Robert F. McMahon

Thermal tolerance, evaporative water loss, air-water oxygen consumption and zonation of intertidal prosobranchs: a new synthesis

Duration of emergence increases with tidal height on rocky shores therefore, emergence adaptations in intertidal species such as littorine and other prosobranch gastropods have been considered correlated with zonation patterns; temperature tolerance, desiccation resistance and aerial respiration rate all commonly assumed to increase progressively with increasing zonation level. Such direct correlations are rarely observed in nature. Maximal aerial gas exchange occurs in mid-shore, not high shore species. Temperature tolerance and desiccation resistance do not increase directly with shore height. Thus, hypotheses regarding physiological correlates of zonation require revaluation. A new hypothesis is presented that the high tide mark presents a single major physiological barrier on rocky shores. Above it, snails experience prolonged emergence and extensive desiccation; below it, predictable submergence and rehydration with each tidal cycle. Thus, desiccation stress is minimal below the high tide mark and maximal above it. Therefore, species restricted below high tide (the eulittoral zone) should display markedly different adaptive strategies to emergence than those above it (the eulittoral fringe). A review of the literature indicated that adaptations in eulittoral species are dominated by those allowing maintenance of activity and foraging in air including: evaporative cooling; low thermal tolerance; elevated aerial O2 uptake rates; and high capacity for radiant heat absorption. Such adaptations exacerbate evaporative water loss. In contrast, species restricted to the eulittoral fringe display adaptive strategies that minimize desiccation and prolong survival of emergence including: foot withdrawal, preventing heat conduction from the substratum; aestivation in air; elevated thermal tolerance reducing necessity for evaporative cooling; position maintenance by cementation to the substratum and increased capacity for heat dissipation. In order to test of this hypothesis the upper thermal limits, tissue and substratum temperatures on emergence in direct sunlight and evaporative water loss and tissue temperatures on emergence in 40 °C were evaluated for specimens of six species of eulittoral and eulittoral fringe gastropods from a granite shore on Princess Royal Harbour near Albany, Western Australia. The results were consistant with adaptation to the proposed desiccation barrier at high tide. The eulittoral species, Austrocochlea constricta, Austrocochlea concamerata, Nerita atramentosa and Lepsiella vinosa, displayed adaptations dominated by maintenance of activity and foraging during emergence while the eulittoral fringe littorine species, Bembicium vittatum and Nodilittorina unifasciata displayed adaptations dominated by minization of activity and evaporative water loss during emergence. The evolution of adaptations allowing tolerance of prolonged desiccation have allowed littorine species to dominate high intertidal rocky shore gastropod faunas throughout the world’s oceans.

11 – MOLLUSCA: BIVALVIA

North American (NA) freshwater bivalve molluscs (class Bivalvia) fall in the subclasses Paleoheterodonta (Superfamily Unionoidea) and Heterodonta (Superfamilies Corbiculoidea and Dreissenoidea). They have enlarged gills with elongated, ciliated filaments for suspension feeding on plankton, algae, bacteria, and microdetritus. The mantle tissue underlying and secreting the shell forms a pair of lateral, dorsally connected lobes. Mantle and shell are both single entities. During development, the right and left mantle lobes extend ventrally from the dorsal visceral mass to enfold the body. Each lobe secrets a calcareous shell valve which remains connected by a mid-dorsal isthmus. Like all molluscs, the shell valves consist of outer proteinaceous and inner crystalline calcium carbonate elements. The lateral mantle lobes secrete shell material marked by a high proportion of crystalline calcium carbonate making them thick, strong and inflexible, while the mantle isthmus secretes primarily protein, forming a dorsal elastic hinge ligament uniting the calcareous valves. The hinge ligament is external in all freshwater bivalves. Its elasticity opens the valves while the anterior and posterior shell adductor muscles run between the valves and close them in opposition to the hinge ligament which opens them on adductor muscle relaxation. The key presented in this chapter, as well as many of the major keys to freshwater bivalves, relies on the integrated use of important anatomical structures and shell characters to identify unionoidean bivalves. This key is artificial, and it is divided into four sections corresponding to geographical provinces to facilitate identification.

Response to temperature and hypoxia in the oxygen consumption of the introduced asiatic freshwater clam Corbicula fluminea (Müller)

Abstract 1. 1. Respiratory response to temperature and hypoxia was determined for specimens of the introduced freshwater Asiatic clam, Corbicula fluminea (Muller). 2. 2. O2 uptake increases with temperature through 25°C in clams acclimated to 10, 20 or 30°C but is severely depressed above it. 3. 3. Clams acclimated to 10°C respond to hypoxia as strict O2 conformers while 20 and 30°C acclimated clams are less than strict conformers. 4. 4. Respiratory adaptation to high temperatures and hypoxia are not as well developed in C. ftuminea as other freshwater bivalve species, reflecting a more recent penetration of freshwater, and limiting its eventual distribution in North America.

Exploration of structure-antifouling relationships of capsaicin-like compounds that inhibit zebra mussel (Dreissena polymorpha) macrofouling.

Abstract Macrofouling of aquatic man-made structures by zebra mussels (Dreissena polymorpha) poses significant economic burdens on commercial freshwater shipping and facilities utilising raw water. The negative environmental impact of some current antifouling technologies has limited their use and prompted investigation of non-organometallic and non-oxidising antifoulants as possible environment-friendly alternatives. The plant-derived natural product capsaicin and 18 other compounds with one or more capsaicin-like structural features were tested for their potential to inhibit zebra mussel byssal attachment at a single high concentration of 30 μM. Of these, three compounds displaying the highest levels of attachment inhibition where selected for further concentration-response testing. This testing revealed that capsaicin (8-methyl-N-vanillyl-trans-6-nonenamide), N-vanillylnonanamide, and N-benzoylmonoethanolamine benzoate all inhibited byssal attachment with potency values (EC50) in the micromolar range. None of these compounds were lethal to adult specimens of the water flea, Daphnia magna, at concentrations that inhibited mussel byssal attachment.

Seasonal respiratory responses to temperature and hypoxia in relation to burrowing depth in three intertidal bivalves

Respiratory responses to temperature and hypoxia in relation to burrowing depth were determined for winter- (W-C) and summer-conditioned (S-C) individuals of Tellina tenuis, Macoma balthica, and Carastoderma edula. These bivalves occur sympatrically on sand-flats but display different sediment burrowing depths (C. edule, <2 cm; M. balthica, 3–8 cm; T. tenuis, 5–20 cm). A range depths over which daily temperature variation and O2 concentration decline rapidly from surface values. Species thermal tolerance limits were found to decrease and to be more greatly temperature compensated with burrowing depth. Oxygen consumption rate (VO2 increased with temperature to 25°C in C. edule, but was, thereafter, regulated (Q10 ca 1.0) up to 40°C while VO2 increased with temperature in M. balthica and T. tenuis until thermally streased at 25° to 30°C. The deposit feeding M. balthica does not acclimate VO2 to temperature while C. edule and T. tenuis, both suspension feeders, show “reverse” acclimation [VO2 (W-C) < VO2 (S-C)] that conserves overwintering energy stores. The shallow burrowing C. edule and deposit feeding M. balthica rarely experience hypoxia and are poor to non-regulators of VO2 in reduced O2 concentrations. In contrast, when winter-conditioned, T. tenuis is a moderate regulator of VO2, the degree of regulation increasing in S-C individuals which are exposed to higher levels of hypoxia.

Effects of Hypoxia and Low-Frequency Agitation on Byssogenesis in the Freshwater Mussel Dreissena polymorpha (Pallas)

The effect of variations in PO2 and agitation rate on byssogenesis, motility, and survival of the zebra mussel (Driessena polymorpha) was investigated. Mussels exposed to a PO2 {le} 15.4 torr exhibited increased mortality, reduced motility, and significant suppression of byssogenesis. At 7.7 and 15.4 torr, mean survival times were 5.2 and 5.8 days, maximum survival times being 15 and 16 days, respectively. After 21 days at a PO2 of 23.1 torr, sample mortality was 33.3% and declined to 18.2% at 30.9 torr. There was no mortality at full air O2 saturation (~ 154.3 torr). Adult zebra mussels exhibited the highest rate of byssogenesis in still water (0 cycles per minute [CPM]). Rate of byssogenesis progressively decreased as agitation rate increased. At 30 and 40 CPM, rate of byssal thread production was significantly lower than at 0 CPM. After 21 days, means of 58.6 and 44.8 byssal threads/mussel were found in the byssal mass of specimens exposed to 30 and 40 CPM, respectively, significantly fewer than the mean of 92.7 threads/mussel recorded in still water. Suppression of byssogenesis in D. polymorpha under hypoxic conditions is a response similar to that reported for the marine mytilid Mytilis edulis; however, suppression of byssogenesis with elevated agitation rate is the opposite response to that reported for M. edulis.

Effects of Artificially Elevated Water Temperatures on the Growth, Reproduction and Life Cycle of a Natural Population of Physa Virgata Gould

Abstr-act. Plhysa virgata Gould is the common pond snail of north central Texas. From two natural populations, one affected by heated discharge from a power plant in Lake Arlington, Texas, regular samples were collected from August 1972 until April 1974. Investigations of growth, reproduction, life cycle and the effects of heated effluent showed that each population had three generations/year. More northern physid populations have a simple annual pattern or at most two complete generations/year. Even without artificially elevated water temperatures the longer growing seasons which occur in southwestern USA allow P. iirgata populations to achieve three generations/year. The autecology of these two P. virgata populations was remarkably similar even though one was in an area affected by thermal effluent. Observed differences include a more rapid rate of growth of the second generation in the discharge area (probably as a result of higher summer water temperatures) and a significant decrease in mean number of eggs per mass laid by the discharge population.

Effluent-induced interpopulation variation in the thermal tolerance of Physa virgata Gould

Abstract 1. 1. Seasonal variation and acclimation of thermal tolerance limits were investigated in two partially isolated populations of the common fresh water pulmonate snail Physa virgata Gould from Lake Arlington in North Central Texas, one of which occurs in an area receiving heated effluent from a steam-electric power plant. 2. 2. In both populations summer-conditioned specimens had thermal tolerance limits 8.4°C higher than winter-conditioned specimens. 3. 3. Specimens of P. virgata from the area receiving heated effluent had significantly higher mean heat coma temperatures than snails from the unaffected area after 2 weeks laboratory acclimation to 10, 20 and 30°C. 4. 4. The possible occurrence of thermal effluent-induced “physiological race” formation in P. virgata is discussed in regard to the effects of thermal pollution on freshwater invertebrates.

Geographical variation in heat coma temperatures in Littorina species (Mollusca: Gastropoda)

Upper critical thermal limits were measured as heat coma temperatures in Littorina species (Mollusca: Gastropoda) from sites around the British Isles. Thermal tolerance was found to be stable within a population but extensive variation occurred between populations and species. The majority of species examined displayed heat coma values of around 30°C and did not show a positive correlation with shore height. The highest capacity for thermal tolerance was found along the coast of South Wales, while the lowest was found on the south-west coast of Ireland and on the east coast of Scotland.

Cannabinoids inhibit zebra mussel (Dreissena polymorpha) byssal attachment: a potentially green antifouling technology

Macrofouling by zebra mussels (Dreissena polymorpha) has serious environmental, economic and legal consequences for freshwater shipping and raw water facilities. Current antifouling technologies, such as organometallics or aggressive oxidisers, have negative environmental impacts limiting their application. As part of an effort to discover antifoulants with a reduced environmental footprint, the endocannabinoid, anandamide and nine other compounds sharing structural or functional features were tested for their ability to inhibit zebra mussel byssal attachment. A byssal attachment bioassay identified six efficacious compounds; four compounds also had no negative impact on mussels at concentrations maximally inhibiting byssal attachment and three of them had no significant cumulative toxicity towards a non-target organism, Daphnia magna. This discovery demonstrates that both naturally occurring and synthetic cannabinoids can serve as non-toxic efficacious zebra mussel antifoulants. Applications with this technology may lead to a new genre of cleaner antifoulants, because the strategy is to prevent attachment rather than to poison mussels.