Gill dimensions in near-term embryos of Amazonian freshwater stingrays (Elasmobranchii: Potamotrygonidae) and their relationship to the lifestyle and habitat of neonatal pups

This comparative study of gill morphometrics in near-term embryos of freshwater stingray potamotrygonids examines gill dimensions in relation to neonatal lifestyle and habitat. In embryos of the potamotrygonids Paratrygon aiereba, Plesiotrygon iwamae, Potamotrygon motoro, Potamotrygon orbignyi, and cururu ray Potamotrygon sp. the number and length of filaments, total gill surface area, mass-specific surface area, water-blood diffusion distance, and anatomical diffusion factor were analysed. In all potamotrygonids, the 3rd branchial arch possessed a larger respiratory surface than the other gill arches. Larger embryos had more gill surface area and large spiracles, which are necessary to maintain the high oxygen uptake needed due to their larger body size. However, the higher mass-specific gill surface area observed in near-term embryos may be advantageous because neonates can use hypoxic environments as refuges against predators, as well as catch small prey that inhabit the same environment. As expected from their benthic mode of life, freshwater stingrays are sluggish animals compared to pelagic fishes. However, based on gill respiratory morphometry (such as gill area, mass-specific gill area, the water-blood diffusion barrier, anatomical diffusion factor, and relative opening of the spiracle), subtypes of lifestyles can be observed corresponding to: active, intermediate, and sluggish species according to Gray's scale.

Filtering capacity and endoscopic analysis of sympatric infaunal and epifaunal bivalves of southern Chile

The bivalves Mytilus chilensis, Venus antiqua, Mulinia edulis and Tagelus dombeii inhabit different levels of the tidal flats in Yaldad Bay, southern Chile. Mytilus chilensis is an epifaunal bivalve, which, at this location, is also farmed in culture rafts; whereas the other species belong to the infauna. The objective of this study was to compare filtering capacity of these species; in order to do this, measurements of food consumption were taken by quantifying clearance rates, as well as taking morphological measurements of their feeding systems (gills and labial palps), and endoscopic observations to determine the velocity of particle transport within the gills. All of these parameters were related to the sediments present in environments occupied by these species. In Mytilus chilensis, both wild and farmed, gill area was significantly higher than in the three infaunal species. Mulinia edulis and Venus antiqua did not show any significant differences in gill area. As far as Tagelus dombeii is concerned, the species presented the lowest values among all species studied. Weight of labial palps was significantly higher in Mulinia edulis compared to the other species. There were no significant differences between labial palps of either wild or farmed Mytilus chilensis or in those of Venus antiqua and Tagelus dombeii. A positive, significant relationship was identified between labial palps size and mud sediment content. Both culture-rafts and intertidal Mytilus chilensis displayed a clearance rate that was significantly higher than the infaunal species, concurring with a significant relationship between clearance rate and gill area in the different species studied. Clearance rates displayed by Venus antiqua, Mulinia edulis and Tagelus dombeii did not show significantly different results among them. Mytilus chilensis from both culture-rafts and intertidal zone displayed higher values of particle transport velocity than the infaunal species. Particle transport was not observed in the dorsal canal of any of the species studied. The conclusion is that in these species filtering capacity is determined mostly by gill size and that labial palp size displays a strong relationship with sediment content in the mud of Yaldad tidal flats.

Variations in pallial organs and eulatero-frontal cirri in response to high particulate matter concentrations in the oyster Crassostrea gigas

Variations in gill, palp, and adductor muscle areas and in the length and spacing of eulatero-frontal cirri (EFC) were investigated in specimens of the Japanese oyster (Crassostrea gigas) within the same bay in two nearby sites (15 km) differing in turbidity. Significant variations occurred on a small geographical scale. In particular, individuals from the higher turbidity zone had smaller gills and larger labial palps. For a standard oyster (1 g dry body weight), gill area was 25% larger in oysters from the low-turbidity zone, while palp area was 66% larger in oysters from the high-turbidity zone. No difference was observed for total adductor muscle area. Intersite differences in the mean length of EFC and in the distance between two adjacent cirri sampled on ordinary filaments in the midregion of the gill were studied by image analysis. Cirri tended to be longer and more evenly spaced in oysters from the higher turbidity zone, although differences were not significant at the 5% probability level. The geometric characteristics of the EFC were apparently not modified in response to high turbidity, although the wide individual variability observed may have prevented detection of ciliary adaptation.

The scaling and potential importance of cutaneous and branchial surfaces in respiratory gas exchange in larval and juvenile walleye

Measurements were made of the surface areas (As) of the skin and gills of larval and juvenile walleye Stizostedion vitreum with a body mass (M) of between 2mg (1 day post hatch) and 2.3g (98 days post hatch). The skin, with a relative surface area (As/M) of approximately 8500mm2g-1, accounted for more than 99.9% of the total surface area (skin + gills) at 1 day post hatch. The relative area of the skin decreased as fish grew at an allometric rate of b-1=-0.32±0.01 (mean ± s.e.m., where b-1 is the specific-mass exponent in the allometric equation YxM-1=aMb-1, in which Y is surface area and a is a constant). The relative surface area of the gills (filaments + lamellae) increased in a hyperbolic fashion from very low levels (approximately 5mm2g-1) at 1 day post hatch to reach a maximum of approximately 1100mm2g-1 at a body mass of approximately 200mg. Thereafter, relative gill area declined at an allometric rate of b-1=-0.19±0.10 (mean ± s.e.m.). Gill area, because it declined at a slower relative rate, finally exceeded skin area at a body mass of approximately 700mg. The relative surface area of the skin and gills combined (total surface area) decreased at a more-or-less constant allometric rate of b-1=-0.21±0.01 (mean ± s.e.m.) throughout the experimental period. On the basis of the allometric rates of expansion, the structural capacity to supply oxygen (b-1=-0.19; total gill area, this study) and metabolic demand for oxygen (b-1~-0.13; mean literature value for routine and resting metabolism) appear to remain fairly closely matched in postlarval walleye (>300mg). The two parameters do not display the same degree of concordance during larval development. In larvae, total respiratory surface area declines on a mass-specific basis at roughly the same rate (b-1=-0.21) as gill area does in older fish but, unlike in older fish, metabolic demand for oxygen does not change (b-1~0.0). This results in a progressive decline in effective respiratory surface area (As/M.O2) but does not affect O2 uptake, probably because larvae are so small that surface area is not the limiting factor in gas exchange. Analysis of data from the literature suggests that surface area typically becomes limiting at a body mass of approximately 100mg. The major function of gills in smaller larvae (<100mg) appears to involve ionoregulation or related aspects of acid­base balance rather than respiratory gas exchange.