REGULAR PAPERS Unusual stable isotope fractionation patterns observed for fish host—parasite trophic relationships

Trophic relationships between four taxa of fish parasite (Cestoda, Nematoda, Isopoda, Copepoda) and their hosts ( Gasterosteus aculeatus, Merlangius merlangus, Boops boops, Platichthys flesus ) were investigated using stable isotopes of carbon and nitrogen. δ¹⁵N differences between parasite and host were unlike those conventionally observed among consumers and their diets. In no case were parasites enriched in 15N with respect to the host organism; endoparasites were significantly and consistently depleted. In no case were parasites enriched in 15N with respect to the host organism; endoparasites were significantly and consistently depleted.     

Dietary differences between two co-occurring calanoid copepod species

Summary Two co-occurring high altitude calanoid copepods, Diaptomus shoshone Marsh and D. coloradensis Forbes are of different size: the former is approximately twice as long as the latter. There are also temporal differences in development: the former matures 2–3 weeks before the latter in most ponds. Gut analyses of copepodids and adults of both species indicate that D. shoshone eats larger food particles but that D. shoshone early copepodids eat particles similar in size to D. coloradensis adults. Morphology of feeding appendages partially explains the dietary differences. These data are discussed from the viewpoint of competition between the species; it is concluded that either temporal differences or size differences of copepodids and adults would allow coexistence and that extreme size differences are probably unimportant compared to morphological differences of feeding appendages and perhaps behavioral differences.Supported in part by a Tufts Faculty Research Award.     

The relationship between species richness and productivity in metazoan parasite communities

Biodiversity is not distributed homogeneously in space, and it often covaries with productivity. The shape of the relationship between diversity and productivity, however, varies from a monotonic linear increase to a hump-shaped curve with maximum diversity values corresponding to intermediate productivity. The system studied and the spatial scale of study may affect this relationship. Parasite communities are useful models to test the productivity-diversity relationship because they consist of species belonging to a restricted set of higher taxa common to all host species. Using total parasite biovolume per host individual as a surrogate for community productivity, we tested the relationship between productivity and species richness among assemblages of metazoan parasites in 131 vertebrate host species. Across all host species, we found a linear relationship between total parasite biovolume and parasite species richness, with no trace of a hump-shaped curve. This result remained after corrections for the potential confounding effect of the number of host individuals examined per host species, host body mass, and phylogenetic relationships among host species. Although weaker, the linear relationship remained when the analyses were performed within the five vertebrate groups (fish, amphibians, reptiles, mammals and birds) instead of across all host species. These findings agree with the classic isolationist-interactive continuum of parasite communities that has become widely accepted in parasite ecology. They also suggest that parasite communities are not saturated with species, and that the addition of new species will result in increased total parasite biovolume per host. If the number of parasite species exploiting a host population is not regulated by processes arising from within the parasite community, external factors such as host characteristics may be the main determinants of parasite diversity.     

Ecological determinants of parasite acquisition by exotic fish species

Disease‐mediated threats posed by exotic species to native counterparts are not limited to introduced parasites alone, since exotic hosts frequently acquire native parasites with possible consequences for infection patterns in native hosts. Several biological and geographical factors are thought to explain both the richness of parasites in native hosts, and the invasion success of free‐living exotic species. However, the determinants of native parasite acquisition by exotic hosts remain unknown. Here, we investigated native parasite communities of exotic freshwater fish to determine which traits influence acquisition of native parasites by exotic hosts. Model selection suggested that five factors (total body length, time since introduction, phylogenetic relatedness to the native fish fauna, trophic level and native fish species richness) may be linked to native parasite acquisition by exotic fish, but 95% confidence intervals of coefficient estimates indicated these explained little of the variance in parasite richness. Based on R²‐values, weak positive relationships may exist only between the number of parasites acquired and either host size or time since introduction. Whilst our results suggest that factors influencing parasite richness in native host communities may be less important for exotic species, it seems that analyses of general ecological factors currently fail to adequately incorporate the physiological and immunological complexity of whether a given animal species will become a host for a new parasite.     

The effect of sediment characteristics on the distribution of two subtidal harpacticoid copepod species

Abundances of two harpacticoid copepod species, Enhydrosoma littorale Wells and Zausodes c.f. arenicolus Wilson, were significantly higher in one of two adjacent subtidal, soft-bottom habitats in St. George Sound, Florida (29°54′N : 84°37′48′′W). For Enhydrosoma littorale, a laboratory-preference experiment indicated that sediment-related factors caused the observed distribution. In a series of preference experiments, differences between the sediments of the two habitats in granulometry and organic matter were shown not to account for the preference. Rather, the preference results from differences in the microbes attached to the sediment particles in the two areas. In contrast, Zausodes c.f. arenicolus did nol prefer sediments from its area of high field abundance in laboratory preference experiments, indicating that factors external to the sediment i.e. hydrographic conditions or biological interactions, were responsible for this species' distribution.     

Genetic diversity and parasite prevalence in two species of bumblebee

Many bumblebee species have been suffering from significant declines across their ranges in the Northern Hemisphere over the last few decades. The remaining populations of the rare species are now often isolated due to habitat fragmentation and have reduced levels of genetic diversity. The persistence of these populations may be threatened by inbreeding depression, which may result in a higher susceptibility to parasites. Here we investigate the relationship between genetic diversity and prevalence of the parasitic mite Locustacarus buchneri in bumblebees, using the previously-studied system of Bombus muscorum and Bombus jonellus in the Western Isles of Scotland. We recorded L. buchneri prevalence in 17 populations of B. muscorum and 13 populations of B. jonellus and related the results to levels of heterozygosity. For B. muscorum, we found that prevalence of the mite was higher in populations with lower genetic diversity but there was no such relationship in the more genetically diverse B. jonellus. In contrast to population-level measures of genetic diversity, the heterozygosity of individual bees was not correlated with infection status. We suggest population-level genetic homogeneity may facilitate parasite transmission and elevate prevalence, with potential consequences for population persistence.     

两种寄生植物营养器官结构的研究

研究了槲寄生和北桑寄生茎、叶和吸器的结构,结果表明：槲寄生三年生茎稍肉质化,无周皮,有极厚的角质层;皮层有7个纤维束,薄壁组织发达;木质部中亦含有大量薄壁组织;韧皮部发达;簇晶及大量粘稠物质在茎中广泛分布。槲寄生叶肉质化,厚1.44 mm; 角质层发达,厚度为9.06～13.4 μm;叶肉无栅栏组织和海绵组织的分化。北桑寄生茎与正常木本植物茎的结构相似,木质部为典型的环孔材;韧皮部含有石细胞;髓部细胞壁厚,单纹孔明显。北桑寄生叶较薄,叶脉发达,无栅栏组织和海绵组织的分化。两种植物的吸器结构相似,由薄壁组织和螺纹导管组成,它们的螺纹导管长度相同,但槲寄生导管的直径是北桑寄生的2.5倍。     

Diapause dynamics of two diaptomid copepod species in a large lake

Skistodiaptomus oregonensis and Leptodiaptomus minutus produce diapausing eggs in a large lake, Oneida Lake, in New York State. The timing of the switch from production of subitaneous (immediately hatching) eggs to diapausing eggs for both species is in October. This timing is consistent with a pattern, reviewed here, for other populations of diaptomid copepods: populations living in large lakes tend to begin production of diapausing eggs later in the season than those living in small lakes. Populations living in temporary ponds tend to switch still earlier in the season. All populations reviewed here live in the north temperate zone. The sediments of Oneida Lake contain densities of diaptomid diapausing eggs on the order of 10⁵ m^–2 per cm below the sediment surface down to 5 cm. Below this sediment depth, egg densities decline. The highest egg densities were found in sediments under the deepest water. Diapausing eggs of L. minutus survive in the sediments at least two years, as shown by the recovery of the population after a year in which no new diapausing eggs were produced, and probably for two or more decades. Long-term dormancy can have the effect of ensuring the continuation of a population through periods of poor recruitment, and can help create conditions for the coexistence of competing species. Other investigators have suggested that S. oregonensis and L. minutus are competitors in other lakes when they co-occur.     

Detritivory in two Amazonian fish species

In floodplain lakes of Central Amazon, the siluriform Liposarcus pardalis and the characiform Prochilodus nigricans consumed detritus with different compositions. Most of the diet of L. pardalis and P. nigricans was made of amorphous organic matter. The food ingested by L. pardalis had relatively higher organic matter (OM) (35–55%) and crude protein (10–19%) than that found in P. nigricans stomachs (10–20% and 2–5%, respectively). Both fish ingested higher quantities of OM and crude protein during the high-water season. The differences between the two species seem to be related to their ability to select different detrital components. Striking differences were also found in the digestibility rate of OM and protein between the two species. Prochilodus nigricans assimilated only 2·3% of its intake compared with 24% in L. pardalis .     

Nonhost species reduce parasite infection in a focal host species within experimental fish communities

The dilution effect describes the negative association between host biodiversity and the risk of infectious disease. Tests designed to understand the relative roles of host species richness, host species identity, and rates of exposure within experimental host communities would help resolve ongoing contention regarding the importance and generality of dilution effects. We exposed fathead minnows to infective larvae of the trematode, Ornithodiplostomum ptychocheilus in minnow‐only containers and in mixed containers that held 1–3 other species of fish. Parasite infection was estimated as the number of encysted worms (i.e., brainworms) present in minnows following exposure. The results of exposure trials showed that nonminnow fish species were incompatible with O. ptychocheilus larvae. There was no reduction in mean brainworm counts in minnows in mixed containers with brook sticklebacks or longnose dace. In contrast, brainworm counts in minnows declined by 51% and 27% in mesocosms and aquaria, respectively, when they co‐occurred with emerald shiners. Dilution within minnow + shiner containers may arise from shiner‐induced alterations in minnow or parasite behaviors that reduced encounter rates between minnows and parasite larvae. Alternatively, shiners may act as parasite sinks for parasite larvae. These results highlight the role of host species identity in the dilution effect. Our results also emphasize the complex and idiosyncratic effects of host community composition on rates of parasite infection within contemporary host communities that contain combinations of introduced and native species.     

Spatiotemporal and gender‐specific parasitism in two species of gobiid fish

Parasitism is considered a major selective force in natural host populations. Infections can decrease host condition and vigour, and potentially influence, for example, host population dynamics and behavior such as mate choice. We studied parasite infections of two common marine fish species, the sand goby (Pomatoschistus minutus) and the common goby (Pomatoschistus microps), in the brackish water Northern Baltic Sea. We were particularly interested in the occurrence of parasite taxa located in central sensory organs, such as eyes, potentially affecting fish behavior and mate choice. We found that both fish species harbored parasite communities dominated by taxa transmitted to fish through aquatic invertebrates. Infections also showed significant spatiotemporal variation. Trematodes in the eyes were very few in some locations, but infection levels were higher among females than males, suggesting differences in exposure or resistance between the sexes. To test between these hypotheses, we experimentally exposed male and female sand gobies to infection with the eye fluke Diplostomum pseudospathaceum. These trials showed that the fish became readily infected and females had higher parasite numbers, supporting higher susceptibility of females. Eye fluke infections also caused high cataract intensities among the fish in the wild. Our results demonstrate the potential of these parasites to influence host condition and visual abilities, which may have significant implications for survival and mate choice in goby populations.     

Length–weight relationship for freshwater fish species from Brazilian Cerrado

Presented are length–weight relationship (LWR) estimates for 15 native fish species captured between 2010 and 2011, in the Munim River, Brazilian Cerrado basin. Samples were collected using gillnets of four different mesh sizes (3.0, 4.0, 5.0 and 6.0 cm), hand nets and a covo‐fish trap. Additionally provided are the first LWRs for eight species, specially the recently described species Charax awa Guimarães, Brito, Ferreira & Ottoni, 2018.     

Length–weight relationship for seven freshwater fish species from Brazil

The present study reports length‐weight relationships for seven native freshwater fish species (Triportheus angulatus, Psectrogaster rhomboides, Prochilodus brevis, Leporinus piau, Cichlasoma orientale, Crenicichla menezesi, and Pimelodella gracilis) captured in a semiarid Brazilian reservoir located in the state of Rio Grande do Norte.     

Enigmatic relationship of two Microsporum species

Unusual cultures of a Microsporum species were isolated from eleven patients who had either tinea capitis or tinea corporis or both. Although the macroconidia produced in the primary cultures resembled those of M. canis, hyphal morphology and colony characteristics were different from any of the more commonly occurring Microsporum species. Careful examination of subcultures showed that the isolates were composed of two species; M. canis and M. distortum, whose relationship remains enigmatic.     

Length–weight and length–length relationship of two endemic snakehead fish species from Brahmaputra river basin,Assam, India

Length–weight relationships (LWRs) and length–length relationships (LLRs) of two endemic Channid fish species, Channa aurantimaculata Musikasinthorn, 2000 and C. stewartii (Playfair, 1867) were studied from Brahmaputra River basin in Assam, India. Sampling was done from May to December, 2017 and a total of 81 fish specimens were collected using bamboo traps. In the LWRs, b value for C. aurantimaculata and C. stewartii were 3.0456 and 2.9172 respectively. Maximum size recorded for C. aurantimaculata in the present study (34.1 cm) was higher than previous record in FishBase (19.1 cm).     

Behavioural thermoregulation in two freshwater fish species

In the presence of a vertical thermal gradient, juvenile three‐spined sticklebacks Gasterosteus aculeatus and minnows Phoxinus phoxinus positioned themselves higher in the water column compared with adult conspecifics. This result was consistent regardless of whether age cohorts were tested separately or together. Furthermore, juveniles but not adult fishes positioned themselves higher in water column in the presence of a thermal gradient compared with those in the absence of a thermal gradient. Juvenile G. aculeatus and adult fish of both species did opt to position themselves higher in the water column in the hours immediately following a feeding event relative to their positions in the same gradient when they had not fed.     

Length–weight relationship of seven freshwater fish species from Turkish Thrace

The length‐weight relationships (LWRs) of Alburnus istanbulensis Battalgil, 1941, Barbus tauricus Kessler, 1877, Cobitis pontica Vasil'eva & Vasil'ev, 2006, Cobitis strumicae Karaman, 1955, Gobio bulgaricus Drensky, 1926, Phoxinus strandjae Drensky, 1926 and Squalius orpheus Kottelat & Economidis, 2006 were analysed. Fish samples were collected by electroshocker (SAMUS 725 G and SAMUS 725 M portable electroshockers; frequency 50–55 Hz; 30–70 cm fishing depth) in some water bodies in Turkish Thrace from March 2012 to August 2017. The values of parameter b in the LWR equations varied from 2.916 (B. tauricus) to 3.336 (P. strandjae). This study also provides a new maximum length data for A. istanbulensis, G. bulgaricus, P. strandjae and S. orpheus.