Robert Price Higgins-Aspects of his life and his contributions to meiobenthic research (1932-2022).

The scientific life of Robert P. Higgins was devoted to meiofauna, microscopically small animals living in aquatic sediments from the intertidal to hadal depths worldwide. He focused on the taxonomy, life-history, and ecology of the marine taxa Kinorhyncha, Tardigrada, and Priapulida and co-discovered the phylum Loricifera. He improved the methods for studying meiofauna and contributed significantly to spreading knowledge about these animals. Aspects of his life are described and lists of his publications, taxa described, contributions to science, and honours received are provided.

Echinoderes unispinosus (Kinorhyncha: Cyclorhagida), a new record from deep-sea sediments in the Gulf of Mexico.

Kinorhyncha is a phylum of exclusively marine, meiofaunal invertebrates (Sørensen Pardos 2008). Currently, the knowledge about the worldwide distribution of most of its species is considerably patchy, as several taxa have been reported from a single or few localities, usually within a limited geographic area (Sørensen Pardos 2008; Yamasaki et al. 2018a). This even becomes more evident for deep-sea kinorhynchs, as noticed by Sørensen et al. (2018). Given the particular value of new records from the deep-sea to increase our understanding in the distribution of meiofaunal organisms plus the observed relationship of morphological variation through the bathymetric gradient, we aim to report the first record of Echinoderes unispinosus Yamasaki, Neuhaus George, 2018 in the southern edge of the Gulf of Mexico (GoM), and contribute with new information about the morphological intraspecific variation of this species. In Mexico, studies of kinorhynch biodiversity are limited to the northern Gulf of California (50-1570m depth) (Álvarez-Castillo et al. 2015, 2018; Cepeda et al. 2019) and the Yucatán Peninsula (Sánchez Martínez 2019). The GoM is a semi-closed basin located in a transition zone with both subtropical and tropical weather. Deep-sea muddy sediment samples of the present study were collected during the expedition XIXIMI-5 (10-24 June 2016) and processed according to Cisterna-Céliz et al. (2019). Kinorhynch specimens were mounted, measured and photographed according to Cepeda et al. (2019). Two adult males were identified as E. unispinosus, one from station B11 (2298 m depth) and another from station B15 (3708m depth). Kinorhynch specimens were deposited at the Aquatic Invertebrates Collection of the Facultad de Ciencias, Universidad Nacional Autónoma de México (UNAM), Mexico, under accession numbers: KGM.001 and KGM.002.

Does sediment composition sort kinorhynch communities? An ecomorphological approach through geometric morphometrics.

Ecomorphology studies the relationship between organisms' morphology and environment features. To better understand whether the shape of the body and the appendages involved in the movement is correlated to sediment composition in meiofaunal organisms, we study the evolved morphological adaptations to environment in selected taxa of the phylum Kinorhyncha: the allomalorhagid families Dracoderidae and Pycnophyidae, and the cyclorhagid genus Echinoderes. The selected taxa include the most diverse groups of Kinorhyncha worldwide, representing the 75.5% of the total phylum diversity. Widened, plump bodies and lateral terminal spines may be adaptive for species living in coarse, more heterogeneous sediments, as they could maintain a more powerful musculature to actively displace the sediment grains applying a greater force. Conversely, slender, vermiform bodies and lateral terminal spines would represent an adaptation of species inhabiting fine, more homogeneous sediments where there would not be much need to exert a high force to displace the sediment particles, and a more vermiform shape would even favour the burrowing of the animal through the smaller interstices. The studied kinorhynch taxa would also be adapted to the higher velocity of the sea-water and the intense erosion and transportation of heterogeneous sediments by possessing more robust bodies, avoiding getting laid off substratum under these conditions. These findings provide evolutionary evidence that body shape in the studied kinorhynch groups is adapted to environment.

Neuroanatomy of mud dragons: a comprehensive view of the nervous system in Echinoderes (Kinorhyncha) by confocal laser scanning microscopy.

BACKGROUND: The Scalidophora (Kinorhyncha, Loricifera and Priapulida) have an important phylogenetic position as early branching ecdysozoans, yet the architecture of their nervous organ systems is notably underinvestigated. Without such information, and in the absence of a stable phylogenetic context, we are inhibited from producing adequate hypotheses about the evolution and diversification of ecdysozoan nervous systems. Here, we utilize confocal laser scanning microscopy to characterize serotonergic, tubulinergic and FMRFamidergic immunoreactivity patterns in a comparative neuroanatomical study with three species of Echinoderes, the most speciose, abundant and diverse genus within Kinorhyncha. RESULTS: Neuroanatomy in Echinoderes as revealed by acetylated α-tubulin immunoreactivity includes a circumpharyngeal brain and ten neurite bundles in the head region that converge into five longitudinal nerves within the trunk. The ventral nerve cord is ganglionated, emerging from the brain with two connectives that converge in trunk segments 2-3, and diverge again within segment 8. The longitudinal nerves and ventral nerve cord are connected by two transverse neurites in segments 2-9. Differences among species correlate with the number, position and innervation of cuticular structures along the body. Patterns of serotoninergic and FMRFamidergic immunoreactivity correlate with the position of the brain neuropil and the ventral nerve cord. Distinct serotonergic and FMRFamidergic somata are associated with the brain neuropil and specific trunk segments along the ventral nerve cord. CONCLUSIONS: Neural architecture is highly conserved across all three species, suggesting that our results reveal a pattern that is common to more than 40% of the species within Kinorhyncha. The nervous system of Echinoderes is segmented along most of the trunk; however, posterior trunk segments exhibit modifications that are likely associated with sensorial, motor or reproductive functions. Although all kinorhynchs show some evidence of an externally segmented trunk, it is unclear whether external segmentation matches internal segmentation of nervous and muscular organ systems across Kinorhyncha, as we observed in Echinoderes. The neuroanatomical data provided in this study not only expand the limited knowledge on kinorhynch nervous systems but also establish a comparative morphological framework within Scalidophora that will support broader inferences about the evolution of neural architecture among the deepest branching lineages of the Ecdysozoa.

Morphology disentangles the systematics of a ubiquitous but elusive meiofaunal group (Kinorhyncha: Pycnophyidae).

Kinorhyncha is a group of benthic, microscopic animals distributed worldwide in marine sediments. The phylum is divided into two classes, Cyclorhagida and Allomalorhagida, congruent with the two major clades recovered in recent phylogenetic analyses. Allomalorhagida accommodates more than one-third of the described species, most of them assigned to the family Pycnophyidae. All previous phylogenetic analyses of the phylum recovered the two genera within Pycnophyidae, Pycnophyes and Kinorhynchus, as paraphyletic and polyphyletic. A major problem in these studies was the lack of molecular data of most pycnophyids, due to the limited and highly localized distribution of most species, often in the Arctic and the deep-sea. We here overcame the problem by adding a morphological partition with data for 79 Pycnophyidae species, 15 of them also represented by molecular data. Model-based analyses yielded seven clades, which each was supported by several morphological apomorphies. Accordingly, Kinorhynchus is synonymized with Pycnophyes and six new genera are described for the remaining recovered clades: Leiocanthus gen. nov., Cristaphyes gen. nov., Higginsium gen. nov., Krakenella gen. nov., Setaphyes gen. nov. and Fujuriphyes gen. nov.

Phylogeny of Kinorhyncha Based on Morphology and Two Molecular Loci.

The phylogeny of Kinorhyncha was analyzed using morphology and the molecular loci 18S rRNA and 28S rRNA. The different datasets were analyzed separately and in combination, using maximum likelihood and Bayesian Inference. Bayesian inference of molecular sequence data in combination with morphology supported the division of Kinorhyncha into two major clades: Cyclorhagida comb. nov. and Allomalorhagida nom. nov. The latter clade represents a new kinorhynch class, and accommodates Dracoderes, Franciscideres, a yet undescribed genus which is closely related with Franciscideres, and the traditional homalorhagid genera. Homalorhagid monophyly was not supported by any analyses with molecular sequence data included. Analysis of the combined molecular and morphological data furthermore supported a cyclorhagid clade which included all traditional cyclorhagid taxa, except Dracoderes that no longer should be considered a cyclorhagid genus. Accordingly, Cyclorhagida is divided into three main lineages: Echinoderidae, Campyloderidae, and a large clade, 'Kentrorhagata', which except for species of Campyloderes, includes all species with a midterminal spine present in adult individuals. Maximum likelihood analysis of the combined datasets produced a rather unresolved tree that was not regarded in the following discussion. Results of the analyses with only molecular sequence data included were incongruent at different points. However, common for all analyses was the support of several major clades, i.e., Campyloderidae, Kentrorhagata, Echinoderidae, Dracoderidae, Pycnophyidae, and a clade with Paracentrophyes + New Genus and Franciscideres (in those analyses where the latter was included). All molecular analyses including 18S rRNA sequence data furthermore supported monophyly of Allomalorhagida. Cyclorhagid monophyly was only supported in analyses of combined 18S rRNA and 28S rRNA (both ML and BI), and only in a restricted dataset where taxa with incomplete information from 28S rRNA had been omitted. Analysis of the morphological data produced results that were similar with those from the combined molecular and morphological analysis. E.g., the morphological data also supported exclusion of Dracoderes from Cyclorhagida. The main differences between the morphological analysis and analyses based on the combined datasets include: 1) Homalorhagida appears as monophyletic in the morphological tree only, 2) the morphological analyses position Franciscideres and the new genus within Cyclorhagida near Zelinkaderidae and Cateriidae, whereas analyses including molecular data place the two genera inside Allomalorhagida, and 3) species of Campyloderes appear in a basal trichotomy within Kentrorhagata in the morphological tree, whereas analysis of the combined datasets places species of Campyloderes as a sister clade to Echinoderidae and Kentrorhagata.

New species of Centroderes (Kinorhyncha: Cyclorhagida) from the Northwest Atlantic Ocean, life cycle, and ground pattern of the genus.

Four new species of Centroderes are described from the Northwest Atlantic Ocean based on light microscopical observations of 153 adult and 26 juvenile specimens and on SEM investigations of 54 adult and 3 juvenile specimens. Centroderes barbanigra n. sp. and C. bonnyae n. sp. can be distinguished from all other species by the existence of a short lateroventral tube on segment 7. The latter species can be separated from the former by an acicular spine in the lateral accessory position on segment 8. The female of both C. readae n. sp. and C. spinosus possesses a female-specific, modified gland cell outlet on segments 7-9, but such an outlet is missing on segment 7 of all other species. The latter species is distinguished from the former by its robust lateroventral spine on segment 8 and by its lack of a laterodorsal sensory spot on segment 4, whereas the former species shows a midlateral sensory spot on segment 8. Centroderes drakei n. sp. agrees with the remaining American species in the possession of a laterodorsal sensory spot on segment 4; the former species can be distinguished from C. readae n. sp. by the lack of a sensory spot sublaterally on segment 1 and midlaterally on segment 8 as well as by the lack of a female-specific, modified gland cell outlet on segment 7; C. drakei n. sp. can be separated from C. barbanigra n. sp. and C. bonnyae n. sp. by its lack of a lateroventral tube on segment 7.        We report anomalies rarely noticed for Kinorhyncha, such as different developmental artifacts in several specimens and a potential tumour in one individual. Evidence is provided that species of Centroderes develop via at least two adult life history stages, but three or more adult stages exist in C. drakei n. sp.; this represents the first record of a more complicated life cycle in Kinorhyncha. This paper also contains the first report of spermatophores in cyclorhagid Kinorhyncha and in both female and male specimens. In addition, characters in the ground pattern of Centroderes are summarized.

Comparative myoanatomy of Echinoderes (Kinorhyncha): a comprehensive investigation by CLSM and 3D reconstruction.

INTRODUCTION: Kinorhyncha is a clade of marine invertebrate meiofauna. Their body plan includes a retractable introvert bearing rings of cuticular spines, and a limbless trunk with distinct segmentation of nervous, muscular and epidermal organ systems. As derived members within the basal branch of Ecdysozoa, kinorhynchs may provide an important example of convergence on the evolution of segmentation within one of three bilaterian superclades. We describe the myoanatomy of Echinoderes, the most specious kinorhynch genus, and build upon historical studies of kinorhynch ultrastructure and gross morphology. This is the first multi-species comparison of a complete organ system by confocal microscopy and three-dimensional reconstruction within Kinorhyncha. RESULTS: Myoanatomy of adult Echinoderes is composed of the following: Head with two mouth cone circular muscles, nine pairs of oral style muscles, ten introvert retractors, one introvert circular muscle, and fourteen introvert circular muscle retractors; Neck with one circular muscle; Trunk showing distinct pairs of ventral and dorsal muscles within segments 1-10, dorsoventral muscles within segments 3-10, diagonal muscles within segments 1-8, longitudinal fibers spanning segments 1-9, three pairs of terminal spine muscles, and one pair of male penile spine muscles; Gut showing a pharynx with ten alternating rings of radial and circular muscle fibers enclosed in a complex sheath of protractors and retractors, an orthogonal grid of longitudinal and circular fibers surrounding the intestine, and paired hindgut dilators. CONCLUSIONS: Myoanatomy is highly conserved between species of Echinoderes. Interspecific variation is observed in the arrangement and number of introvert fibers and the composition of pharyngeal muscles. Segmented trunk musculature facilitates the movements of articulated cuticular plates along the anterior-posterior axis. Intersegmental muscle fibers assist with dorsoventral and lateral trunk movements. Protractors, retractors and circular muscles coordinate eversion and retraction of the introvert and mouth cone, and relocation of the pharynx during locomotion and feeding behaviors. Pairs of posterior fibers suggest independent movements of terminal spines, and male penile spines. Within Scalidophora, myoanatomy is more similar between Kinorhyncha and Loricifera, than either group is to Priapulida. Kinorhynch myoanatomy may reflect a convergent transition from vermiform to segmented body plans during the early radiation of Ecdysozoa.

Comparative morphology of serotonergic-like immunoreactive elements in the central nervous system of kinorhynchs (Kinorhyncha, Cyclorhagida).

Cycloneuralian taxa exhibit similar organ system architectures, providing informative characters of metazoan evolution, yet very few modern comparative descriptions of cellular and molecular homologies within and among those taxa are available. We immunolabeled and characterized elements of the serotonergic nervous system in the kinorhynchs Echinoderes spinifurca, Antygomonas paulae, and Zelinkaderes brightae using confocal laser scanning microscopy. Fluorescent markers targeting DNA were combined with observations of auto-fluorescent structures to guide interpretations of the internal and external anatomy in each species. Results show a common pattern of the central nervous system with a circumenteric brain divided into ring-shaped anterior and posterior neuronal somata and a central neuropil connected to a multi-stringed, longitudinal ventral nerve cord. Structural similarities and differences in the nervous systems of these species were observed and described, stressing the incomplete ring nature of the anterior region of the kinorhynch brain, the functional relationship between the brain and the movable introvert, and the number and arrangement of nerve strings and somata of the ventral nerve cord. The ventral cord ends in two ventrolateral cell bodies in E. spinifurca, and forms a terminal loop associated with a midterminal spine in A. paulae and Z. brightae. The possible functional and phylogenetic significance of these features and arrangements are discussed.

Gut-associated cells of Derocheilocaris remanei (Crustacea, Mystacocarida).

The gut-associated cells (GA-cells) of the mystacocarid Derocheilocaris remanei were investigated by transmission electron microscopy. These cells are characterized by a dense cytoplasm, the presence of clear vesicles adjacent to the gut epithelium, glycogen, and lipid droplets. GA-cells envelop the midgut and hindgut and send blunt cytoplasmic extensions to the gut epithelium through its basal lamina. The GA-cells also extend dorsolateral projections to the body wall by means of intermediate cells. In addition to a mechanical function of suspending and stabilizing the gut, these cells may affect the flow of the hemocoelic fluid and may be implicated in the processes of transport, assimilation, and storage of nutrients.

Ultrastructural observations on the phoronid nervous system.

Standard transmission electron microscopy (TEM) techniques have been applied to three species of phoronids to describe cytological features of their nervous systems. Four types of neurons have been distinguished by morphological and ultrastructural criteria, mainly involving their kinds of vesicles. Glial cells with characteristic polymorphic dense granules are abundant in the plexus, with possible roles in neuronal nourishment, mechanical support, and neurosecretion. Synaptic contacts and neuromuscular junctions across the connective tissue are described. The different structural features of the trunk giant axons in the three species mainly concern the enveloping sheath, and are described and discussed. Data on the innervation of internal organs, such as gut and nephridia, are also given. A detailed description of the structure, arrangement, and relationships of tentacular sensory cells, probably mechanoreceptors, is included, with a tentative hypothesis concerning the functions of these cells. © 1996 Wiley-Liss, Inc.

Ultrastructure of the labrum and foregut of Derocheilocaris remanei (Crustacea, Mystacocarida).

The cuticle-lined foregut of Derocheilocaris remanei consists of the mouth with its associated labrum, and an undifferentiated esophagus. It is separated from the midgut by an esophageal valve. The labrum is a conspicuous structure moved by five pairs of muscles (four dorsoventral and one longitudinal). Four pairs of subcuticular glands open to its inner face forming two longitudinal, lateral rows of cuticular pores. Each secretory unit is composed of a glandular component (with one or two secretory cells), a neck cell, and a duct cell. In addition, a single gland cell opens mesially into the buccal cavity. The ventrally located mouth is a complex structure characterized by a filter-like system, a sensory organ, and epithelial cells with highly developed microvilli. The esophagus is a simple tube with a characteristic curvature following the mouth. It has a rounded cross section and a triradiate lumen. A layer of circular musculature surrounds this region. The end of the esophagus protrudes into the midgut lumen forming the so-called esophageal valve. The ultrastructural features of the foregut, with the presence of a mucus-trapping mechanism, a relatively well-developed filter system and associated structures and an esophagus lacking glands confirm the microphagic feeding habits of mystacocarids. © 1996 Wiley-Liss, Inc.