Nymphon Gracile Research Articles

The Pycnogonids (Arthropoda, Pycnogonida) of Torrevaldaliga (Tyrrhenian Sea), Italy, with data on Endeis biseriata, new record for the Mediterranean Sea

New data about the Mediterranean pycnogonids are given. For this contribution, we analysed 315 samples that until now had remained unidentified, collected in 1979–1980 in the Tyrrhenian Sea close to Torrevaldaliga, Italy. We found six species: Ammothella appendiculata, Anoplodactylus californicus, Endeis biseriata, Endeis spinosa, Nymphon gracile and Tanystylum conirostre. Two of them are the subject of taxonomical and faunal remarks: morphological features of Anoplodactylus californicus specimens are reported and discussed, and the first record of Endeis biseriata for the Mediterranean Sea is reported.

Studying sources of incongruence in arthropod molecular phylogenies: Sea spiders (Pycnogonida) as a case study

In this report, we analyze the phylogeny of Pycnogonida using the three nuclear and three mitochondrial markers currently sequenced for studying inter- and intrafamilial relationships within Arthropoda: 18S and 28S rRNA genes, Histone H3, cytochrome c oxidase subunit 1 (CO1), 12S and 16S rRNA genes. We identify several problems in previous studies, due to the use of inappropriate sequences (taxonomic misidentification, DNA contamination, sequencing errors, missing data) or taxa (outgroup choice). Our analyses show that most markers are not powerful to study the phylogeny of sea spiders. The results suggest however a recent diversification of the group (Mesozoic rather than Paleozoic) and the early divergence of Austrodecidae, followed by Colossendeidae, Pycnogonidae and Rhynchothoracidae. Except Ammotheidae and Callipallenidae, all other families were recovered as monophyletic. Analyses of synonymous sites in CO1 sequences reveal an extreme heterogeneity of nucleotide composition within sea spiders, as six unrelated species show a reverse strand-specific bias. We therefore suggest that several independent reversals of asymmetric mutational constraints occurred during the evolution of Pycnogonida, as a consequence of genomic inversions involving either the control region or a fragment containing the CO1 gene. These hypotheses are supported by the comparison of two complete mitochondrial genomes of sea spiders ( Achelia bituberculata and Nymphon gracile) with that of Limulus.

The complete mitochondrial genome of the sea spider Achelia bituberculata (Pycnogonida, Ammotheidae): arthropod ground pattern of gene arrangement.

BackgroundThe phylogenetic position of pycnogonids is a long-standing and controversial issue in arthropod phylogeny. This controversy has recently been rekindled by differences in the conclusions based on neuroanatomical data concerning the chelifore and the patterns of Hox expression. The mitochondrial genome of a sea spider, Nymphon gracile (Pycnogonida, Nymphonidae), was recently reported in an attempt to address this issue. However, N. gracile appears to be a long-branch taxon on the phylogenetic tree and exhibits a number of peculiar features, such as 10 tRNA translocations and even an inversion of several protein-coding genes. Sequences of other pycnogonid mitochondrial genomes are needed if the position of pycnogonids is to be elucidated on this basis.ResultsThe complete mitochondrial genome (15,474 bp) of a sea spider (Achelia bituberculata) belonging to the family Ammotheidae, which combines a number of anatomical features considered plesiomorphic with respect to other pycnogonids, was sequenced and characterized. The genome organization shows the features typical of most metazoan animal genomes (37 tightly-packed genes). The overall gene arrangement is completely identical to the arthropod ground pattern, with one exception: the position of the trnQ gene between the rrnS gene and the control region. Maximum likelihood and Bayesian inference trees inferred from the amino acid sequences of mitochondrial protein-coding genes consistently indicate that the pycnogonids (A. bituberculata and N. gracile) may be closely related to the clade of Acari and Araneae.ConclusionThe complete mitochondrial genome sequence of A. bituberculata (Family Ammotheidae) and the previously-reported partial sequence of Endeis spinosa show the gene arrangement patterns typical of arthropods (Limulus-like), but they differ markedly from that of N. gracile. Phylogenetic analyses based on mitochondrial protein-coding genes showed that Pycnogonida may be authentic arachnids (= aquatic arachnids) within Chelicerata sensu lato, as indicated by the name 'sea spider,' and suggest that the Cormogonida theory – that the pycnogonids are a sister group of all other arthropods – should be rejected. However, in view of the relatively weak node confidence, strand-biased nucleotide composition and long-branch attraction artifact, further more intensive studies seem necessary to resolve the exact position of the pycnogonids.

The complete mitochondrial genome of the sea spider Nymphon gracile (Arthropoda: Pycnogonida)

BackgroundMitochondrial genomes form units of genetic information replicating indepentently from nuclear genomes. Sequence data (most often from protein-coding genes) and other features (gene order, RNA secondary structure) of mitochondrial genomes are often used in phylogenetic studies of metazoan animals from population to phylum level. Pycnogonids are primarily marine arthropods, often considered closely related to chelicerates (spiders, scorpions and allies). However, due to their aberrant morphology and to controversial results from molecular studies, their phylogenetic position is still under debate.ResultsThis is the first report of a complete mitochondrial genome sequence from a sea spider (Nymphon gracile, class Pycnogonida). Gene order derives from that of other arthropods so that presumably 10 single tRNA gene translocations, a translocation of the mitochondrial control region, and one large inversion affecting protein-coding genes must have happened in the lineage leading to Nymphon gracile. Some of the changes in gene order seem not to be common to all pycnogonids, as those were not found in a partial mitochondrial genome of another species, Endeis spinosa. Four transfer RNAs of Nymphon gracile show derivations from the usual cloverleaf secondary structure (truncation or loss of an arm). Initial phylogenetic analyses using mitochondrial protein-coding gene sequences placed Pycnogonida as sister group to Acari. However, this is in contrast to the majority of all other studies using nuclear genes and/or morphology and was not recovered in a second analysis where two long-branching acarid species were omitted.ConclusionExtensive gene rearrangement characterizes the mitochondrial genome of Nymphon gracile. At least some of the events leading to this derived gene order happened after the split of pycnogonid subtaxa. Nucleotide and amino acid frequencies show strong differences between chelicerate taxa, presumably biasing phylogenetic analyses. Thus the affinities between Pycnogonida and Acari (mites and ticks), as found in phylogenetic analyses using mitochondrial genes, may rather be due to long-branch attraction and independently derived nucleotide composition and amino acid frequency, than to a real sister group relationship.

Hox genes in sea spiders (Pycnogonida) and the homology of arthropod head segments

The pycnogonids (or sea spiders) are an enigmatic group of arthropods, classified in recent phylogenies as a sister-group of either euchelicerates (horseshoe crabs and arachnids), or all other extant arthropods. Because of their bizarre morpho-anatomy, homologies with other arthropod taxa have been difficult to assess. We review the main morphology-based hypotheses of correspondence between anterior segments of pycnogonids, arachnids and mandibulates. In an attempt to provide new relevant data to these controversial issues, we performed a PCR survey of Hox genes in two pycnogonid species, Endeis spinosa and Nymphon gracile, from which we could recover nine and six Hox genes, respectively. Phylogenetic analyses allowed to identify their orthology relationships. The Deformed gene from E. spinosa and the abdominal-A gene from N. gracile exhibit unusual sequence divergence in their homeodomains, which, in the latter case, may be correlated with the extreme reduction of the posterior region in pycnogonids. Expression patterns of two Hox genes (labial and Deformed) in the E. spinosa protonymphon larva are discussed. The anterior boundaries of their expression domains favour homology between sea spider chelifores, euchelicerates chelicerae and mandibulate (first) antennae, in contradistinction with previously proposed alternative schemes such as the protocerebral identity of sea spider chelifores or the absence of a deutocerebrum in chelicerates. In addition, while anatomical and embryological evidences suggest the possibility that the ovigers of sea spiders could be a duplicated pair of pedipalps, the Hox data support them as modified anterior walking legs, consistent with the classical views.

Food and Feeding Behaviour of <i>Nymphon gracile </i>(Pycnogonida: Nymphonidae) around the Mumbles Pier area of Swansea, United Kingdom

The feeding behaviour of the “sea-spider” Nymphon gracile (Pycnogonida: Nymphonidae) and its association with other organisms, around the Mumbles pier area of Swansea, United Kingdom is described and compared to the reported cases in other families of the order Pycnogonida. Journal of Aquatic Sciences Volume 16, April 2001, pp. 33-34 KEY WORDS: Feeding behaviour, Pycnogonida, Nymphon gracile, Swansea, United kingdom.

The Littoral Pycnogonida of the Swansea Area of the United Kingdom

A survey was carried out at the Mumbles Pier Ares of Swansea in the United Kingdom to determine whether or not oil spills and other human activity in the region affected the distribution of Pycnogonids (Sea spiders) within the area. Results indicate that the oil spills have no effect on the presence or absence of these organisms. Questions are raised as to how the sea spiders are able to evade the negative effects of an oil spill. Journal of Aquatic Sciences Volume 15, April 2000, pp. 63-65 KEY WORDS: Pycnogonida, Nymphon gracile, Achelia echinata, Swansea, United Kingdom.

The response of the PycnogonidsNymphon gracileleach andPycnogonum littorale(Strom) to light stimuli of different wavelength and intensity

It is generally agreed that the eyes are the only perspicuous sense organs in Pycnogonids. Their structure has previously been described by the present authors when they were shown to be of the non‐inverted, prebascillar type. The present study investigates the photo‐response in Nymphon gracile (leach) and Pycnogonum littorale (Strom), two common British Pycnogonid species. Various aspects of the photoresponse are described and comparisons are made between the responses to light of different wavelengths, and also between animals of different ages. The light responses are discussed in the context of the general behavioural biology of the two species. Both species exhibit similar optimum sensitivities between wavelengths of 530–545 μ. This is similar to many marine invertebrates which occupy a similar habitat. The response in each species is such that the animal tends to remain in a favourable environment.

Ecological distribution of pycnogonids on the Catalan coast

Thirty-three samples, collected from seven stations between Cadaqués and Casas d'Alcanar on the Catalan coast, yielded 870 specimens of 20 species and subspecies of Pycnogonida for a study of shallow water pycnogonocoenosis. Achelia echinata and Ammothella longipes are the numerically dominant species. Amm. uniunguiculata, Callipallene emaciata spectrum and Tanystylum orbiculare are, in order, the next most abundant forms, although occurring in low numbers. Nymphon gracile, Ach. langi, T. communis, Nymphonella tapetis, Ascorhynchus casteRi, Call. ph. phantoma, Call. brevi-rostris producta, Call. e. emaciata, Anoplodactylus petiolatus, An. angulatus and An. virescens are rare forms, each comprising less than 1% of communities. Communities occurring on Halopteris scoparia and Posidonia oceanica were compared for species diversity (Shannon-Weaver index) and statistical testing revealed a higher diversity on the more stable, latter substratum. Some substrata influence community composition, but some pycnogonids do not find the substratum limiting, probably reflecting wide feeding preferences. There appear to be several reproductive cycles in the year, although individual populations remain in their biotopes and do not undergo reproductive migrations. It is not clear what are the durations of reproductive cycles, nor whether single populations undergo more than one reproductive cycle in a year. The sites of larval development remain undiscovered and the patterns of grouping of adults and juveniles remain to be elucidated.

The energy cost of off-shore migration in Nymphon gracile (Pycnogonida)

The transportation off-shore of the littoral pycnogonid Nymphon gracile during the winter months is facilitated by the ebb tide swimming of this otherwise benthic animal. The energy cost of migration has been estimated from measurements of oxygen consumption and drag forces acting on the different segments of the leg during swimming. The swimming patterns of N. gracile both under constant conditions and in response to tidal pressure cycles are considered in relation to the animals' energy reserves.

Experiments on food preference and chemical sense in Pycnogonida

Food choice experiments were carried out with three species of pycnogonids: Nymphon rubrum, N. gracile, and Endeis spinosa. Running sea-water that had circulated over different possible food-sources (hydroids, actiniarians, octocorals, synascidians) was offered to the pycnogonids at one side of a basin, pure sea-water or se-water that had flowed over a different source of food at the other. The experiments, though few in number, seemed to indicate that: (1) Pycnogonids are capable of detecting the presence or absence of various coelenterate species. (2) They are able to discriminate between different food-sources, i.e. the nature of the chemical substances produced by the food determines the degree of attraction. (3) The preferential sequence for the three pycnogonids tested is: Nymphon rubrum prefers Dynamena over Tubularia and Laomedea; these are preferred over Actinia and Metridium; there is no attraction to Alcyonium. Nymphon gracile prefers Laomedea over Dynamena. Endeis spinosa prefers Laomedea over Dynamena; these are preferred over Actinia; there is no attraction to Morchellium. (4) Experimental amputation of appendages indicates a high probability that the chemo-receptors of the pycnogonids are not located on the chelifores, palps, or ovigers. They may be located (a) all over the body, possibly in innervated epidermal bristles; and/or (b) in the distal part of the proboscis, bearing sensory setae; and/or (c) on the walking legs.

Salinity and temperature tolerance by Nymphon gracile (Leach) and Achelia echinata (Hodge) (Pycnogonida)

The tolerance of two species of pycnogonid, Achelia echinata (Hodge) and Nymphon gracile (Leach) to a range of salinities, temperatures and relative humidities has been examined. In most instances N. gracile is more resistant than A. echinata and indeed is found higher in the littoral zone during the warmer months and thus likely to be subjected to greater environmental fluctuation.

The Swimming of Nymphon Gracile (Pycnogonida): The Energetics of Swimming At Constant Depth

ABSTRACT The mechanical power required by Nymphon for swimming at constant depth has been calculated from drag forces acting on the legs. For an adult male this was found to be 3·4 W kg. Only about 60 % of this is used to support the animal’s weight in water. The metabolic rate fluctuates spontaneously over a tidal cycle, being greatest during the ebb-tide period. The mean rate of oxygen consumption during the animals least active phase was found to be about 0·1 µl O2 mg−1 h−1. The total carbohydrate and lipid immediately available for combustion have been estimated at 4·64 and 16 µg/mg wet wt respectively. These quantities should be adequate for about 42 h periodic swimming in an adult Nymphon.

The Swimming of Nymphon Gracile (Pycnogonida): the Vertical Lift Forces Generated While Swimming at Constant Depth

ABSTRACT The vertical lift forces generated by the legs of Nymphon while swimming at constant depth have been estimated graphically using drag constants determined by a sedimentation method. Drag both normal and tangential to the different segments of the leg has been considered. When holding station Nymphon characteristically employs a low elevation beat in which the upward force produced during the power stroke of each leg only just exceeds the predominantly downward force generated during the recovery. An upward lift component is also produced late during the recovery stroke. The legs beat in a vertical plane and an investigation of the moments at the joints of the leg suggests that much of the power stroke is gravity assisted. The upward lift produced by all eight legs agreed fairly well on average with the sinking force due to the animal ‘s weight in water, and the vertical lift fluctuates rhythmically throughout the leg beat cycle. The relationship between the swimming gait and the amplitude of these fluctuations has been investigated. During the gait most frequently used by the animal fluctuations in vertical lift were found to be minimal.

Endogenous tidal rhythmicity in the littoral pycnogonid Nymphon gracile (Leach)

The pycnogonid Nymphon gracile (Leach) has been shown to have an endogenous activity rhythm of tidal frequency, the peak of activity occurring at, or just after, expected high water. Animals collected littorally show some rhythmicity at all times of the year. The rhythm drifts in most individuals when recorded in the laboratory at 20°C, but some animals collected in winter and recorded at 9°C, did not show any drift.

Ultrastructure of the Photoreceptors in the Pycnogonid species,Nymphon gracile(Leach), andPycnogonum littorale(Strom)

In British pycnogonids the eyes are four in number and are sited on a tubercle which is situated on the dorsal surface of the cephalothorax. The shapes of the eye tubercles in some British species are described, together with the ultrastructure of the eyes in Nymphon gracile. The eye is simple with a cup‐shaped, non‐inverted retina similar to that found in a number of arachnids, with a single lens. The ultrastructure of the cuticle, hypodermal cells, tapetum, pigment cells and retinal cells is described. It is similar to that of other simple eyes from a variety of invertebrates.

Reproduction and development in the pycnogonid Pycnogonum littorale

Aspects of the reproductive biology of Pycnogonum littorale (Strom) are examined. Vitellogenesis occurs between November and August, the process resembling that in Nymphon gracile, annelids, and Limulus polyphemus, in which the majority of the yolk is synthesized within the oocyte, with only a small contribution from outside, thus contrasting with the method in insects. The mode of formation of the mature yolk platelets shows similarities with the process described in Cambarus spp. Growth occurs in the post larva by a series of moults, and sometimes in the adult by a process not involving moults. The male usually has 9 moults and the female 10 or 11.

(RESPONSES OF NYMPHON GRACILE (PYCNOGONIDA) TO PRESSURE CYCLES OF TIDAL FREQUENCY.

ABSTRACT The responses of Nymphon gracile to changes in hydrostatic pressure are described. (M, the unit of pressure used here, denotes 1 metre sea water.) A sudden increase in pressure tended to inhibit swimming, while a reduction promoted it, a response opposite in sense to that normally associated with depthregulation. Nymphon showed no adaptive response to cyclical pressure changes similar to those produced by onshore waves. Pressure changes of similar range (1M) but with a 2 sec. period usually induced swimming in previously sluggish individuals. Pressure cycles of 8M range and periods ranging from 14 to 160 min. evoked a depth-regulatory type of response in several individuals, but the majority responded in the opposite way. Nymphon responded to cyclical pressure changes of approximately tidal range (8M) and frequency, by swimming most actively during the late ebb or low water of the artificial cycles. The data of Fage (1932) support the hypothesis that Nymphon swims most often when stimulated by decreasing pressure as the tide ebbs.