Rick Hochberg

Phylogeny of Gastrotricha: a morphology-based framework of gastrotrich relationships

Currently, the phylum Gastrotricha is divided into the orders Macrodasyida and Chaetonotida, with the structure of the myoepithelial pharynx being an important distinguishing feature. Macrodasyida currently has six recognized families, and Chaetonotida comprises seven families. However, within-group relationships are poorly understood. To arrive at a better understanding of gastrotrich systematics and phylogeny, we performed the first cladistic analysis of nearly all known gastrotrich genera using 71 morphological characters. Results suggest that the Gastrotricha is a monophyletic group (supported by 82% of bootstrap replications) with its most primitive taxa distributed among the families Dactylopodolidae and Neodasyidae. Monophyly of Macrodasyida and Chaetonotida was supported by 90% and 52% bootstrap replications, respectively. Within the Macrodasyida, the families Dactylopodolidae, Turbanellidae, Macrodasyidae, and Thaumastodermatidae all formed monophyletic clades. The families Planodasyidae and Lepidodasyidae were paraphyletic. Among the Chaetonotida, the marine family Xenotrichulidae was monophyletic, supported by 51% of bootstrap replications. A second clade containing all freshwater families was supported by 62% bootstrap values. However, Chaetonotidae were paraphyletic. Using this analysis as a framework, we now can explore possible patterns of evolution within it, and arrive at a consensus of the gastrotrich ground pattern. Moreover, in future molecular studies of metazoan phylogeny, we will be able to select gastrotrich species that are more appropriate representatives of the phylum.

Gastrotricha: A Marine Sister for a Freshwater Puzzle

Background Within an evolutionary framework of Gastrotricha Marinellina flagellata and Redudasys fornerise bear special interest, as they are the only Macrodasyida that inhabit freshwater ecosystems. Notwithstanding, these rare animals are poorly known; found only once (Austria and Brazil), they are currently systematised as incertae sedis. Here we report on the rediscovery of Redudasys fornerise, provide an account on morphological novelties and present a hypothesis on its phylogenetic relationship based on molecular data. Methodology/Principal Findings Specimens were surveyed using DIC microscopy and SEM, and used to obtain the 18 S rRNA gene sequence; molecular data was analyzed cladistically in conjunction with data from 42 additional species belonging to the near complete Macrodasyida taxonomic spectrum. Morphological analysis, while providing new information on taxonomically relevant traits (adhesive tubes, protonephridia and sensorial bristles), failed to detect elements of the male system, thus stressing the parthenogenetic nature of the Brazilian species. Phylogenetic analysis, carried out with ML, MP and Bayesian approaches, yielded topologies with strong nodal support and highly congruent with each other. Among the supported groups is the previously undocumented clade showing the alliance between Redudasys fornerise and Dactylopodola agadasys; other strongly sustained clades include the densely sampled families Thaumastodermatidae and Turbanellidae and most genera. Conclusions/Significance A reconsideration of the morphological traits of Dactylopodola agadasys in light of the new information on Redudasys fornerise makes the alliance between these two taxa very likely. As a result, we create Anandrodasys gen. nov. to contain members of the previously described D. agadasys and erect Redudasyidae fam. nov. to reflect this novel relationship between Anandrodasys and Redudasys. From an ecological perspective, the derived position of Redudasys, which is deeply nested within the Macrodasyida clade, unequivocally demonstrates that invasion of freshwater by gastrotrichs has taken place at least twice, in contrast with the single event hypothesis recently put forward.

A new species, genus and family of marine Gastrotricha from Jamaica, with a phylogenetic analysis of Macrodasyida based on molecular data

Gastrotricha is a phylum of aquatic microinvertebrates counting about 850 species divided into the two orders Chaetonotida and Macrodasyida. Currently, Chaetonotida includes 8 families and 31 genera while Macrodasyida includes 9 families and 32 genera; however, systematics is in a state of flux due to phylogenetic incongruences of the classical systematization. The process of re-systematization will benefit from additional surveys of insufficiently known taxa and especially from the discovery of new species bearing novel characteristics that could help to recognize plesiomorphy in these morphologically diverse animals. Herein, a new and interesting macrodasyidan species from the northwestern shore of Jamaica is described. Specimens up to 1170 μm in length have: a vermiform body with numerous epidermal glands; naked cuticle; head weakly demarked; posterior end in the form of two caudal pedicles; TbA, arranged in two diagonal rows, inserting directly on the cuticle; TbL/TbVL scarce; TbD and TbV absent; TbP at the tip of each caudal pedicle and along its inner margins; mid-sized terminal mouth; pharynx with pores at the base; PhIJ at U20; intestine rectilinear, anus ventral; hermaphroditic sexual apparatus; gonads paired: female anterior, male posterior; gametes maturing in a caudo-cephalic direction; spermatozoa relatively short, with spiralled head and spiralled tail; sperm ducts directed posteriorly, converging ventrally on the midline and joining the sac-like caudal organ; and frontal organ, dorsal to the intestine, not obviously muscularized. External morphology and layout of the reproductive system appear so unique among Gastrotricha to grant the establishment of a new taxon, for which the name Hummondasys jamaicensis gen. et sp. nov. is proposed. Furthermore, to allocate the new genus, the creation of the new family Hummondasyidae is proposed based both on the morphological peculiarities and results of phylogenetic analyses based on the 18S rRNA gene, which involved all of the relevant macrodasyidan taxa. http://zoobank.org/urn:lsid:zoobank.org:pub:595575EA-5ADF-4D0C-AD8B-8BC87E3D3905

A Muscular Double Helix in Gastrotricha

Abstract Studies on the muscular system of gastrotrichs revealed a new muscle arrangement. Helicoidal muscles, in right- and left-hand spirals, were found in all 9 species of Gastrotricha examined. The macrodasyidans Acanthodasys aculeatus , Tetranchyroderma papii and Thaumastoderma heideri had helicoidal muscles along the entire digestive tract (pharynx and intestine), whereas Macrodasys caudatus and Turbanella cornuta displayed helicoidal muscles mainly along the pharynx. Representatives of the order Chaetonotida, Chaetonotus aculifer , Draculiciteria tessalata, Lepidodermella squamata , and Xenotrichula intermedia , all had helicoidal muscles on the intestine, while pharyngeal helicoidal muscles were not observed in C. aculifer or L. squamata . Helicoidal muscles are the thinnest-diameter muscles in the gastrotrich body and generally surround splanchnic circular muscles and longitudinal bands. The proposed functions of helicoidal muscles include antagonization of radial dilations of the digestive tract during feeding, pressurization of the intestine and closure of the pharyngeal-intestinal valve in chaetonotidans, and prevention of intestinal kinking during changes in body shape. The presence of this unique muscle orientation may be a synapomorphy for Gastrotricha.

The muscular system of Dactylopodola baltica and other macrodasyidan gastrotrichs in a functional and phylogenetic perspective

The gastrotrich muscular system is characterized by band‐like muscles arranged in orientations that reflect both function and phylogeny. To better understand the evolution of the Dactylopodolidae, a putative primitive lineage and potential sister group to other extant macrodasyidans, we have used a fluorescent phalloidin stain to visualize muscle patterns in the marine gastrotrich Dactylopodola baltica and eight other species of Macrodasyida from four families. The musculature of D. baltica is arranged as a series of circular, helicoidal and longitudinal bands around the digestive tract. Circular muscles and longitudinal muscles were found in splanchnic and somatic positions. Helicoidal muscles, in 50–60° angles with respect to the longitudinal body axis, surrounded circular and longitudinal splanchnic muscles in a spiralling orientation. The largest longitudinal muscles were the ventrolateral bands composed of numerous cross‐striated myocytes arranged in parallel arrays. The overall arrangement of the muscular system of D. baltica showed several similarities to other macrodasyidan gastrotrichs, including the presence and location of circular, helicoidal and longitudinal muscles, their orientation with respect to the longitudinal body axis and their points of insertion. Unique to D. baltica is the anterior and posterior arrangement of the ventrolateral muscles and the orientation of muscle branches that supply the ventral and dorsal aspects of the pharynx. Muscle data from observations of D. baltica and eight additional species were coded as phylogenetic characters, mapped onto a cladogram and compared to an existing phylogeny of the order. The direction of evolutionary change in specific muscle groups was inferred, as was the ground pattern of muscles for the Macrodasyida.

Hexamethyldisilazane for Scanning Electron Microscopy of Gastrotricha

We evaluated treatment with hexamethyldisilazane (HMDS) as an alternative to critical-point drying (CPD) for preparing microscopic Gastrotricha for scanning electron microscopy (SEM). We prepared large marine (2 mm) and small freshwater (100 μm) gastrotrichs using HMDS as the primary dehydration solvent and compared the results to earlier investigations using CPD. The results of HMDS dehydration are similar to or better than CPD for resolution of two important taxonomic features: cuticular ornamentation and patterns of ciliation. The body wall of both sculpted (Lepidodermeila) and smooth (Dolichodasys) gastrotrichs retained excellent morphology as did the delicate sensory and locomotory cilia. The only unfavorable result of HMDS dehydration was an occasional coagulation of gold residue when the solvent had not fully evaporated before sputter-coating. We consider HMDS an effective alternative for preparing of gastrotrichs for SEM because it saves time and expense compared to CPD.

Two new species of Pseudostomella (Gastrotricha: Macrodasyida) with scaled triancres from the east coast of Australia

Abstract Two new species of Pseudostomella from North Stradbroke Island, Australia, P. klauserae n. sp. and P. megapalpator n. sp., are characterised by scaled triancres, cuticular structures that until now were known only from one other member of the genus. The structure of the scaled triancres in both species is new: the triancre base is diamond‐ or triangular‐shaped with three shafts that arise to form feather‐like scales. P. klauserae n. sp. is characterised by scales arising from a diamond‐shaped triancre base, the distribution of cephalic papillae, and the abundance and distribution of anterior and lateral adhesive tubes. P. megapalpator n. sp. is distinguished by scales arising from a rounded‐triangular triancre base, large dorsal, digitiform papillae, and ventral adhesive tubes. The presence of two new species with scaled triancres allows for the formation of a novel species complex within the genus. An amended key to the species is included.

The musculature of Draculiciteria tessalata (Chaetonotida, Paucitubulatina) : implications for the evolution of dorsoventral muscles in Gastrotricha

The muscular system of the marine interstitial gastrotrich Draculiciteria tessalata (Chaetonotida, Paucitubulatina) was analyzed with fluorescent phalloidin. Muscles in circular, longitudinal, helicoidal and dorsoventral orientations were found. Circular muscles were present as discreet rings on the pharynx only. Five pairs of longitudinal muscles were found in dorsal, lateral and ventral positions. One of the two pairs of lateral muscles is newly described for the species. Helicoidal muscles, external to the circular muscles and some longitudinal bands, spiraled around the pharynx and anterior portion of the intestine. Two pairs of segmentally-arranged dorsoventral muscles were also present. Lateral dorsoventral muscles extended from the base of the pharynx to the anterior part of the caudal furca. Medial dorsoventral muscles extended from the pharyngeal-intestinal junction into each ramus of the caudal furca. A hypothesis on the evolution of dorsoventral muscles in D. tessalata is proposed which includes a splitting of circular muscles into separate somatic and splanchnic components with a further displacement of both muscle sets into a dorsoventral orientation.

Acanthodasys caribbeanensis sp. n., a new species of Thaumastodermatidae (Gastrotricha, Macrodasyida) from Belize and Panama

Abstract We describe one new species of Acanthodasys (Gastrotricha, Macrodasyida, Thaumastodermatidae) collected from sublittoral sites around Carrie Bow Cay, Belize and Isla Colón in the Bocas del Toro archipelago, Panama. Though eight species of Acanthodasys are currently recognized, no species has yet been reported from the Caribbean. Acanthodasys caribbeanensis sp. n. is characterized by the lack of lateral adhesive tubes, the presence of ventrolateral adhesive tubes, and with cuticular armature in the form of both spineless and spined scales. The spineless scales are not elliptical as in other species of Acanthodasys, but are instead variable in shape and closely resemble the spineless scales of species of Diplodasys. Spined scales bear uniancres up to 50 µm long and are the largest reported in the genus. Uniancres are arranged dorsally around the mouth rim and distributed in five distinguishable columns. Adult size varies from 325–625 µm long.

Functional morphology of muscles in Tetranchyroderma papii (Gastrotricha)

Abstract Movement in gastrotrichs is powered by an interaction of ventral cilia and muscles. In interstitial gastrotrichs, movement among sand grains often requires the additional use of adhesive tubules that allow for behaviors such as escape responses and changes in body position. In this study, we describe orientations and possible mechanical actions of muscles during locomotion in the gastrotrich, Tetranchyroderma papii (Macrodasyida). Fluorescently labeled phalloidin was used to stain F-actin of muscles and visualize muscle patterns. Muscles are arranged in circular, longitudinal, and helicoidal orientations. Circular muscles were in the form of discreet rings around the pharynx and intestine, and contribute to the structure of the oral hood. Longitudinal muscles are largely concentrated on the ventral and ventrolateral sides of the body, where they aid in body flexion, including directional changes during ciliary swimming, body torsion, and escape responses. Helicoidal muscles, present as myocytes in left- and right-hand orientations, lie external of the circular bands and some of the longitudinal bands, and are hypothesized to counteract dilations of the pharynx and intestine during feeding. Extraordinary muscle orientations with undetermined functions include a pair of crossover muscles and a single semicircular muscle band at the caudal end.