Osmotic and Ionic Regulation of North American Zebra Mussels (Dreissena polymorpha)

SYNOPSIS. Unlike other freshwater bivalves that survive formonths in deionized water, Dreissena polymorpha requires minimalconcentrations of Na, K, Mg, and Cl in the bathing medium forlong-term survival. Although ion transport rates are higherin D. polymorpha compared to other freshwater bivalves, theytend to have lower blood solute concentrations. D. polymorphahas an unusually "leaky" epithelium with a high paracellularpermeability to solutes. Thus, even with high transport rates,it may not be possible for zebra mussels to retain higher bloodsolutes because of the extensive passive loss of ions. Undera hyperosmotic stress, D. polymorpha will rapidly osmoconform(about 12 hr) due primarily to the diffusion of solutes andpartially to the osmotic loss of water. D. polymorpha is notcapable of surviving an imbalance of Na/K in the external medium.In the absence of K the cells will tend to lose volume to achieveisosmotic balance with the blood, but the animals usually diewithin a few days. If D. polymorpha is exposed to excess K inthe environment (1 mM), they will accumulate K in the blood.If the K enters the cells, cellular volume would expand dueto increase in osmolyte concentration, yet, if K remains inthe blood, there will be an electrochemical imbalance. In eithercase, the animal cannot survive much longer than a day. WhenNa and K are present in the medium in a balanced combinationapproximated by artificial seawater (ASW), D. polymorpha willsurvive an acute transfer to 100 mosm ASW indefinitely (months).Our preliminary studies have shown that D. polymorpha will toleratestep-wise acclimation to solutions >250 mosm provided thechanges in salinity do not exceed 50–100 mosm. Freshwaterbivalves, unlike the marine bivalves, have limited free aminoacids in their body fluids and must rely on inorganic ions forosmotic regulation. The free amino acids serve as an importantosmolyte buffer for volume regulation when an animal experiencesan environment of changing salinity. The inability of Dreissena,and perhaps other freshwater bivalves, to tolerate hyperosmoticallyinduced dehydration may be due, in part, to the inability toaccumulate or retain sufficient intracellular K to facilitateregulatory volume adjustments.     

The Importance of Body Stiffness in Undulatory Propulsion

During steady swimming in fish, the dynamic form taken by theaxial undulatory wave may depend on the bending stiffness ofthe body. Previous studies have suggested the hypothesis thatfish use their muscles to modulate body stiffness. In orderto expand the theoretical and experimental tools available fortesting this hypothesis, we explored the relationship betweenbody stiffness, muscle activity, and undulatory waveform inthe mechanical context of dynamically bending beams. We proposethat fish minimize the mechanical cost of bending by increasingtheir body stiffness, which would allow them to tune their body'snatural frequency to match the tailbeat frequency at a givenswimming speed. A review of the literature reveals that theform of the undulatory wave, as measured by propulsive wavelength,is highly variable within species, a result which calls intoquestion the use of propulsive wavelength as a species-specificindicator of swimming mode. At the same time, the smallest wavelengthwithin a species is inversely proportional to the number ofvertebrae across taxa (r² = 0.21). In order to determine ifintact fish bodies are capable of increasing bending stiffness,we introduce a method for stimulating muscle in the body ofa dead fish while it is being cyclically bent at physiologicalfrequencies. The bending moment (N m) and angular displacement(radians) are measured during dynamic bending with and withoutmuscle stimulation. Initial results from these whole body workloops demonstrate that largemouth bass possess the capabilityto increase body stiffness by using their muscles to generatenegative mechanical work.     

Comparison of regeneration following burning,clearing or mineral sand mining at Tomago,NSW: I. Structure and growth of the vegetation

Abstract We have begun a long-term ecological research project to address questions about the impact of multiple disturbances on the species richness of communities and whether multiple disturbances are additive or interactive. A protected water catchment area was chosen, which is subjected to fires, sand mining and clearing, and for which detailed records are available. The study area, at Tomago (32°52′S, 151°45′E), has forest, woodland, shrubland and swamp on a sand substrate, with the vegetated dunes forming part of a coastal embayment. Forty-four sites were located in forested areas that had undergone disturbance by either fire, sand mining or clearing. Sites of each disturbance type were grouped into four age classes: less than 1 year since disturbance, nominally 1991; 5 years, nominally 1986; 11 years, nominally 1980; and 17 years, nominally 1974. A set of burned sites, with the time of the last fire matched to the times of the other disturbances, was used as the control response. In this paper we describe the study area and sites, then examine the effects of each single disturbance on vegetation structure. Canopy cover increased with time and type of disturbance, with 17 year old cleared or mined sites similar to the cover of 11 year old burned sites. In the first two years after disturbance, burned sites had significantly more understorey vegetation than cleared or mined sites, but by 5 years all three were similar. The data presented here show that regeneration of mined sites at Tomago is substantially slower than regeneration following disturbance by fire, with the regeneration of cleared sites intermediate but closer to mining than fire. After 17 years regeneration, cleared and sand mined sites had not returned to the vegetation structure of the pre-disturbance state. Understorey height and the amount of vegetation on cleared or mined sites have not achieved the levels in the original forest, although canopy cover did seem to have reached pre-disturbance levels. Current rehabilitation techniques are more sophisticated than those used 17 years ago and continued monitoring of sites currently being rehabilitated may show a faster return to pre-disturbance states. Having established the hierarchy and nature of the response to each single disturbance here, we are now in a position to investigate the impact of multiple disturbances.     

Post-Anthesis Economy of Carbon in a Cultivar of Cowpea

Budgets for transfer of carbon from individual leaves and othersource organs to fruits and nodulated roots were constructedfor stages of the post-flowering development of symbiotically-dependentcowpea (Vigna unguiculata L. Walp. cv. Vita 3-Rhizobium strainCB756). Exportable surpluses of carbon from sources, assessedfrom net exchanges of CO₂ and changes in carbon content, wereallocated to sink organs in proportion to carbon consumption(growth and respiration) and the ability of each sink organto attract assimilates from the sources, as demonstrated by¹⁴C-feeding. The first 10 d after flowering showed high sinkactivity by roots, stem and petioles, low consumption by fruits,with the upper three trifoliate blossom leaves providing thebulk of the required assimilates. The next 10 d showed a sharpdecline in photosynthesis of the leaf subtending the oldestfruit followed by similar declines in leaves at the other fruitingnodes. All leaflets at fruiting nodes abscised during the final10 d period, while the two lower leaves, not subtending fruits,remained green and supplied most of the carbon required by developingfruits and roots. Throughout fruiting all currently-active sourcessupplied all sinks, with only slight evidence of blossom leavesspecializing in nourishing their subtended fruits. Of the carbontranslocated from leaves during fruiting 32% came from the topmostleaf, 28% from the leaf below this, 16% from the next leaf,and the remaining 24% from the lowest three leaves. Some 80%of the fruit's total intake of carbon came from leaves, therest from mobilization of stored carbon (partly sugars and starch)fromother vegetative parts. Key words: Carbon, Translocation, Cowpea     

Effect of Substituted Pyridazinones on Chloroplast Structure and Lipid Metabolism in Greening Barley Leaves

The effect of the substituted pyridazinone herbicides, Sandoz9785 and Sandoz 6706, on lipid metabolism was studied in greeningbarley leaves. The herbicides had no effect on chlorophyll formationbut caused an altered chloroplast morphology during greening.In leaves supplied with {¹⁴C} acetate, Sandoz 9785 decreasedincorporation of radioactivity into linolenate while Sandoz6706 decreased incorporation into both linolenate and trans-3-hexadecenoate.Decreased linolenate labelling was accompanied by an accumulationof {¹⁴C}linolenate in diacylgalactosylglycerol. {¹⁴C}Palmitateaccumulated in phosphatidylglycerol when synthesis of trans-3-hexadecenoatewas inhibited. The results are discussed in relation to thefunction of acyl lipids in fatty acid desaturation and the roleof lipids in chloroplast morphology. Key words: Chloroplast structure, Lipid synthesis, Substituted pyridazinones, Fatty acid desaturation     

Gonad pouches in a new clam-like monoplacophoran from the Silurian of Sweden

Beyrichidiscus fecundatus n. gen. n. sp. is described from the Upper Silurian of Gotland, Sweden. The minute, isostrophically coiled mollusc exhibits profound allometry and is considered to have been a monoplacophoran on the basis of functional interpretation of apertural structures. A clam-like, semi-infaunal mode of life is suggested. A pair of prominent lateral swellings in the adult shell are interpreted as reflecting the location of the gonads.