Shell Erosion Research Articles

Epibiont communities on mussels in relation to parasitism and location in the rocky intertidal zone.

The factors shaping host-parasite interactions and epibiont communities in the variable rocky intertidal zone are poorly understood. California mussels, Mytilus californianus, are colonized by endolithic cyanobacterial parasites that erode the host shell. These cyanobacteria become mutualistic under certain abiotic conditions because shell erosion can protect mussels from thermal stress. How parasitic shell erosion affects or is affected by epibiotic microbial communities on mussel shells and the context dependency of these interactions is unknown. We used transplant experiments to characterize assemblages of epibiotic bacteria and endolithic parasites on mussel shells across intertidal elevation gradients. We hypothesized that living mussels, and associated epibacterial communities, could limit colonization and erosion by endolithic cyanobacteria compared with empty mussel shells. We hypothesized that shell erosion would be associated with compositional shifts in the epibacterial community and tidal elevation. We found that living mussels experienced less shell erosion than empty shells, demonstrating potential biotic regulation of endolithic parasites. Increased shell erosion was not associated with a distinct epibacterial community and was decoupled from the relative abundance of putatively endolithic taxa. Our findings suggest that epibacterial community structure is not directly impacted by the dynamic symbiosis between endolithic cyanobacteria and mussels throughout the rocky intertidal zone.

Organismal Responses to Coastal Acidification Informed by Interrelating Erosion, Roundness and Growth of Gastropod Shells.

urrent understanding of how calcifying organisms respond to externally forced oceanic and coastal acidification (OCA) is largely based on short-term, controlled laboratory or mesocosm experiments. Studies on organismal responses to acidification (reduced carbonate saturation and pH) in the wild, where animals simultaneously interact with a range of biotic and abiotic circumstances, are limited in scope and interpretation. The present study aimed to better understand how gastropod shell attributes and their interrelations can inform about responses to coastal acidification. We investigated shell chemical erosion, shell roundness, and growth rate of Planaxis sulcatus snails, which are locally exposed to acidified and non-acidified rocky intertidal water. We tested a new approach to quantifying shell erosion based on the spiral suture length (EI, erosion index) and found that shell erosion mirrored field acidification conditions. Exposure to acidification caused shells to become rounder (width/length). Field growth rate, determined from apertural margin extension of marked and later recaptured snails, was strongly negatively related to both shell erosion and shell roundness. Since different shell attributes are indicative of different relationships-shell erosion is an extrinsic passive marker of acidification, and shell roundness and growth rate are intrinsic performance responders-analyzing their interrelations can imply causation, enhance predictive power, and bolster interpretation confidence. This study contributes to the methodology and interpretation of findings of trait-based field investigations to understand organismal responses to coastal acidification.

Tracking coastal acidification from erosion of gastropod shells: spatial sensitivity and organism size effect.

The rapidly changing marine environmental chemistry associated with growing industrialisation, urban population expansion, and the unabated rise in atmospheric CO2 necessitates monitoring. Traditional approaches using metres, dataloggers, and buoys to monitor marine acidification have limited application in coastal oceans and intertidal zones subjected to direct wave action. The present study trialled a system to biomonitor coastal acidification (carbonate ion and pH) based on the dissolution of living gastropod shells. We extended on an approach that ranked shell erosion (SER) in Nerita chamaeleon (Nc) in environments where such erosion was found to correlate with exposure to acidified water. We assessed the spatial scale at which the Nc-SER marker could detect change in acidification along rocky shores, and whether snail body size affected this marker. We found that proportional and unique Nc-SERs not only varied between acidified and non-acidified reference shores at a coarse spatial scale (10km), but also in predictable ways at fine scales (metres), vertically and horizontally within a shore. Differences between acidified and reference shores in the relationship for snail size and Nc-SER were accentuated by less weathered shells at reference localities, highlighting the value of including small, juvenile snails in monitoring protocols. Gastropod shells are shown to be useful for assessing point sources of acidification and the spatial area of affected coastal zones. This cost-effective and easy-to-use approach (potentially even by citizen-scientists) offers an early warning system of acidification of rocky shore ecosystems, where the deployment of instruments is precluded.

Ectoparasites reduce scope for growth in a rocky-shore mussel (Perna perna) by raising maintenance costs

Endolithic cyanobacteria are ubiquitous colonisers of organic and inorganic carbonate substrata that frequently attack the shells of mussels, eroding the shell to extract carbon, often with population infestation rates of >80%. This reduces host physiological condition and ultimately leads to shell collapse and mortality, compromising the services provided by these important ecosystem engineers. While the ecological implications of this and similar interactions have been examined, our understanding of the underlying mechanisms driving the physiological responses of infested hosts remains limited. Using field and laboratory experiments, we assessed the energetic costs of cyanobacterial infestation to the intertidal brown mussel (Perna perna). In the field we found that growth (measured as both increase in shell length and rate of biomineralization) and reproductive potential of clean mussels are greater than those of infested individuals. To explore the mechanisms behind these effects, we compared the energy allocation of parasite-free and infested mussels using the scope for growth (SFG) framework. This revealed a lower SFG in parasitized mussels attributed to an energetic imbalance caused by increased standard metabolic rates, without compensation through increased feeding or reduced excretion of ammonia. Separate laboratory assays showed no differences in calcium uptake rates, indicating that infested mussels do not compensate for shell erosion through increased mineralization. This suggests that the increased maintenance costs detected reflect repair of the organic component of the inner nacreous layer of the shell, an energetically more demanding process than mineralization. Thus, parasite-inflicted damage reduces SFG directly through the need for increased basal metabolic rate to drive shell repair without compensatory increases in energy intake. This study provides a first perspective of the physiological mechanisms underlying this parasite-host interaction, a critical step towards a comprehensive understanding of the ecological processes driving dynamics of this intertidal ecosystem engineer.

Morphological and molecular evidence support the intertidal barnacle Octomeris intermedia Nilsson-Cantell, 1921 (Thoracica, Chthamalidae) as a valid species in Indo-Pacific waters.

Octomeris is a chthamalid intertidal barnacle with eight shell plates. There are currently two species of such barnacles: O. brunnea Darwin, 1854 (type locality in the Philippines), common in the Indo-Pacific region, and O. angulosa Sowerby, 1825, only recorded in South Africa. Octomeris intermedia Nilsson-Cantell, 1921, identified from the Mergui Archipelago in Myanmar, was considered to be conspecific with O. brunnea by Hiro (1939) based on samples collected in Taiwan. The morphological differences in shell and opercular plates between O. brunnea and O. intermedia are believed to be intra-specific variations due to different degrees of shell erosion. In the present study, the genetic and morphological differentiations of Octomeris in the Indo-Pacific region were examined. This study found two molecular clades (with inter-specific differences) based on the divergence in the COI genes, and the species also have distinct geographical distributions. The Octomeris brunnea clade covers samples collected from the Philippines and Taiwan waters and the other clade, which we argue is O. intermedia, is distributed in Phuket and Krabi, Thailand and Langkawi, Malaysia. Phuket and Krabi are located approximately 300 km south of the Mergui Archipelago, the type locality of O. intermedia. The morphology of samples collected from Thailand fits the type description of O. intermedia in Nilsson-Cantell (1921). Our study concludes that O. intermedia is a valid species based on morphological and molecular evidence.

Biomonitoring acidification using marine gastropods.

Ocean acidification is mainly being monitored using data loggers which currently offer limited coverage of marine ecosystems. Here, we trial the use of gastropod shells to monitor acidification on rocky shores. Animals living in areas with highly variable pH (8.6-5.9) were compared with those from sites with more stable pH (8.6-7.9). Differences in site pH were reflected in size, shape and erosion patterns in Nerita chamaeleon and Planaxis sulcatus. Shells from acidified sites were shorter, more globular and more eroded, with both of these species proving to be good biomonitors. After an assessment of baseline weathering, shell erosion can be used to indicate the level of exposure of organisms to corrosive water, providing a tool for biomonitoring acidification in heterogeneous intertidal systems. A shell erosion ranking system was found to clearly discriminate between acidified and reference sites. Being spatially-extensive, this approach can identify coastal areas of greater or lesser acidification. Cost-effective and simple shell erosion ranking is amenable to citizen science projects and could serve as an early-warning-signal for natural or anthropogenic acidification of coastal waters.

What is the matter with Corbicula fluminea (Mollusca: Bivalvia: Cyrenidiae)? Occurrence of unusual shell erosion in Northern Italy

ABSTRACTCorbicula fluminea is a bivalve considered to be among the 100 worst invasive species in Europe due to its economic and ecological impacts in freshwater ecosystems. Here, we report for the first time a phenomenon of shell erosion observed in two distinct populations of C. fluminea collected in 2015 from two semi-natural small streams of the River Ticino hydrographic basin (Northern Italy). We measured several biometric variables in order to quantify the shell damage: length and thickness of the right valve and the shell free dry weight of each specimen and compared results with those of ‘normal’ populations from another stream of the same area. Our results confirmed that there was abnormal shell erosion and a reduction in shell thickness in both populations. We hypothesised several potential causes for the observed erosion, but none of them could provide sufficient explanation: neither the pH of the water, nor the level of calcium or total hardness of the water or granulometry of the substrates differed among streams. Similarly, predation pressure cannot be considered a reasonable explanation, since in the study area only a few animals actively prey on these bivalves and the marks they leave on the shells are completely different from the ones observed on our specimens. Lastly, we hypothesised that the bivalves may have been affected by a disease or parasitic infestation that had weakened their shells or prevented them from self-repairing and identified a fungal parasite that could possibly cause a disease to C. fluminea, namely the Ascomycota Geotrichum candidum. However, this species occurred only in one of the two abnormally eroded populations. In conclusion, a valid explanation for the morphological alteration that our study revealed has yet to be found.

Effects of CO2 enrichment on cockle shell growth interpreted with a Dynamic Energy Budget model

The increase in human induced atmospheric CO2 level leads to an increase in ocean acidification (OA). Mitigation of this increase by storage of CO2 in abandoned marine oil and gas reservoirs is seen as an interesting cost effective solution. However, this involves a risk of CO2 loss causing localised reductions in seawater pH. In this paper we report on the effects of CO2 enhancement on the growth of the bivalve Cerastoderma edule in mesocosms. The experiments show significant reductions in shell length, shell weight and cockle flesh dry weight at increased CO2 level suggesting both direct (shell erosion) and indirect (metabolic) effects. Indirect effects were analysed and interpreted using a Dynamic Energy Budget model by describing changes in 3 metabolic processes: assimilation, maintenance, and growth. Based on cockle size data only we could not differentiate between these processes, however, by using variability of DEB parameter values in 11 bivalve species, we showed growth to be the least relevant process.

Impact of trematodes on the population structure and shell shape of the estuarine mud snail Hydrobia ulvae from a Southern European estuary

AbstractThe occurrence of trematodes within the gastropod Hydrobia ulvae was studied in two areas (a Zostera noltii bed and a eutrophic area) in the Mondego Estuary, Portugal. The aim was to assess trematode infections across snail age classes, as well as to investigate any influence of parasitism on the shell shape and size of these gastropods. In the Z. noltii bed, infection occurred in snails with shell height of 1.5–6 mm, but a higher prevalence was seen in intermediate size classes (3–5 mm). Infected individuals were never found among the elder gastropods. In the eutrophic area, infection was commonly observed within small individuals (1.6–2.5 mm), as large individuals were rarely found. The warm temperate climate in this coastal system may cause infected specimens to die before they can grow to larger dimensions, in contrast to Northern European populations. The frequency of snails with thin, distorted and corroded shells was higher among infected than non‐infected snails. Infected individuals were frequently colonized by epibionts that may have additional detrimental effects and probably promote shell erosion. Infection with trematodes and their influence on snail size needs consideration in the evaluation of parameters related to population dynamics and population structure. If the implications of parasite infection are ignored, inadequate decisions about ecosystem management may be taken, resulting mainly from erroneous assessment of population structure, age composition and ecological longevity. The role of parasites is especially important to consider for systems under environmental stress.

Fusarium avenaceum causes burn spot disease syndrome in noble crayfish (Astacus astacus)

Burn spot disease has been causing epidemics both in the Estonian mainland and in Saaremaa Island in the threatened noble crayfish (Astacus astacus) stocks. To study the cause of the disease, we isolated several Fusarium spp. from Estonian noble crayfish (A. astacus) populations suffering from burn spot disease syndrome. We first identified fungi directly from melanised cuticle by their ITS sequences. Then we isolated Fusarium spp. from melanised spots of crayfish showing burn spot disease symptoms, such as melanisation and shell erosion, from two different crayfish populations and watercourses in Estonia. The isolates were then identified based on ITS and EF1α-gene sequences. Isolates of Fusarium spp. taken from two separate Estonian noble crayfish populations were used in infection studies. Koch postulates confirmed that the studied agent was causing burn spot disease symptoms including shell erosion in the noble crayfish, which were significantly more severe after molts. After the infection period, an identical Fusarium spp. was re-isolated from carapace lesions and was thus shown to be the disease agent causing burn spot disease syndrome and shell erosion in noble crayfish. Based on GenBank database searches, the isolates causing burn spot disease symptoms were identified as Fusarium avenaceum in mainland Estonia and F. solani in Saaremaa crayfish.

Evaluation of Passive Integrated Transponders for Abalone: Tag Placement, Retention, and Effect on Survival

Since 1969, abalone populations have declined globally more than 50%, with many species now recognized as threatened, endangered, or species of concern. As monitoring progresses and restoration efforts evolve to include population supplementation, a reliable and robust method of tagging individual abalone is needed. Current abalone tagging methods are unsatisfactory, particularly for long-term studies as a result of tag loss, shell erosion, and encrustation. Observing tag numbers of cryptically positioned abalone can be difficult. To obviate these issues, we evaluated passive integrated transponders (PITs) as tags for pinto abalone (Haliotis kamtschatkana kamtschatkana). We applied 9-mm PITs with cyanoacrylate glue to the dorsal exterior of the shell and to the ventral anterior of the shell, and by injection into the foot muscle of small adults (trial 1), and applied PITs to the ventral anterior of the shell of juveniles (trial 2). We subsequently tracked growth, survival, and tag retention over 15 mo in trial 1 and 6 mo in trial 2 in captivity. Among small adults (trial 1), differences in relative growth rate and survival were not significant. PIT retention by adhesion to the ventral anterior and dorsal exterior was significantly greater than injection into the foot in trial 1. Between controls and tagged animals in trial 2, differences in survival were not significant. There was no significant difference in ventral anterior tag retention between trial 1 and trial 2. Gluing PITs on the ventral anterior of the shell is a promising method because abalone quickly formed nacre over the tags, incorporating them into the shell, which does not appear to affect tag detection by the PIT reader. Trials are underway to characterize PIT retention in natural habitats, to determine tag longevity, and to use PITs to track adults reintroduced to aggregations.

The effect of calcium and pH on Florida apple snail, Pomacea paludosa (Gastropoda: Ampullariidae), shell growth and crush weight

Pomacea (Ampullariidae) snails, commonly referred to as apple snails, serve as prey for many freshwater-dependent predators, and some species are highly invasive. Identifying limits to apple snail distribution and abundance are pertinent to understanding their ecology. Calcium (Ca2+) availability and pH generally influences freshwater snail populations, yet scant data exist for Pomacea snails. We measured 6-week change in shell length (ΔSL) in P. paludosa in two laboratory experiments with varying Ca2+ and pH levels. ΔSL was significantly higher in ≥28 mg Ca2+/l compared with treatments ≤14 mg/l. Snails from populations living in low Ca2+/pH waters did not appear genetically predisposed at growing faster in these conditions. Smallest ΔSL was in snails treated with 3.6 mg Ca2+/l and pH < 6.5 water; these snails had signs of shell erosion. Shell crush weights (CWs) were lowest for snails grown in the lowest Ca2+/pH treatment. Smaller shells and lower CWs have implications for predation vulnerability and reproductive success. Our results are consistent with reports associating relatively low snail densities with relatively low Ca2+/pH waters, and they are consistent with the geographic distribution of P. paludosa as related to the underlying water chemistry as influenced by geology.

Variability of growth, health, and population turnover within mussel beds of Perna canaliculus in northern New Zealand

The growth and health of intertidal mussel (Perna canaliculus) populations were investigated along an exposed coastline at Ninety Mile Beach, northern New Zealand. A marked and recaptured experiment of young mussels (15–35 mm in shell length) over a year at three sites resulted in significant differences in recovery rates, shell growth, condition indices, age, and degree of shell parasitism. A high recovery rate of fast growing mussels coincided with a high condition index and low shell parasitism. Conversely, a low recovery rate of slow growing populations was associated with a low condition index and a high level of shell parasitism. Anomalous shell conditions included bored shells, wear and sloughing of periostracum, shell erosion, and nacre pits and bulges. Aging of adult mussels (70–80 mm) indicates that in areas of fast population turnover rates, mussels reach adulthood in only 1.1 years compared to the 2.5-year-old mussels found in low turnover areas. These differences are attributed to variations in environmental conditions (e.g., food availability, sediment load) throughout the study area, and highlight the importance of site-specific variability for intertidal populations.

Correlations between gastropod shell dissolution and water chemical properties in a tropical estuary

Although poorly reported in the scientific literature, acidic waters characterize many South East Asian estuaries. The observation of shell dissolution in a typically marine gastropod whelk ( Thais) prompted investigation into determining relationships between shell properties of this whelk and the water chemistry (including pH) of the Sungai Brunei estuary (Borneo) in which it occurs. Shell weight, shell length and topographical shell features were determined for populations of Thais gradata distributed along a gradient of pH and salinity ranging between 5.78 and 8.3 pH units, and 3.58 and 31.2 psu. Shell weight varied independently of the co-varying acidity, salinity and calcium levels experienced. In contrast, shell length and a semi-quantitative variable based on shell sculpturing (shell erosion rank, SER) were significantly correlated with these water chemistry variables. This study brings attention to the potential use of estuarine organisms and systems in investigating current marine acidification questions.

Are there hot spots for Elliptio complanata in the shallow littoral zone of a large Canadian Shield lake?

The variability in habitat conditions in large lake basins may provide areas favourable for unionid recruitment and growth. Here we test whether the distribution, body size, and growth rate of Elliptio complanata differ between upwind and downwind sites in a 5.8 km long lake basin. We measured density and morphological characteristics of E. complanata at nine upwind and nine downwind sites. We also measured growth rates and stable isotope (δ13C, δ15N) signatures at three upwind and three downwind sites. We found no difference in density or body size of E. complanata between upwind and downwind sites, despite clear differences in water temperature and food availability. Contrary to our expectations, the growth rate of E. complanata was higher (by 62%) at upwind sites. The reason for this difference is uncertain, but higher shell erosion and a negative relationship between δ13C and C/N ratio suggest that E. complanata may be more stressed at downwind sites. The δ15N of E. complanata did not differ systematically between upwind and downwind sites, but increased with increasing body size. It would therefore be prudent to standardize the size of E. complanata used to measure the baseline of lake food webs.

Corrasion of a remoulded cohesive bed by saltating littorinid shells

Corrasion of a standard cohesive bed due to saltating gastropod shells of the species Littorina has been examined in a laboratory mini flume. The purpose of the study was to examine the effects of shell size and number on bed erosion rate. The movement of shells by flows explain why intertidal, glacial clays in the Bay of Fundy (which are covered in places with Littorinid shells) suffer erosion because bed erosion rate increased up to 20-fold with the introduction of a single shell to the flume (27 m −2). The standard bed was made of potters clay which had an erosion threshold of 0.19 Pa and a fluid-induced erosion rate E o=0.072( U y − U crit) gm −2 s −1, where U y is the azimuthal current speed at height y=0.10 m. Shells of seven differing sizes ( d s) were used to define the process of erosion by shell impacts. The threshold for shell motion ( U gcrit) was linearly related to shell size in the form: U gcrit=9.17×10 −3 d s−0.22 m s −1. Motion began by intermittent rolling, followed by continuous rolling and then by saltation. The shell speed in saltation was 68% that of U y , thus 32% of the horizontal shell momentum was transmitted to the bed. The length/height ratio of saltations was 6.3, and was constant for all sizes, and the mean saltation frequency was 1.7 s −1. The shell erosion rate ( E s) increased with shell diameter for both the rolling and saltating phases. During the rolling phase, E s was up to 5 times greater than E o at the same current speed. During saltation, E s was up to 20 times greater than E o at the same current speed. The effect of shell number (1–7) was examined for the 7–10 mm size class. During rolling, E s increased linearly with shell number. For the saltating phase, E s increased in an asymptotic fashion, suggesting that groups of saltating shells affect the erosion process differently than single shells. The ballistic momentum flux ( T) of saltating shells is highly dependent on the area of impact ( A g), which in the case of the littorinids, is along the rounded outer lip of the aperture (measured to be 0.5% of the nominal cross-sectional area of the shell). T increased with shell mass ( M g) and current speed ( U y). Below 0.29 m s −1, T< τ o; for 0.29< U y <0.35 m s −1, T≈ τ o; and for U y >0.35 m s −1, T> τ o where τ o is the fluid-induced shear stress. The total erosion rate ( E tot= E s+ E o) was compared to the excess total bed shear stress ( τ tot− τ crit) and found to be linearly related in the form: E tot =0.010 [τ tot −τ crit ] gm −2 s −1. E s appeared to be in linear continuity with E o if the impact area was set to 0.05% of the estimated shell nominal cross-sectional area.

How costly is molluscan shell erosion? A comparison of two patellid limpets with contrasting shell structures

The energetic cost of replacing shell loss due to erosion has been overlooked in previous research on molluscan energy budgets, although several studies have shown that shell breakage incurs a significant short-term cost and may reduce growth. We measured the rate of shell erosion for two species of patellid limpets: Patella granatina, which has a shell predominated by calcite, and P. argenvillei, in which aragonite is dominant. Highest rates of erosion of P. granatina shells (36% p.a.) were recorded when the shells were grazed by a co-occurring limpet, P. granularis; in their absence, erosion was 15.2%. P. argenvillei experienced a slightly lower rate of erosion (32.4% p.a.) when its shells were grazed by juvenile conspecifics, whereas ungrazed shells lost 12.5%. P. argenvillei shells are often coated with an encrusting coralline, and infested by a boring lichen, but neither had a significant effect on the rate of erosion. By modelling the energetic costs of shell erosion, we showed that the shell mass produced by P. argenvillei each year is substantially greater than that produced by P. granatina, as is the loss of mass due to erosion. However, the energetic costs of shell production, and the losses due erosion, are higher for P. granatina, reflecting its faster growth and the higher energy content of its shells. For P. argenvillei, the cost of erosion increases with age from about 8–20% of the total energy devoted to “production” (somatic growth, shell production and gonadial output). For P. granatina it is 8–12%. Experimental elimination of erosion failed to yield higher somatic, visceral or gonadial masses. This suggests that shell erosion does not carry a short-term cost equivalent to that experienced when shells are broken. Rather, it seems that compensation for shell erosion is an ongoing process involving a long-term cost. As a more explicit test of this hypothesis, we coated the shells of some individuals to protect them against erosion, left others unprotected, and coated one side of the shells of a third series of animals. Protected shells or sides of shells became thicker than those that were unprotected. Deposition of shell continued whether the shells (or portions of shells) were being eroded or not. Thus, compensation for shell erosion is an process of continual maintenance that is not simply initiated where and when erosion occurs, and will carry long-term costs.

A practical chronology for the abalone Haliotis fulgens

The abalone Haliotis fulgens deposits fine (non-pigmented) and dark (pigmented) rings in the spire that are useful for aging the shell. A dark ring is deposited annually, and in the first 4–5 years is preceded by one or more fine rings which are deposited with decreasing frequency over time. External shell erosion causes the loss of about one ring per 32 mm shell length over about 80 mm length. The El Niño events of 1982–3 and 1991–2 caused growth checks in shells of H. fulgens at La Natividad and Bahia Tortugas, Baja California and were used as a time-stamp to validate the rate of deposition of dark rings in the shell. In about 9% of the shells examined more or less dark rings are deposited than expected. In these cases counts of fine rings as well as dark rings can reduce the chance of error.

Life History and Habitat of the Endangered Dwarf Wedgemussel Alasmidonta heterodon (Bivalvia:Unionidae)

The reproductive cycle, population demographics, and habitat use of the dwarf wedgemussel (Alasmidonta heterodon) were studied in populations from the Tar River, North Carolina, and Aquia Creek, Virginia, during 1991 and 1992. The mussel is a long-term brooder, spawning in late summer and becoming gravid in fall. Laboratory infestations with glochidia on 15 species of fish confirmed three hosts for A. heterodon: tessellated darter (Etheostoma olmstedi), Johnny darter (E. nigrum), and mottled sculpin (Cottus bairdi). Six species of sunfishes, four minnows, one sucker, and one catfish tested in the laboratory did not serve as hosts. Age and growth characteristics were calculated using a thin sectioning technique on valves collected from the Tar River, Aquia Creek, and Neversink River, New York. As computed by the von Bertalanffy equation, the dwarf wedgemussel reaches a maximum length of 38.70 mm, 45.26 mm, and 45.84 mm in the Tar River, Aquia Creek, and Neversink River, respectively. There were significant differences in growth rates of A. heterodon from the Tar River and Aquia Creek and between populations in the Neversink River and Aquia Creek. There was no significant difference in growth rate of populations from the Tar and Neversink rivers. Age-class distribution of live mussels collected at the three sites was estimated using age-length keys. The Tar River population included year classes from 4 to >6 years. Excessive shell erosion prevented the aging of older individuals and thus, a key to include larger live individuals. The age class distribution of the Aquia Creek population ranged from 2 to 9 years, while the Neversink River population was represented by age 1 to 8 year classes. The von Bertalanffy growth equations for the Tar River, Aquia Creek, and Neversink River populations were as follows: L_t = 38.70 mm (1-e^{-0.299(t+0.251)}; L_t = 45.26 mm (1-e^{-0.260(t+0.244)}); and L_t = 46.84 mm (1-e^{-0.207(t+0.335)}), respectively. Habitat-suitability experiments were conducted in the laboratory to determine preferred substratum size and water velocity. Dwarf wedgemussels always preferred the finer substratum offered, but little preference was shown for lotic versus lentic flow regimes.

Population structure, shell morphology, age and condition of the freshwater mussel Hyridella menziesi (Unionacea: Hyriidae) from seven lake and river sites in the Waikato River system

Populations of the freshwater mussel Hyridella menziesi were compared from six lakes and one river site along the Waikato River system, North Island, New Zealand. Estimated densities ranged from several hundred per square metre in Lake Taupo to less than one per square metre in Lake Karapiro. Length-frequencies were similar at all sites, with unimodal distributions and a notable absence of any individuals less than 20 mm long. The extent of shell erosion varied from sites where most mussels had less than 1% of the right valve affected (i.e., Ohakuri & Karapiro), to Lake Aratiatia where most shells had erosion over 1–20% of their surface. Taupo and Ohakuri mussels showed an unusually high incidence (over 50%) of internal shell abnormalities (including dulling, deformities and protuberances). Canonical discrimination based on shell length, height, width and weight clearly separated Taupo and Ohakuri mussels from all other sites, and ANCOVA on morphological characteristics confirmed significant differences between sites. Physical condition indices (based on dry flesh weight per shell weight and shell volume) showed an increasing downstream trend, but this was not significantly correlated with food availability (i.e., chlorophyll a concentration) because of unexpectedly low condition in lakes Ohakuri and Karapiro. Length v. age relationships contained considerable scatter and did not show between-site differences. Measured differences in external erosion, internal shell abnormalities and morphology were attributed to physical, chemical and biological factors. Use of mussels for environmental monitoring must take these ‘background’ differences into account.