, with values for visual pigment λ(max) of 427 and 518 nm. Retinal spectral sensitivity varied with electrode position along the distal-proximal axis. Chromatic adaptation of distal and proximal photoreceptors resulted in sensitivity peaks at 430 and 522 nm, respectively. In accordance with identified visual pigments and spectral sensitivity, T. longicornis photobehavioral responsivity covered a broad range (420-580 nm). Collectively, a dual-pigment visual system underlies wavelength-specific behavior in T. longicornis, with the short-wavelength pigment likely to be localized in the distal R5 retinular cell. While response-intensity functions did not change over the diel cycle at the retinal level, behavioral photoresponsiveness varied between day and night. At a wavelength used by T. longicornis for celestial orientation (420 nm), photobehavior was heightened at night, potentially aiding in nocturnal orientation. By contrast, at a wavelength used to entrain its circadian rhythm (520 nm) and for routine visual tasks, photobehavior was heightened during the day, and spectral sensitivity matched to the twilight spectrum, facilitating crepuscular vision and entrainment by irradiance at sunrise and sunset.

Larval biology of the crab Rhithropanopeus harrisii (Gould): a synthesis.

This synthesis reviews the physiological ecology and behavior of larvae of the benthic crab Rhithropanopeus harrisii, which occurs in low-salinity areas of estuaries. Larvae are released rhythmically around the time of high tide in tidal estuaries and in the 2-h interval after sunset in nontidal estuaries. As in most subtidal crustaceans, the timing of larval release is controlled by the developing embryos, which release peptide pheromones that stimulate larval release behavior by the female to synchronize the time of egg hatching. Larvae pass through four zoeal stages and a postlarval or megalopal stage that are planktonic before metamorphosis. They are retained near the adult population by means of an endogenous tidal rhythm in vertical migration. Larvae have several safeguards against predation: they undergo nocturnal diel vertical migration (DVM) and have a shadow response to avoid encountering predators, and they bear long spines as a deterrent. Photoresponses during DVM and the shadow response are enhanced by exposure to chemical cues from the mucus of predator fishes and ctenophores. The primary visual pigment has a spectral sensitivity maximum at about 500 nm, which is typical for zooplankton and matches the ambient spectrum at twilight. Larvae can detect vertical gradients in temperature, salinity, and hydrostatic pressure, which are used for depth regulation and avoidance of adverse environmental conditions. Characteristics that are related to the larval habitat and are common to other crab larval species are considered.

Larval release behaviors in the Caribbean spiny lobster, Panulirus argus: role of peptide pheromones.

Larval release in the Caribbean spiny lobster Panulirus argus is highly synchronous and is controlled by a "pumping pheromone" released from the hatching eggs. The pheromone induces a parent female to undergo stereotypical larval release behaviors, including rapid abdominal extensions and pleopod pumping. These behaviors help to break open the egg membranes and result in the synchronous release of larvae. Based on previous studies on larval release in brachyuran crabs, we hypothesized that larval release behaviors are induced by pheromones composed of small peptides. We quantified pleopod pumping activity upon exposure to a range of synthetic peptides to identify compounds that will induce larval release behaviors. Chemically cued pumping behavior was described in terms of the threshold concentration for response, maximum percentage response, and effective concentration range. Pleopod pumping behavior was evoked by di- and tripeptides with a neutral amino acid at the amino terminus and a basic amino acid at the carboxy terminus and also by the basic-basic dipeptide Lys-Arg. All carboxy-terminal arginine peptides tested produced a significant pumping response, with the exception of Trp-Ile-Arg. Response concentration thresholds ranged from 10(-9) M for the most potent peptide (Gly-Arg) to 10(-4) M for the least potent (Gly-His-Lys). The maximum percentage of lobsters responding was largely independent of the threshold concentration and ranged from 24.3 to 58.3%. Effective concentration ranges for the peptides were variable from 1 to 4 orders of magnitude. Pumping response usually declined with increasing concentration beyond the concentration that evoked the maximum response of the peptides. Our results support the conceptual model that larval release in subtidal crustaceans is controlled by small peptides that act as pheromones.

Larval release rhythm of the mole crab Emerita talpoida (Say).

Ovigerous mole crabs Emerita talpoida (Say) were monitored in the laboratory to determine if the time of larval release is synchronous and under endogenous control. To determine the time of larval release, ovigerous females were placed under a 14:10 light/dark cycle simulating the ambient photoperiod. Hatching was rhythmic, occurring as a quick burst lasting about 5-15 min shortly after the onset of darkness. An individual mole crab will release batches of larvae for up to three successive nights, suggesting that the rhythm is under endogenous control. Mole crabs monitored under constant low-level red light displayed the same release pattern with hatching occurring near the time of expected sunset, indicating the presence of a circadian rhythm in larval release. To investigate whether the female or the embryos control hatching, a portion of the egg mass (50-100 embryos) was separated from the female. The time of hatching of the detached embryos subjected to either a still or shaken treatment was compared with the hatching time of embryos still attached to the female. Detached eggs in both treatments hatched within 1.5-2 h of the time of the female-attached eggs, which suggests that embryos control the timing of hatching.

Spectral sensitivity of vertically migrating marine copepods.

Light is a critical factor in the proximate basis of diel vertical migration (DVM) in zooplankton. A photobehavioral approach was used to examine the spectral sensitivity of four coastal species of calanoid copepod, representing a diversity of DVM patterns, to test whether species that migrate (nocturnal or reverse DVM) have response spectra that differ from non-migratory surface dwellers. The following species were given light stimuli at wavelengths from 350 to 740 nm, and their photoresponses were measured: Centropages typicus (nocturnal migrator), Calanopia americana (nocturnal migrator), Anomalocera ornata (reverse migrator), and Labidocera aestiva (non-migrator). Centropages typicus and A. ornata had peak responses at 500 and 520 nm, respectively, while Calanopia americana had maximum responses at 480 and 520 nm. Thus, the species that undergo DVM have peak photobehavioral responses at wavelengths corresponding to those available during twilight in coastal water, although the range of wavelengths to which they respond is variable. Non-migratory surface-dwelling L. aestiva had numerous response peaks over a broad spectral range, which may serve to maximize photon capture for vision in their broad-spectrum shallow-water habitat.

Behavioral Responses of Crustacean Larvae to Rates of Salinity Change.

The ontogeny of behavioral responses of larvae of the crabs Rhithropanopeus harrisii and Neopanope sayi to rates of change in salinity were analyzed with a video system. A salinity increase evoked an ascent in both species. For R. harrisii the threshold rate of increase was 1.1 x 10-3 ppt s-1 for the first and last zoeal stages and changed little with acclimation salinity. N. sayi larvae were more sensitive, as thresholds were 2.8 x 10-4 ppt s-1 for Stage I zoeae and 7.0 x 10-4 ppt s-1 for Stage IV. This difference in sensitivity may relate to the magnitude of salinity gradients in the estuarine/coastal areas inhabited by the larvae. At threshold rates of salinity increase the absolute amount of change before a response was lower for Stage I zoeae (0.09-0.11 ppt) than Stage IV zoeae (0.21-0.29 ppt) for both species. Decreases in salinity did not induce the expected descent response in either species at rates up to 4.7 x 10-2 ppt s-1. The different responses in a salinity gradient may have resulted because the rate threshold and absolute amount of change before a response to a salinity increase were below those for a salinity decrease. Considering larval sinking rates and normal environmental salinity gradients, larvae of both species can respond to rates and amounts of salinity increase in their environment. The ascent response may be important for keeping larvae up in the water column and reducing the likelihood that they will encounter the bottom.

THE ONTOGENY AND SPECTRAL SENSITIVITY OF POLAROTAXIS IN LARVAE OF THE CRAB RHITHROPANOPEUS HARRISI (GOULD).

1. To determine the ontogeny plus the intensity and spectral characteristics of polarotaxis in larvae of the crab, Rhithropanopeus harrisi, swimming paths in relation to the e-vector direction of linearly polarized light were monitored and recorded with a closed circuit video system. 2. All four zoeal stages were examined. Only stages II and III responded with significant directional swimming (stage II, perpendicular to the e-vector, stage III, parallel to the e-vector; Figs. 3 and 4). Stages I and IV appeared indifferent to the polarization plane (Figs. 2 and 5). Swimming under unpolarized light was consistently random (Table I). 3. Polarotaxis disappeared following dark adaptation of stage II larvae (Fig. 6). 4. The lower intensity threshold at 499-nm light for polarotaxis was found to be between 10-4 and 10-3 W/m2, while the upper threshold is in the range 10-1 to 2 x 10-1 W/m2 (Figs. 3 and 4). 5. Examination of selected wavelengths from 420-620 nm yielded greatest responsiveness at 499 nm (Fig. 7). A rise evident at 420 nm suggests a secondary maximum in the violet. 6. Stage II larvae showed significant positive phototaxis under depolarized light when a collar of alternating black and white quadrants surrounded the experimental vessel. With the e-vector of polarized light aligned parallel to the white sectors, phototaxis was reduced by polarotactic swimming into the black sectors (approximately perpendicular to the e-vector). 7. A comparison with phototaxis (Forward and Costlow, 1974) provides two items of evidence that polarotaxis and phototaxis are indeed separate responses. First, phototaxis is essentially unchanged during larval development, while polarotaxis appears only at stages II and III. Secondly, the responses have different optimum intensities.