Localized invasion of the North American Harris mud crab, Rhithropanopeus harrisii, in the Panama Canal: implications for eradication and spread

As the rate of biological invasions continues to increase, a growing number of aquatic introduced species are becoming globally widespread. Despite this ubiquitous phenomenon, rarely do we discover aquatic invaders early enough to allow the possibility of eradication. Recently, the North American Harris mud crab (Rhithropanopeus harrisii) was found in the waters adjacent to the Panama Canal and herein we provide an assessment of the crab’s distribution in Panama to evaluate the possibility of eradication. Using salinity tolerance experiments, we also evaluate the potential for further spread of this crab within the Canal. Our results suggest that populations of R. harrisii are currently limited to two manmade lagoons which are adjacent to the Panama Canal. Our experiments suggest that both juvenile and adult R. harrisii can survive in salinities found outside its current range in Panama. Although it is difficult to predict the potential for future spread and impacts in Panama, current management strategies could reduce the probability for spread locally as well as elsewhere in the world given the intensity of shipping in this region. The current containment of this invader suggests that a localized eradication may be possible.     

Complex interactions among fish, snails and macrophytes: implications for biological control of an invasive snail

The golden apple snail (Pomacea canaliculata), a native of freshwater wetlands of South America, has invaded many Asian countries and grazed heavily in agricultural and wild areas. Common carp (Cyprinus carpio) has been proposed as a biological control agent against this snail, but little is known about its impact on non-target aquatic plants and animals. In a 8-week enclosure experiment, we quantified the impact of common carp on three species of aquatic macrophytes and nine species of snails, including the apple snail, in a shallow pond. The results showed that the apple snail or carp alone significantly reduced the plant biomass, although the apple snail had a stronger overall herbivorous effect than the carp. The carp completely removed juvenile apple snails, but had only a weak predatory effect on larger apple snails and no effect on the adults’ oviposition frequency. Furthermore, the carp significantly reduced the populations of most species of other snails that occurred naturally in the pond. Our results thus indicate that common carp can be an effective biological control agent against the invasive apple snail, but caution should be taken about its potential to reduce wetland floral and faunal diversity.     

Control methods for snail-borne zoonoses

All trematodes which cause infections and diseases in humans (zoonoses) require snails for their first intermediate host. Some have additional intermediate hosts such as crustaceans, fishes and frogs. In this paper I have discussed the use of various procedures for controlling snail populations thereby reducing the population of trematodes whose cercariae penetrate the skin of man, or which are acquired by eating raw aquatic vegetation such as watercress, or which cause pathology in fish. Biological, chemical, and physical snail control techniques are discussed. The most promising molluscucides are copper compounds, Bayluscide, biocidal rubber and molluscicidal bait.     

Effect of single-species and mixed-species leaf leachate on bacterial communities in biofilms

Bacterial communities in two shallow eutrophic aquatic ecosystems (eastern China) were studied using culture-dependent methods, and their correlations with the other water parameters were analyzed. Although the values of the comprehensive trophic state index in Xizi Lake and Hangzhou Canal were almost identical, the abundances of cultivable bacterial communities, such as protein-hydrolyzing bacteria (PHB), fecal coliforms (FC), nitrogen-utilizing bacteria, phosphate-mineralizing bacteria, and cellulose-decomposing bacteria (CDB), differed significantly. They were much less in Xizi Lake than in Hangzhou Canal. Correlation analyses indicated that the abundances of physiological groups of bacteria were determined mainly by the biomasses of phytoplankton and zooplankton, rather than by the utilized substrates. Xizi Lake was an algae-dominated aquatic ecosystem, a situation that mainly arose from the influx of inorganic nutrients, and the Hangzhou Canal was bacteria-dominated due to the influx of organic sewage. Molecular characterization and phylogenetic analysis of isolates showed that there were 6 phylogenetic lineages out of 15 isolates screened from Xizi Lake, including γ-proteobacteria (5), β-proteobacteria (3), α-proteobacteria (2), actinobacteria (1), firmicutes (4), and bacteroidetes (1). While in Hangzhou Canal there were only 4 bacterial groups among 22 isolates, γ-proteobacteria comprised about 82%, α-proteobacteria made up 9%, firmicutes and bacteroidetes made up only 4.5%, respectively, and no β-proteobacteria were found. Enterobacteriaceae were the principal bacteria components in the two aquatic ecosystems, especially in the sewage-polluted Hangzhou Canal. It can be concluded preliminarily that the bacterial diversity in Xizi Lake is richer than that in Hangzhou Canal. Handling editor: J. Padisak     

Drivers of symbiont diversity in freshwater snails: a comparative analysis of resource availability,community heterogeneity,and colonization opportunities

Decades of community ecology research have highlighted the importance of resource availability, habitat heterogeneity, and colonization opportunities in driving biodiversity. Less clear, however, is whether a similar suite of factors explains the diversity of symbionts. Here, we used a hierarchical dataset involving 12,712 freshwater snail hosts representing five species to test the relative importance of potential factors in driving symbiont richness. Specifically, we used model selection to assess the explanatory power of variables related to host species identity, resource availability (average body size, host density), ecological heterogeneity (richness of hosts and other taxa), and colonization opportunities (wetland size and amount of neighboring wetland area) on symbiont richness in 146 snail host populations in California, USA. We encountered a total of 23 taxa of symbionts, including both obligatory parasites such as digenetic trematodes as well as more commensal, mutualistic, or opportunistic groups such as aquatic insect larvae, annelids, and leeches. After validating richness estimates per host population using species accumulative curves, we detected positive effects on symbiont richness from host body size, total richness of the aquatic community, and colonization opportunities. Neither snail density nor the richness of snail species accounted for significant variation in symbiont diversity. Host species identity also affected symbiont richness, with higher gamma and average alpha diversity among more common host species with higher local abundances. These findings highlight the importance of multiple, concurrent factors in driving symbiont richness that extend beyond epidemiological measures of host abundance or host diversity alone.     

螺类牧食对伊乐藻与苦草种间关系的影响

动物牧食可以调节植物种间的相对竞争能力,从而改变物种在群落中的竞争地位.外来植物伊乐藻生长迅速、具有较强的光竞争能力,土著种苦草根系发达、具有较强的地下资源竞争能力.选择这两种各具竞争特色的沉水植物为模式生物,通过室外受控实验研究了螺类牧食对两种沉水植物种间关系的影响.结果表明:不管有无螺类牧食,伊乐藻的相对生长率为苦草的2～5倍,伊乐藻具有明显的竞争优势.苦草低密度种植时,螺类牧食与种间竞争对其生长没有显著影响;高密度种植时,螺类牧食活动促进了苦草的生长,种间竞争则使苦草的生长率明显降低.无论伊乐藻种植密度如何,螺类牧食均使其生长率明显降低,混栽高密度的苦草也能抑制伊乐藻生长.探讨了螺类牧食对沉水植物的种间竞争关系的作用机理.     

Biomass and energy transfer by a specialized predator of aquatic snails

The predatory larvae of Sepedon fuscipennis (Diptera: Sciomyzidae) feed only on aquatic pulmonate snails. Under laboratory conditions these larvae utilized ingested snail tissue more efficiently when a lower snail biomass was ingested at lower snail densities. The adaptive value of this response to fluctuating prey availability is suggested.     

Aquatic fungi in relation to the physical and chemical parameters of water quality in the Augustów Canal

The objective of this study was to determine the effects of physical and chemical water quality parameters in the Augustów Canal on the abundance and taxonomic diversity of aquatic fungi. The studies were conducted in the hydrological year 2012 along the environmentally valuable area of the Augustów Canal. The mean abundance of fungi was 2 160 ± 1840 CFU ml⁻¹. The lowest abundances were observed in the uppermost part of the canal, and the highest abundances in the southern part. A total of 38 species were identified, including 13 potentially pathogenic fungi, which predominated in the central region of the canal, called the natural lake part. Redundancy analysis (RDA) indicated that variation in water chemistry caused significant changes to fungal species diversity. The results showed that fungal species diversity and abundance in the canal were particularly dependent on the extent of water pollution, as indicated by chloride and sulphate(VI) ions and electrolytic conductivity. Other water quality parameters that influenced the development of aquatic fungi were organic matter and algal biomass, responsible for water eutrophication. We propose that mycoplankton be considered a sensitive biological indicator of water quality.     

Effects of irradiation on the reproductive ability of Zonitoides arboreus, a snail pest of orchid roots

The effect of irradiation on the reproductive ability of the orchid snail, Zonitoides arboreus, a serious pest of potted orchids in Hawaii, was investigated. Weekly egg production averaged between 0.8 and 1.9 over a 9–wk period for snails not exposed to irradiation, and egg hatch averaged 61%. In comparison to untreated controls, irradiation of snails at the lowest dose tested (34–37 Gy) reduced egg production and egg hatch by 63% and 94%, respectively over a 9–wk period. None of the snails treated with levels of irradiation ≥ 69 Gy produced viable eggs. This is the first study measuring the effect of ionising irradiation on a terrestrial snail or slug species using sterilising doses. Overall, the results show that the reproductive ability of this snail species is affected by irradiation in a similar manner as for Biomphalaria glabrata, an aquatic snail for which the effects of irradiation have been studied in detail.     

京杭运河杭州段城市景观格局对河网水环境的影响

近年来人类活动显著改变了陆地生态环境,城市化作为人类活动的重要组成部分,对陆地生态环境中的水环境也产生了深刻的影响。以杭州市作为研究区,以京杭运河杭州段为研究对象,在对水质监测点建立不同半径缓冲区的基础上,分析了城市景观格局对河网水环境的影响。结果表明：（1）京杭运河杭州段城市景观和农村景观镶嵌分布,河网水系发达,景观破碎化程度高,斑块形状复杂多变,城乡交错带特征典型,人类活动强度高低分化明显;（2）各种景观类型沿河分布的空间特征明显,景观格局指数随缓冲半径的增大而变化,当缓冲半径达到1000m以后格局指数维持相对稳定状态;（3）京杭运河杭州段水系的氮源污染物含量整体处于较高水平,总磷含量处于低水平。水质受到建设用地和农业用地的影响较高;（4）不同的景观格局指数对不同的水质指标在不同的时期具有一定的解释能力,且同一景观指数在汛期和非汛期对水环境的影响作用差异较大甚至相反,景观格局在河网密集型城市区域对河流水环境影响的整体利弊还需进一步权衡量化。     

Distribution,abundance, and density of harbor porpoises in Hood Canal,Washington

Harbor porpoises (Phocoena phocoena) are the only cetaceans routinely sighted in Hood Canal, a narrow fjord that comprises the western edge of Puget Sound, Washington, USA. Harbor porpoises are sensitive to anthropogenic sounds, including noise from recreational and commercial vessel traffic, and the United States Navy, which conducts military training and testing within Hood Canal that can include underwater sound sources. This study was funded as part of the Navy monitoring program to assess potential impacts of naval activities on cetaceans. We conducted vessel-based line-transect surveys for harbor porpoises in Hood Canal in 2022–2023 to derive seasonal estimates of abundance and density. We carried out surveys over 37 days and surveyed the entire canal twice per season totaling 2,176 km of on-effort track line. We recorded 809 on-effort harbor porpoise groups and 1,385 individuals. Seasonal abundance estimates were lowest in winter (308 animals, 95% CI = 189–503) and gradually increased through spring and summer to a peak of 1,336 animals (95% CI = 826–2,160) in fall. Overall porpoise density was highest in central Hood Canal, an area that includes a designated United States Navy training range, though porpoise sightings were notably absent in a 21-km² area adjacent to the naval submarine base within this otherwise high-density region. Though we collected only a single year of data, these results suggest that harbor porpoise abundance in Hood Canal increased significantly since it was last estimated (2013–2015). The notable seasonal fluctuation of harbor porpoise abundance suggests Hood Canal may host a larger percentage of the overall Washington Inland Waters stock during the fall season, raising important management considerations.     

福寿螺的寄主植物及其对福寿螺体重的影响

为了明确福寿螺对寄主植物的选择性,进一步掌握其可能的危害及入侵暴发机制,对稻田水域及周边的24科43种植物进行了室外取食试验,分别在取食后3、5d对受试样品及成螺称重,比较其无选择条件下的取食偏好性及其对成螺增长率的影响。结果表明:福寿螺对受试的24科43种植物(样品)的根、茎、叶、果实以及块根(茎)均能取食,对水生(湿生)和陆生植物没有偏好性;在取食过程中,优先取食细嫩的植株和部分,硬老部分需经水浸泡软化后才能被取食;辣椒、苦瓜等果实的辣苦味对福寿螺无驱避作用;取食率与体重增长率呈显著正相关,取食越多生长越快;但体重与植物的总酚含量呈负相关。福寿螺的广食性、取食量、植物总酚含量、植物丰盛度等种群和生态系统特征是影响其快速扩散的重要原因,同时可以借助福寿螺对植物的不同利用类型防范其扩散和危害。     

Lethal and non-lethal effects of multiple indigenous predators on the invasive golden apple snail (Pomacea canaliculata)

1. We investigated the individual and combined effects of two predators (the climbing perch, Anabas testudineus, and the wetland crab, Esanthelphusa nimoafi) indigenous to wetlands in Laos, on the behaviour and survival of the invasive South American golden apple snail (Pomacea canaliculata). The snail is considered a pest, consuming large amounts of rice and other aquatic vegetation in the region. 2. Snail avoidance reactions to released predator chemical cues were investigated in aquaria while the effects of predators on a mixed snail population were studied in field enclosures that contained native aquatic plants (Salvinia cucullata, Ludwigia adscendens and Ipomoea aquatica). 3. In the aquaria experiment, neonate (2–3 mm) and medium‐sized snails (8–10 mm) responded to fish chemical cues by going to the surface, whereas adult snails (35–40 mm) went to the bottom. In contrast, no size class of snails reacted to chemical cues released by crabs. 4. In the field experiment, fish reduced the abundance of neonate snails, and crabs reduced the abundance of all size classes. The effect of the combined predators could not be predicted from the mortality rate observed in single predator treatments. The survival of neonate and medium‐sized snails was greater and of adults less than expected. The presence of predators did not affect egg production. Snails consumed significant amounts of plants despite the presence of predators. 5. Our findings suggest that some indigenous Asian predators have lethal and sublethal effects on P. canaliculata that depend on snail size and predator type. When in the presence of several predators the response of snails to one predator may either increase or decrease the vulnerability of snails to the others.     

Crayfish,catfish and snails: the perils of uncontrolled biological control

A recent proposal that the Australian redclaw crayfish Cherax quadricarinatus and hybrid catfish could potentially control the snail hosts of schistosomiasis has been criticised on the grounds that crayfish pose a severe threat to aquatic ecosystems into which it might be introduced. This note examines the issue further, pointing out that both lack the host-specificity requirement to be a successful biological control agent. The catfish Clarias gariepinus is an omnivore and snails form only a small proportion of its diet; there is no evidence to suggest that it controls snail populations anywhere in Africa. The same applies to other species that have been proposed as biological control agents. Simple laboratory experiments are not an adequate guide to the efficiency of an animal as a biological control agent and detailed ecological investigations would usually demonstrate that few African fish species have this capability.     

The experimental production of Fasciola hepatica metacercariae from three aquatic populations of Galba truncatula

Laboratory investigations on three aquatic populations of Galba truncatula, originating from the Peruvian Altiplano and French Massif Central, were carried out during three successive snail generations to determine if these populations might be successfully used for the metacercarial production of Fasciola hepatica under experimental conditions. High numbers of surviving snails at day 30 post-exposure (>70%), high prevalences of F. hepatica infections (>60%), and prolonged productions of cercariae for a mean period of 35 to 47 days were observed in the three populations, whatever the snail generation. In the Peruvian population, metacercariae of F. hepatica significantly decreased in numbers from a mean of 251 in the parent snails to 124 per snail in the F2 generation, whereas no significant variation was observed in the two French populations. As these aquatic snails rarely emerged out of water, the use of these populations for the commercial production of F. hepatica metacercariae was of great interest, because the daily time spent watching the breeding boxes of snails was clearly shorter, thereby reducing the cost of producing metacercariae compared with using amphibious snails reared with romaine lettuce.     

Impacts of an invasive snail (Tarebia granifera) on nutrient cycling in tropical streams: the role of riparian deforestation in Trinidad, West Indies

Non-native species and habitat degradation are two major catalysts of environmental change and often occur simultaneously. In freshwater systems, degradation of adjacent terrestrial vegetation may facilitate introduced species by altering resource availability. Here we examine how the presence of intact riparian cover influences the impact of an invasive herbivorous snail, Tarebia granifera, on nitrogen (N) cycling in aquatic systems on the island of Trinidad. We quantified snail biomass, growth, and N excretion in locations where riparian vegetation was present or removed to determine how snail demographics and excretion were related to the condition of the riparian zone. In three Neotropical streams, we measured snail biomass and N excretion in open and closed canopy habitats to generate estimates of mass- and area-specific N excretion rates. Snail biomass was 2 to 8 times greater and areal N excretion rates ranged from 3 to 9 times greater in open canopy habitats. Snails foraging in open canopy habitat also had access to more abundant food resources and exhibited greater growth and mass-specific N excretion rates. Estimates of ecosystem N demand indicated that snail N excretion in fully closed, partially closed, and open canopy habitats supplied 2%, 11%, and 16% of integrated ecosystem N demand, respectively. We conclude that human-mediated riparian canopy loss can generate hotspots of snail biomass, growth, and N excretion along tropical stream networks, altering the impacts of an invasive snail on the biogeochemical cycling of N.     

Experimental quantification of long distance dispersal potential of aquatic snails in the gut of migratory birds

Many plant seeds and invertebrates can survive passage through the digestive system of birds, which may lead to long distance dispersal (endozoochory) in case of prolonged retention by moving vectors. Endozoochorous dispersal by waterbirds has nowadays been documented for many aquatic plant seeds, algae and dormant life stages of aquatic invertebrates. Anecdotal information indicates that endozoochory is also possible for fully functional, active aquatic organisms, a phenomenon that we here address experimentally using aquatic snails. We fed four species of aquatic snails to mallards (Anas platyrhynchos), and monitored snail retrieval and survival over time. One of the snail species tested was found to survive passage through the digestive tract of mallards as fully functional adults. Hydrobia (Peringia) ulvae survived up to five hours in the digestive tract. This suggests a maximum potential transport distance of up to 300 km may be possible if these snails are taken by flying birds, although the actual dispersal distance greatly depends on additional factors such as the behavior of the vectors. We put forward that more organisms that acquired traits for survival in stochastic environments such as wetlands, but not specifically adapted for endozoochory, may be sufficiently equipped to successfully pass a bird's digestive system. This may be explained by a digestive trade-off in birds, which maximize their net energy intake rate rather than digestive efficiency, since higher efficiency comes with the cost of prolonged retention times and hence reduces food intake. The resulting lower digestive efficiency allows species like aquatic snails, and potentially other fully functional organisms without obvious dispersal adaptations, to be transported internally. Adopting this view, endozoochorous dispersal may be more common than up to now thought.     

蓝藻堆积和螺类牧食对苦草生长的影响

设计了双因素四组处理(对照组,加螺组,加藻组,螺藻组)的室外受控实验,模拟湖泊沿岸带水华蓝藻的堆积以及底栖螺类的牧食活动对沉水植物苦草生长的影响。结果表明:蓝藻堆积(水体叶绿素a浓度为220μg/L)对苦草的生长具有明显的抑制作用,和对照组以及加螺组相比,加藻组和螺藻组中苦草的相对生长率分别下降了40.9%和36.4%,分株数也分别下降了56.4%和64.1%,分析认为蓝藻在水体表层堆积所产生的遮光可能是抑制底层苦草生长的主要原因。然而,环棱螺能在一定程度上促进苦草的生长,加螺组和螺藻组中苦草的相对生长率和分株数分别要明显高于对照组和加藻组,这可能要归因于螺类的牧食去除了沉水植物表面附着生物。实验中蓝藻堆积和螺类牧食对苦草的各项生长指标均无显著的交互作用,但蓝藻对苦草生长的抑制作用要远大于螺类对植物生长的促进作用。研究证实了在富营养浅水湖泊中,水华蓝藻在湖泊沿岸带的堆积会严重胁迫沉水植物的生长,而底栖螺类的牧食活动则能在一定程度上提高植物在不良环境下的生存能力。     

Colonisation of Heliconia caribaea by aquatic invertebrates: resource and microsite characteristics

Abstract.  1. Colonisation of ephemeral aquatic habitats via oviposition by invertebrates may be influenced by a variety of factors, such as the quality of aquatic habitat and the characteristics of the surrounding terrestrial environment. The water-holding bracts of Heliconia caribaea , a subtropical herb that produces ephemeral aquatic habitats, are colonised by a variety of aquatic invertebrates. To date, no experiments have been conducted to identify the cues that affect colonisation patterns via oviposition selection in Heliconia .
2. Artificial bracts were used to assess the influence of two types of resources found in bracts (plant produced carbohydrates and terrestrial snail faeces) on oviposition site-selection by invertebrate taxa via a replicated factorial design at four locations in the Luquillo Experimental Forest of Puerto Rico, U.S.A. Eleven microsite characteristics thought to affect oviposition were measured for each experimental container.
3. Most taxa responded in a minor way to microsite characteristics, whereas site selection by the most numerically dominant groups (e.g. Syrphidae) were influenced principally by resources within artificial bracts. Overall, the greatest response by particular taxa was to the presence of snail faeces. At the community level, total abundance, richness, and evenness of invertebrates increased with increasing biomass of faeces. Variation in sugar produced a more complex response.
4. In general, the terrestrial matrix surrounding these aquatic habitats was only a secondary determinant of population and community attributes; the principal factor affecting site selection was the quality of the aquatic habitat.