Richard B. Forward

Spectral sensitivity of larvae from intertidal crustaceans

SummaryResponse spectra for positive phototaxis of the planktonic first stage larvae of 7 species of estuarine intertidal crabs were measured. Species living highest intertidally as adults generally have larvae with good ultraviolet (UV) and blue/green sensitivity, while those from adults living lower intertidally lack the UV sensitivity. Comparison of the measured spectra with previous studies of adults indicates that there is probably no change in spectral sensitivity throughout development. This sensitivity is well adapted to the adult intertidal environment, but is not adapted for a planktonic existence in estuarine areas.

Photoresponses of crab megalopae in offshore and estuarine waters: Implications for transport

Abstract This study tested the hypothesis that fiddler crab ( Uca spp. ) and blue crab [ Callinectes sapidus (Rathbun)] megalopae have separate behaviors in offshore and estuarine waters that are evoked by chemical cues. Past field studies indicate that blue crab megalopae are frequently in the neuston during the day offshore, while fiddler crab megalopae are at middle depths. Both species are rarely collected in the water column during the day in estuaries but are abundant during rising tides at night. Since photoresponses contribute to the day distributions of megalopae, we hypothesize that photobehavior changes dramatically between offshore and estuarine areas and used this as an assay for the effects of water-borne chemical cues. Photoresponses were tested in a light field that mimics the underwater angular light distribution and were quantified by measuring the proportion of megalopae swimming in the upper 1/3 of a water column. For blue crab megalopae collected offshore, swimming was significantly greater in offshore than in estuarine water and was suppressed in estuarine water at high light intensities. Thus, photoresponses contribute to depth maintenance near the surface during the day which is important for onshore transport by wind-generated surface currents. When megalopae were collected in an estuary, both species had greater swimming in offshore water than in estuarine water at all light intensities and in darkness. Thus, the absence of megalopae in the water column during the day in estuaries results from light suppression of swimming. Since the water only differed in chemical composition, the changes in photoresponses were induced by chemical cues. Tests with aged estuarine water indicated the chemical cues were relatively stable or initially present at high concentrations. These results support the hypothesis and suggest that megalopae have separate behaviors in coastal/shelf areas and in estuaries, which are induced by chemical cues in offshore and estuarine waters.

THE EFFECT OF SALINITY UPON PHOTOTAXIS AND GEOTAXIS IN A LARVAL CRUSTACEAN

1. Experiments were conducted to determine the effect of salinity on phototaxis and geotaxis by Stages I and IV zoeae of the crab, Rhithropanopeus harrisii.2. Larvae were exposed to sudden salinity changes and stimulated with various intensities of 500-nm light in the horizontal plane. Although the pattern of phototaxis of larvae exposed to 40‰ was unchanged from that at 20‰ (acclimation salinity), the level of positive phototaxis to higher intensities was significantly greater and the level of negative phototaxis to low intensities significantly lower at 40‰. Exposure to low salinity sea water (5‰) generally reverses the sign of phototaxis, since a significantly higher level of negative phototaxis and lower level of positive phototaxis occurs at light intensities above 10-2 Wm-2.3. The minimum amount of salinity decrease from the acclimation salinity that induces a reversal in phototactic sign from positive to negative phototaxis at 0.19 Wm-2 ranges from 1 to 2‰, and appears to be independent of acclimat...

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.

CONTROL OF EGG HATCHING IN THE CRAB RHITHROPANOPEUS HARRISII (GOULD)

Ovigerous females of the crab Rhithropanopeus harrisii were collected from an estuary having irregular tides. When monitored under constant conditions in the laboratory, the crabs have a circadian rhythm in larval release. Eggs removed from the female within 2 days of hatching hatched at about the same time as larvae were released by the female. Hatching became increasingly desynchronized with longer removal times. Upon exposure to water in which the larvae hatched, ovigerous females diplayed increased abdomen pumping, a behavior observed at the time of larval release. The active substance was released at the time of egg hatching but not by newly hatched larvae. Homogenized eggs of different ages and homogenized larvae induced similar behavior. There was no change in female sensitivity with clutch age or time of day. Active pumping by the female only induced hatching at times predicted by the larval release rhythm, not at other times during the solar day. These results indicate that an interaction between...

A SHADOW RESPONSE IN A LARVAL CRUSTACEAN

1. A shadow response consisting of oriented movement to light and gravity was studied by means of a closed circuit television system for stage I zoeae from the crab Rhithropanopeus harrisii.2. If larvae are irradiated at an intensity that induces positive phototaxis and the light is extingushed, both light-and dark-adapted larvae show a descent. Since this response involves passive sinking, it is termed a sinking response. The minimum time that the light must be extinguished to evoke the response is 20 to 30 milliseconds.3. If the light intensity is reduced to a level that should induce negative phototaxis, light-adapted larvae show a sinking response followed by a negative phototaxis. The time delay between the responses is related to the initial stimulus intensity and duration.4. The minimum decrease in intensity that induces the sinking response is equivalent to a reduction by a 0.5 O.D. neutral density filter while the maximum response occurs at optical densities of 1.0 to 1.1 and greater. These value...

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.

TIDAL RHYTHMS OF ACTIVITY AND PHOTOTAXIS OF AN ESTUARINE CRAB LARVA

1. Tidal rhythms in swimming speed and phototaxis were measured in stage III zoeae of the crab Rhithropanopeus harrisii. Larvae were captured in the Newport River estuary, North Carolina, and maintained under constant conditions in the laboratory.2. Swimming speed increases during rising tide with a maximum several hours after low tide, and decreases to a minimum about 2 hr after high tide. It is argued that this pattern is the basis of the endogenous tidal vertical migration, in which the larvae ascend on rising tides and descend on falling tides.3. The tidal rhythm in phototaxis consists of an increase in negative responsiveness which is strongest around the middle of the rising tide, and a decline to a minimum during falling tides. Positive phototaxis shows the opposite pattern. There is no convincing evidence that phototaxis participates in vertical migration. It is hypothesized that the phototactic pattern represents a tidal rhythm in a shadow response used for predator avoidance.

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.

Phototaxis in a dinoflagellate: Action spectra as evidence for a two-pigment system

SummaryAction spectra were determined in the UV region of the spectrum for the first phase of the phototactic response (stop response) and for the phytochrome pigment associated with this response in the dinoflagellate Gyrodinium dorsum Kofoid. Differences between these action spectra indicate the participation of two pigments in phototaxis. Following R (620 nm) irradiation of the phytochrome, the stop response maxima occur at 470 and 280-nm; after FR irradiation they shift to 490 and 300–310 nm. These maxima suggest that the photoreceptor pigment for phototaxis is a carotenoprotein. The action spectrum shift following the different phytochrome conversions may represent a trans to cis isomer change by the carotenoid. The absorption maximum of PR in the UV appears to be at 320 nm, which is consistent with the shift of the R absorption maximum to shorter wavelengths (620 nm) as compared to higher plants. The PFR absorption maximum appears as a broad band between 360 and 390 nm. Comparison of PR to PFR conversions by different intensities of 620-nm and 320-nm light indicates that at lower intensities the logarithm of the threshold for the stop response is inversely proportional to the logarithm of the intensity of the sensitizing light. The ratio of response activation by R and UV light is about 4:1.