Visual physiology of the Antarctic amphipod Abyssorchomene plebs

Although the visual systems of animals living in the cold, dark water of the deep sea have been investigated for some time, little is known about vision in animals inhabiting polar oceans, where temperatures are even colder and irradiance fluctuates dramatically with ice cover and season. Physiology of the compound eye of the amphipod Abyssorchomene plebs (Gammaridea: Lysianassoidea), a common Antarctic benthic scavenger, was studied electrophysiologically by electroretinography. A. plebs has a monochromatic visual system with a spectral sensitivity maximum at 487 nm, and higher sensitivity at ultraviolet wavelengths than predicted by a visual pigment template. While irradiance sensitivity determined from V/log I curves is comparable to that of mesopelagic crustaceans, temporal resolution calculated from response waveform dynamics and as determined by critical flicker fusion frequency suggest that the A. plebs eye is slower than that of crustaceans from the deep sea. A. plebs photoreceptors are physiologically adapted for a slow lifestyle in a low-light environment, where maximizing photon capture occurs at the expense of detecting fast events in the visual scene.     

Complete mitochondrial genome of the Antarctic amphipod Gondogeneia antarctica (Crustacea, amphipod)

The complete sequence of the mitochondrial genome of the Antarctic amphipod Gondogeneia antarctica was determined to be 18,424 bp in length, and to contain 13 protein-coding genes (PCGs), 22 tRNA genes, and large (rrnL) and small (rrnS) rRNA genes. Its total A+T content is 70.1%. The G. antarctica mitogenome is the largest known among those of crustaceans, due to the existence of two relatively large intergenic non-coding sequences. The PCG arrangement of G. antarctica is identical to that of the ancestral pancrustacean ground pattern, although the tRNA arrangement differs somewhat. The complete mitogenome sequences of 68 species of pancrustacea have been added to the NCBI database, only 4 of which represent complete mitogenome sequences from amphipods. This is the first report of a mitogenome sequence of an Antarctic amphipod, and provides insights into the evolution of crustacean mitochondrial genomes, particularly in amphipods.     

The Mouthparts in Some North Atlantic Species of the Genus Orchomene Boeck (Crustacea, Amphipoda)

The mouthparts of five North Atlantic species of the genus Orchomene have been studied in detail. The species are: O. serrata (Boeck, 1861), O. amblyops G. O. Sars, 1890, O. crispata (Goes, 1866). O. humilis (A. Costa, 1853) and O. pectinate. G. O. Sars, 1882. The structure of the mandibular molar has been studied with the aid of the scanning electron microscope. Some features of the mouthparts, common to the species examined, are interpreted as generic criteria which should justify retaining the genus Orchomene Boeck, 1870, separated from the closely related genera OrchomeneUa G. O. Sars, 1890, and Orchomenopsis G. O. Sars. 1891.     

On the nutrition of two Antarctic acanthonotozomatidae (Crustacea: Amphipoda)

Summary Two species of Antarctic acanthonotozomatid amphipods (Echiniphimedia hodgsoni and Maxilliphimedia longipes) have been investigated in regard to their food preference. Examinations of food remains in foreguts and midguts indicate that Echiniphimedia hodgsoni feeds on sponges and Maxilliphimedia longipes ingests Cnidaria. These foregut analyses are supported by a study of the functional morphology of the mouthparts. The structure of the mandibles in particular can be interpreted as an adaptation to the presumed food source. The right mandible of Echiniphimedia hodgsoni bears a lacinia mobilis which works as an additional cutting edge. By means of this cutting mechanism Echiniphimedia hodgsoni should be able to bite out tough sponge material. The mandibular endites of Maxilliphimedia longipes are medially excavated, the incisors are regulary toothed and the molars vestigial. Such kinds of mandibles are adapted to cut large fragments of soft food, i.e. the mucous tissue of Cnidaria.     

Adaptability of the amphipod Pontoporeia affinis (Crustacea: Amphipoda) to salinity changes

Salinity tolerance of the amphipod Pontoporeia affinis from the Baltic Sea was examined after acclimation to increased and decreased salinity. There were adaptive shifts in tolerance related to ambient salinity. A sharp change in salinity tolerance of amphipods occurred after acclimation to extremely low salinity.     

Physiological responses to hyposmotic stress in the supralittoral amphipod Talitrus saltator (Crustacea: Amphipoda)

We investigated the effect of hyposmotic stress upon osmotic capacity (OC), heart rate (HR) and heart rate variability (HRV) of an Atlantic population of the sandhopper Talitrus saltator. Strictly consecutive monitoring of cardiac activity and measurement of OC was achieved by employing a non-invasive infrared technique to monitor heart activity and direct cardiac puncture to collect the haemolymph. Body mass (range 60-160 mg) had no effect on mean HR nor the mean HRV in adult individuals of both sexes. Both OC and HR, but not HRV, were influenced by the interaction between osmotic stress and exposure time. In addition, OC and HR were coupled within a moderate range of osmotic stress, beyond which this association breaks down, and bradycardia and a positive correlation between HR and HRV occur. A possible rise in metabolic cost, indicated by HR increase, was described for hyper-osmotic regulation. This is the first assessment of the relationship OC and HR in a non-decapod crustacean, and the results indicate that, in sandhoppers, the disruptions of cardiac activity may lead to impairment in maintaining an efficient haemolymph osmotic regulation.     

Redescription of the freshwater amphipod Hyalella faxoni from Costa Rica (Crustacea: Amphipoda: Hyalellidae)

Hyalella faxoni Stebbing, 1903 from Costa Rica is redescribed. The species was previously in the synonymy of Hyalella azteca (Saussure, 1858). The morphological differences between these two species are discussed.     

A new species of Caprellinoides (Crustacea: Amphipoda: Phtisicidae) from the Antarctic

A new caprellid species, Caprellinoides singularis, is described and illustrated based on the material collected on the Polarstern Cruise ANT XVII/3 from the Branfield Strait. The most striking characteristic of this species is the presence of bilobed gills on pereonites 3 and 4. The genus Caprellinoides is revised. Caprellinoides antarctica Schellengerg, 1926 and Caprellinoides spinosus Barnard, 1930 are considered junior synonyms of Caprellinoides tristanensis Stebbing, 1888 and Caprellinoides mayeri (Pfeffer, 1888), respectively. The new species, C. singularis, is compared with the remaining species in the genus Caprellinoides: C. tristanensis and C. mayeri, which are illustrated in detail. Received in revised form: 4 June 2001 Electronic Publication     

Comparative fore-gut morphology of Antarctic Amphipoda (Crustacea) adapted to different food sources

The fore-gut morphology of ten species of Antarctic amphipods utilizing different food sources was investigated. There are considerable differences in shape, relative lengths of the stomachs and their structures. Relative lengths of the stomachs range from more than 30% to 2% compared to the total body lengths. The relative length of the anterior rough filter corresponds in general with the relative stomach length. Stomachs with long rough filter share in general a small fine filter area.Interspecific differences of stomach lateralia might be used for phylogenetic analysis, but are apparently not related to different food sources.Different speculative selective pressures that might have had influence on the evolution of fore-guts are discussed.     

Intracerebral ocelli in an amphipod: extraretinal photoreceptors of the sandhopper Talitrus saltator (Crustacea, Amphipoda)

Abstract. No morphological clues on the amphipod head indicate the existence of ocelli. However, as in several isopod species studied so far, two rudimentary photoreceptors are integrated into the medio-dorsal part of the brain. This electron microscopical study of the photoreceptors is the first report on the presence of ocelli in amphipods. Each ocellus is made up of 3 receptor cells which contribute to the formation of a photoreceptive surface (the rhabdom) formed by tightly packed microvilli. The rhabdoms are twisted and irregular in outline. Membrane turnover is suggested by the presence of different kinds of lysosomes. Lacking dioptric lenses, these photoreceptors are not likely to be involved in image formation but may function as appraisers of ambient light intensity. Physiological and behavioral studies will, henceforth, have to take into account these unexpected ocelli, which may represent remnants of the naupliar eye.     

Respiration of Orchomene plebs (Hurley, 1965) and Waldeckia obesa (Chevreux, 1905) from Admiralty Bay (South Shetlands Islands,Antarctic)

Metabolism of Orchomene plebs from Admirably Bay is higher than that of Waldeckia obesa, and similar to that of P. plebs from McMurdo Sound. The range of variation in respiration is highest below freezing which means that these are optimum temperatures for the Amphipoda species under study.     

Global diversity of amphipods (Amphipoda; Crustacea) in freshwater

Amphipods are brooding peracaridan crustaceans whose young undergo direct development, with no independent larval dispersal stage. Most species are epibenthic, benthic, or subterranean. There are some 1,870 amphipod species and subspecies recognized from fresh or inland waters worldwide at the end of 2005. This accounts for 20% of the total known amphipod diversity. The actual diversity may still be several-fold. Amphipods are most abundant in cool and temperate environments; they are particularly diversified in subterranean environments and in running waters (fragmented habitats), and in temperate ancient lakes, but are notably rare in the tropics. Of the described freshwater taxa 70% are Palearctic, 13% Nearctic, 7% Neotropical, 6% Australasian and 3% Afrotropical. Approximately 45% of the taxa are subterranean; subterranean diversity is highest in the karst landscapes of Central and Southern Europe (e.g., Niphargidae), North America (Crangonyctidae), and Australia (Paramelitidae). The majority of Palearctic epigean amphipods are in the superfamily Gammaroidea, whereas talitroid amphipods (Hyalella) account for all Neotropic and much of the Nearctic epigean fauna. Major concentrations of endemic species diversity occur in Southern Europe, Lake Baikal, the Ponto-Caspian basin, Southern Australia (including Tasmania), and the south-eastern USA. Endemic family diversity is similarly centered in the Western Palearctic and Lake Baikal. Freshwater amphipods are greatly polyphyletic, continental invasions have taken place repeatedly in different time frames and regions of the world. In the recent decades, human mediated invasions of Ponto-Caspian amphipods have had great impacts on European fluvial ecosystems. Guest editors: E. V. Balian, C. Lévêque, H. Segers and K. Martens Freshwater Animal Diversity Assessment     

A preliminary molecular and morphological phylogeny of the Antarctic Epimeriidae and Iphimediidae (Crustacea, Amphipoda)

The phylogenetic relationships of 14 species of the Antarctic amphipod families Epimeriidae and Iphimediidae were investigated using 553bp of the gene for the mitochondrial cytochrome oxidase subunit I (COI) and 98 morphological characters. Both families are dominant members of the Antarctic benthic amphipod community. In contrast to previous studies, our molecular and morphological data suggest that the families Epimeriidae and Iphimediidae may not be sister taxa. Our study suggests that Iphimediidae are more closely related to Eusirus (Eusiridae) than to Epimeria (Epimeriidae). Phylogenetic analyses based on maximum parsimony (MP) and maximum likelihood (ML) indicate that the genera Iphimediella and Gnathiphimedia are not monophyletic.     

Seasonal variability in the grazing potential of the invasive amphipod Gammarus tigrinus and the native amphipod Gammarus salinus (Amphipoda: Crustacea) in the northern Baltic Sea

Mesograzers are known to reduce the biomass of their host plant and modify the structure of the whole macrophyte community in many ecosystems. Thus, the introduction of an efficient mesograzer may destabilize macrophyte community and also affect the native grazers. We estimated how large proportion of macrophyte production are consumed by the alien gammarid G. tigrinus and the native gammarid G. salinus in the species poor ecosystem of the northern Baltic Sea. We analysed whether G. tigrinus consumes different diet as the native G. salinus and whether the effect of G. tigrinus on the survival of the native G. salinus is macrophyte species specific. Grazing experiments showed that there was a clear difference in the grazing rates of gammarids among the studied macrophyte species in summer and autumn but not in spring. The grazing rates were significantly higher in the prevailing macrophyte Pilayella littoralis as compared to other macrophytes. The grazing was inversely related to the diurnal net photosynthetic values of macrophytes. The gammarid amphipods potentially removed only a minor part of plant primary production except for summer and autumn when grazing of a few perennial species exceeded macrophyte production. Macrophyte species and presence of G. salinus had no effect on the survival of G. tigrinus. The presence of G. tigrinus, however, reduced the survival of the native gammarids within P. littoralis in summer. To conclude it is likely that both native and alien gammarid amphipods do not exert significant pressure on the macroalgal communities in the northern Baltic Sea. Competitive interactions between G. tigrinus and G. salinus within the prevailing macrophyte P. littoralis is the likely explanation of the decline of the native gammarid amphipods after the establishment of G. tigrinus in the northern Baltic Sea.     

The geometry of natural selection in cave and spring populations of the amphipod Gammarus minus Say (Crustacea: Amphipoda)

The shape of the fitness function was investigated in a spring population (Davis Spring) and a cave population (Organ Cave) of the amphipod Gammarus minus in southern West Virginia. The first three principal components of a series of size, antennal and eye measurements and two fitness components (amplexus and number of eggs) were investigated. Smoothing splines using generalized cross-validation scores were found as were least-squares quadratic regression equations. Confidence intervals on shape, as determined by a smoothing parameter, were calculated using a bootstrap approach. Out of 18 cases, there were six cases of no selection, six cases of directional (monotonic) selection, one case of stabilizing selection, one case of disruptive selection, and four cases of selection with multiple internal maxima and minima. Selection on the cave population was frequently directional (5), resulting from strong directional selection for increased size, larger antennae, and smaller eyes. Selection on the spring population, the ancestral habitat for the species, was frequently non-monotonic (five cases) and rarely directional (one case). In both populations, stabilizing selection was rare and no more common than disruptive selection.     

Visual pigment,dark adaptation and rhodopsin renewal in the eye of Pontoporeia affinis (Crustacea,Amphipoda)

The benthic amphipod Pontoporeia affinis lives in the Baltic sea and in northern European lakes in an environment where very little light is available for vision. The eyes, consisting of 40–50 ommatidia, are correspondingly modified. Microspectrophotometric recordings on isolated eyes show the presence of at least two kinds of screening pigments in the ommatidia with maxima at 540–580 nm and 460–500 nm. Difference spectra obtained from the rhabdoms after exposure to red and blue light, respectively, give evidence of a single rhodopsin with its maximum at 548 nm and a 500-nm metarhodopsin. In ERG recordings sensitivity in the dark-adapted state, after saturating exposures to blue and to red light, stabilizes at levels determined by the rhodopsin concentration. No change is observed during 10–14 h after the beginning of dark adaptation. However, using animals pre-exposed with a strong red light and then kept in darkness, it is found that after a delay of 20–40 h sensitivity of the dark-adapted eye begins to increase and finally, after 5–6 days reaches a level corresponding to 100% rhodopsin. Thus, a slow renewal of rhodopsin appears to occur in darkness, where a photoisomerization of metarhodopsin is excluded.Abbreviations ERG electroretinogram - IR infrared - MSP microspectrophotometry     

Observations on the feeding behaviour of the Antarctic Gammarid Eusirus perdentatus Chevreux, 1912 (Crustacea: Amphipoda) in aquaria

Summary The feeding behaviour of adult Eusirus perdentatus was observed in aquaria. These observations and the analysis of gut contents of preserved specimens demonstrate that this species is a carnivorous predator feeding on polychaetes, amphipods and other small crustaceans. Apparently the localization and capture of prey in aquaria follows a fixed pattern.