Morphometric and taxonomic approach to describe Heterospio variabilis (Annelida, Longosomatidae), a new species with three size-dependent morphotypes, from the Gulf of California, Eastern Pacific.

The Longosomatidae, a poorly known polychaete family, includes only 23 recognized species; in this study, based on morphometric and taxonomic analyses, we describe a new species with three morphotypes: Heterospio variabilis from the Gulf of California, Mexico. The specimens examined exhibit large morphological variations but were clearly separated from close species due to a unique combination of morphological characters: chaetiger 9 as the first elongated chaetiger, four to eight branchial pairs; chaetae from chaetiger 10 forming rings in two rows, posterior row with thin and robust capillaries, anterior row with subuluncini, aristate spines, acicular spines and thick acicular spines. With the discriminant analysis, carried out on 11 morphometric characters, the presence of three morphological groups were recognized (Wilks' lambda= 0.093, p = 0.0001). However, the variables selected to discriminate the specimens (partial Wilks' lambda > 0.57) were correlated to their size: number of branchiae, body width, prostomium width, rate length CH9/CH1-CH8, length CH1-CH8 and length CH9 (r > 0.5). So, we concluded that they belong to a single species with three morphotypes: morpho A with eight branchial pairs, morpho B with 5-6-7 pairs and morpho C with 4 pairs. No correlations between the distribution of the distinct morphotypes along the eastern gulf shelf and the environmental conditions where they settle were detected.

The development of the adult nervous system in the annelid Owenia fusiformis.

BACKGROUND: The evolutionary origins of animal nervous systems remain contentious because we still have a limited understanding of neural development in most major animal clades. Annelids - a species-rich group with centralised nervous systems - have played central roles in hypotheses about the origins of animal nervous systems. However, most studies have focused on adults of deeply nested species in the annelid tree. Recently, Owenia fusiformis has emerged as an informative species to reconstruct ancestral traits in Annelida, given its phylogenetic position within the sister clade to all remaining annelids. METHODS: Combining immunohistochemistry of the conserved neuropeptides FVamide-lir, RYamide-lir, RGWamide-lir and MIP-lir with gene expression, we comprehensively characterise neural development from larva to adulthood in Owenia fusiformis. RESULTS: The early larval nervous system comprises a neuropeptide-rich apical organ connected through peripheral nerves to a prototroch ring and the chaetal sac. There are seven sensory neurons in the prototroch. A bilobed brain forms below the apical organ and connects to the ventral nerve cord of the developing juvenile. During metamorphosis, the brain compresses, becoming ring-shaped, and the trunk nervous system develops several longitudinal cords and segmented lateral nerves. CONCLUSIONS: Our findings reveal the formation and reorganisation of the nervous system during the life cycle of O. fusiformis, an early-branching annelid. Despite its apparent neuroanatomical simplicity, this species has a diverse peptidergic nervous system, exhibiting morphological similarities with other annelids, particularly at the larval stages. Our work supports the importance of neuropeptides in animal nervous systems and highlights how neuropeptides are differentially used throughout development.

Locomotory Palp Function in Interstitial Annelids.

AbstractThe interstitial environment of marine sediments is a complex network of voids and pores that is inhabited by a diverse and abundant fauna. Animals living within these interstitial spaces show widespread functional adaptations to this environment and have developed many strategies for moving and navigating through small spaces. Interstitial annelids demonstrate a remarkable level of morphologic diversity, and some possess dexterous, filiform palps (tentacle-like appendages common across Annelida). The function(s) of these palps in interstitial spaces has not been closely examined, and we propose that they serve a sensory role in the navigation of interstitial spaces. We investigated the locomotory function of long, dexterous palps in three families of interstitial annelids to determine their role in interstitial navigation. We observed two species of protodrilids (Protodrilidae), Pharyngocirrus eroticus (Saccocirridae), and Protodorvillea recuperata (Dorvilleidae), as they moved through two transparent sand analogs: cyolite and glass beads. All four species of annelids consistently used their palps to probe the interstitial environment while locomoting, and the distance probed with their palps was greater than the distance traveled with their heads, indicating a sensory form of palp-based navigation. The functionality of palps as sensory organs in the interstitial environment raises interesting questions about interstitial navigation and how fauna without appendages map their surroundings. The discovery of this previously undocumented function was possible only through the direct observation of interstitial behavior and emphasizes the importance of developing new techniques to study these animals in more natural habitats.

Jaw morphology and function in Drilonereis cf. filum (Oenonidae, Annelida).

Representatives of the extant family Oenonidae (Annelida, Eunicida) have a prionognath jaw apparatus, with maxillae having forceps-like elements, a number of asymmetrical dentate plates and long slender carriers, which is characteristic of some fossil forms known from the Paleozoic epoch. Therefore, data on the fine structure and functional morphology of Oenonidae jaws are helpful for the interpretation of fossil materials. The fine structure of the jaw apparatus and the ventral pharyngeal organ is studied in one species of the Oenonidae (Annelida)-Drilonereis cf. filum. The material was collected in the soft bottom of Marseille Bay (Mediterranean) and examined with the help of TEM and histological techniques. A three-dimensional (3D) reconstruction was made from a complete series of semithin sections. The entire jaw apparatus is about 500 µm in length; it includes ventral mandibles and four pairs of maxillae, connected with long paired dorsal carriers and an unpaired ventral carrier. While retracted, it reaches the VIII-XI chaetigers. The most solid part of the maxillary apparatus, that is, maxillae I and II, are 2.5-5 µm thick. The plate consists of a monolithic array of merged scleroprotein granules in which perforations, that is, spaces remaining from microvilli, are visible; the basal part of the maxillary plate is a layer of loosely arranged collagen fibers penetrated with microvilli and has no signs of sclerotization. A study of the jaws of Drilonereis cf. filum showed the presence of common jaw patterns in Eunicida order. Like the jaws of Dorvilleidae, Eunicidae, Onuphidae, and Lumbrineridae, the jaws of Drilonereis are formed at the basis of a typical annelid cuticle's transformation with epi- and basicuticular layers, and its impregnation by merging scleroprotein granules. Through the nature of sclerotization, the jaws of D. cf. filum are similar to those of Dorvilleidae, Histriobdellidae, and the juvenile jaws of Mooreonuphis stigmatis (Onuphidae). Analysis of the 3D-reconstructions of the D. cf. filum jaw apparatus shows that the MxI of this species, and probably of other Oenonidae with dorsal and ventral carriers, can make grasping motions by fixing the joint of the right and left MxI in the two-door hinge type. In general, the overall structure of the jaw apparatus of D. cf. filum and the mechanics of its work shows greater similarity with that of Dorvilleidae than with the jaw apparatus of extant Labidognatha and Simmetrognatha (Onuphidae, Eunicidae, Lumbrineridae). The need for compactization of the jaw apparatus when moving in dense sediment or in the burrows is probably one of the factors determining its structure.

Annelid functional genomics reveal the origins of bilaterian life cycles.

Indirect development with an intermediate larva exists in all major animal lineages1, which makes larvae central to most scenarios of animal evolution2-11. Yet how larvae evolved remains disputed. Here we show that temporal shifts (that is, heterochronies) in trunk formation underpin the diversification of larvae and bilaterian life cycles. We performed chromosome-scale genome sequencing in the annelid Owenia fusiformis with transcriptomic and epigenomic profiling during the life cycles of this and two other annelids. We found that trunk development is deferred to pre-metamorphic stages in the feeding larva of O. fusiformis but starts after gastrulation in the non-feeding larva with gradual metamorphosis of Capitella teleta and the direct developing embryo of Dimorphilus gyrociliatus. Accordingly, the embryos of O. fusiformis develop first into an enlarged anterior domain that forms larval tissues and the adult head12. Notably, this also occurs in the so-called 'head larvae' of other bilaterians13-17, with which the O. fusiformis larva shows extensive transcriptomic similarities. Together, our findings suggest that the temporal decoupling of head and trunk formation, as maximally observed in head larvae, facilitated larval evolution in Bilateria. This diverges from prevailing scenarios that propose either co-option9,10 or innovation11 of gene regulatory programmes to explain larva and adult origins.

Architecture and tube structure of Sabellaria spinulosa (Leuckart, 1849): Comparison with the Mediterranean S. alveolata congener.

The Atlantic-Mediterranean polychaete Sabellaria spinulosa (Leuckart, 1849) lives in agglutinated tubes forming discrete reef-like bioconstructions on shallow-water bottoms beaten by waves where sediment particles are constantly resuspended. Tubes are built with sand grains glued by a proteinaceous cement. Analyses of a S. spinulosa reef sample of this worm collected off the Casarza coast (central Adriatic Sea) allowed the description of its tube architecture and gluing modality. The tube consists of three layers of agglutinated sand: (a) a thin inner layer with sandy particles arranged side by side with a flat side facing the tube lumen (b) a thick middle layer with larger isodiametric and squat grains with empty pores in between; and (c) a thin discontinuous outer layer of heterometric clasts, prevalently large and flat, diverging towards the opening. This fits the general tube construction known for S. alveolata and in general for tubes of the sabellariidae family, but compared to Sabellaria alveolata, S. spinulosa possesses a smaller tube with a wall about 1/3 thick; the agglutinated sandy elements are finer, and some number of muddy particles participates in the tube construction. Morphological and epifluorescence observations revealed that biocement portions are irregularly and haphazardly distributed compared with those of S. alveolata that consist of drops and strips of glue carefully placed. Adjacent tubes leave empty interspaces in between them only locally filled by loose sand, extremely reduced to absent in S. alveolata.

Phylogenetic position of the enigmatic genus Atherospio and description of Atherospio aestuarii sp. nov. (Annelida: Spionidae) from Japan.

Background: There are currently two species within the small enigmatic genus Atherospio Mackie & Duff, 1986, which belongs to the Pygospiopsis-Atherospio group in the family Spionidae Grube, 1850. The taxonomic relationship of the genus Atherospio with other spionid or spioniform genera is currently not well understood due to its unusual morphological characteristics. Methods: Here, we describe a new Atherospio species, Atherospio aestuarii sp. nov., based on materials collected from three localities in Japan: Hirota Bay (Iwate Prefecture), Ago Bay (Mie Prefecture), and Yakushima Island (Kagoshima Prefecture). We have also evaluated the possible systematic position of this new species by conducting molecular phylogenetic analyses using the nuclear 18S, 28S, and mitochondrial 16S rRNA gene sequences. Results: The morphology of A. aestuarii sp. nov. resembles that of A. disticha Mackie & Duff, 1986 and A. guillei (Laubier & Ramos, 1974) in having branchiae fused to the notopodial lamellae on a restricted number of segments from chaetiger 7, modified neurochaetae on chaetiger 5, and at least some bidentate neuropodial hooks with the secondary tooth below the main fang. The form and arrangement of the modified aristate neurochaetae in double vertical rows closely resemble those found on chaetigers 4 and 5 of A. disticha. The new species lacks the occipital antenna present in A. disticha. In this respect it resembles A. guillei, however, that species differs in having robust neuropodial spines on chaetiger 5 and peristomial papillae, and a preponderance of unidentate neurochaetae. Both A. guillei and the new species have slender needle-like notochaetae in their posteriormost chaetigers. Atherospio aestuarii sp. nov. is distinguished from both congeneric species by its branchial and neuropodial hook distributions. The new species is also unique in that it was recorded at relatively shallow depths, which included intertidal zones. The results of our molecular phylogenetic analysis indicate that the new species was included in a clade that included the genera of the Polydora complex, Pygospio Claparède, 1863, Glandulospio Meißner, Bick, Guggolz, Götting, 2014, Spio Fabricius, 1785, Microspio Mesnil, 1896, Marenzelleria Mesnil, 1896, Rhynchospio Hartman, 1936, Scolelepis Blainville, 1828, Dispio Hartman, 1951, and Malacoceros Quatrefages, 1843 with robust statistical support. The new species formed a clade with Dispio and Scolelepis, however, statistical support for the node was not significant.

Long forgotten: Eunice woodwardi Baird, 1869 (Annelida, Eunicidae) revisited, with an insight on internal anatomy.

Eunice woodwardiBaird, 1869, originally described from the Ría de A Coruña (NW Iberian Peninsula), has been overlooked and never reported from the Atlantic coast of Spain after original description and the subsequent redescription of the holotype. In the present study, we revised comparatively the holotype, newly collected specimens of this species and specimens identified as Eunice vittata (Delle Chiaje, 1829) from western Mediterranean Sea. The validity of E. woodwardi is supported and previous descriptions are complemented after a throughout study of the external morphology by means of light compound microscopy and scanning electron microscopy, and that of the internal anatomy by histological sectioning and micro-computed tomography. The presence of eyes, nuchal organs, dorsal and ventral ciliary organs on parapodial cirri and paired nephridia in most segments is confirmed in E. woodwardi; the digestive tract is clearly regionalized and divided into pharynx, oesophagus, stomach, fore, mid- and hind intestine. The presence of E. woodwardi in the Ría de Ferrol is also reported, and we suggest that previous records of E. vittata in NW Iberian Peninsula should be reviewed. Eunice woodwardi is distinguished by a set of characters such as having non-articulated and non-constricted cephalic appendages, the maxillary formula, the range of branchial distribution, maximum number of branchial filaments, number of limbate and compound falciger chaetae per parapodium, the presence of an apical mucro in the guard of falciger chaetae blades and the number of teeth in pectinate chaetae. Epibiont Ciliophora on branchiae are also reported.

Chaetal arrangement and type diversity in two Magelona species (Magelonidae, Annelida) with ultrastructural details of the internal support chaetae.

The systematics of Annelida has repeatedly been changed based on morphological data, but more recently established transcriptomic approaches yielded a stable and widely accepted phylogenetic tree, placing Magelonidae and Oweniidae as sister group to all remaining annelids. This led to an increased interest in these groups in terms of morphological traits and their phylogenetic significance. As one of the most characteristic morphological characters of annelids, the chaetae of Magelonidae are well investigated regarding their shape, but phylogenetically relevant aspects like their general arrangement are still poorly studied. Furthermore, some species possess abdominal internal support chaetae that no study has addressed in detail thus far. The chaetal arrangement and position of formative sites were studied in the differently expressed parapodia of the thorax, the ninth chaetiger and the abdomen of Magelona mirabilis and Magelona johnstoni. Our results show that all chaetigers primarily bore one row of chaetae per parapodial ramus, each with a single formative site. We also present the first histological as well as ultrastructural data on the magelonid support chaetae, showing that they represent internal hooded hooks, with which they share a common chaetal sac. Their distribution within Magelonidae, however, still requires a broader examination to evaluate their presence as a convergent or homologous trait.

Ultrastructure of proboscis blood vessels in females of Bonellia viridis (Annelida: Bonellinae): New reconstructions.

Many data on echiurid anatomy and ultrastructure are obtained for Bonellia viridis and extrapolated to other species. The ultrastructure of the axial blood vessels, which has been described as an "osmotic pump," is regarded as one of the unusual features of echiurids. In this study, the ultrastructure of the proboscis blood vessels in females of B. viridis is described, illustrated by accurate schemes, and a new reconstruction of the axial blood vessel is suggested. The walls of the axial and lateral vessels of the proboscis are formed by myoepithelial cells, which are connected to each other via adherence junctions, underlined by basal lamina, and therefore form a true epithelium. Apical, middle, and basal parts of the myoepithelial cells form long, thin projections, which extend to the connective tissue (in axial vessel) or coelomic canals (in lateral vessels) and to the lumen of the vessels. The presence of such projections may evidence active cellular transport. Similarity in the fine structure of the myoepithelial cells of axial and lateral blood vessels evidence their common origin from myoepithelial cells of the coelomic lining. However, in evolution, the coelomic canals were retained around the lateral vessels and disappeared around the axial vessel. The reduction of a hypothetical ancestral axial coelom may be caused by the extensive development of the connective tissue and muscles in the central part of the proboscis, where the axial vessel extends.

A new freshwater fabriciid species (Sabellida, Fabriciidae) from the Uruguay River, Argentina, with some remarks on the relationships of Monroika Hartman, 1951, Manayunkia Bourne, 1884 and Brandtika Jones, 1974.

Between the years 2007 and 2009 a hitherto unknown fabriciid species was found in the Lower Uruguay River, Argentina. Initially, it was assumed that this species could represent Manayunkia speciosa, a freshwater species originally described from North America. However, re-examinations have revealed that these specimens are clearly different from M. speciosa but they resemble Monroika africana. However, differences were also found that clearly distinguish this South American species from the African species so that it is described here as M. clarae sp. nov. The characters found in Monroika clarae sp. nov. make it also possible to compare and discuss these characters with those from the West African species Monroika africana, the Southeast Asian species Brandtika asiatica as well as Manayunkia species in terms of their systematic position. This mainly concerns the structure of the radiolar crown, the number of abdominal chaetigers, the presence of transitional chaetae as well as the structure of the thoracic and abdominal uncini. Potential origins of the freshwater Fabriciidae are also discussed.

Redescription of Pomatostegus stellatus (Abildgaard, 1789) and P. kroyeri Mörch, 1863 (Polychaeta: Serpulidae) from the Tropical American coasts.

Pomatostegus Schmarda, 1861 is a serpulid genus with three recognized species, mostly from tropical waters. Pomatostegus stellatus (Abildgaard, 1789), was described from the Caribbean Sea, and has been widely recorded in the Pacific and Indian oceans; P. kroyeri Mörch, 1863 was described from Puntarenas, Costa Rica; however, most records from the Tropical Eastern Pacific were referred to P. stellatus. In this work, the two species are redescribed using morphological characters. The main differences are in the shape of the Spirobranchus-type collar chaetae, opercular plate shape and the number of "free" circlets of spines without accompanying of opercular plates. Comments about characters of P. actinoceras Mörch, 1863, described from Philippines, and an identification key for the three species, are included.

Morphological convergence and adaptation in cave and pelagic scale worms (Polynoidae, Annelida).

Across Annelida, accessing the water column drives morphological and lifestyle modifications-yet in the primarily "benthic" scale worms, the ecological significance of swimming has largely been ignored. We investigated genetic, morphological and behavioural adaptations associated with swimming across Polynoidae, using mitogenomics and comparative methods. Mitochondrial genomes from cave and pelagic polynoids were highly similar, with non-significant rearrangements only present in cave Gesiella. Gene orders of the new mitogenomes were highly similar to shallow water species, suggestive of an underlying polynoid ground pattern. Being the first phylogenetic analyses to include the holopelagic Drieschia, we recovered this species nested among shallow water terminals, suggesting a shallow water ancestry. Based on these results, our phylogenetic reconstructions showed that swimming evolved independently three times in Polynoidae, involving convergent adaptations in morphology and motility patterns across the deep sea (Branchipolynoe), midwater (Drieschia) and anchialine caves (Pelagomacellicephala and Gesiella). Phylogenetic generalized least-squares (PGLS) analyses showed that holopelagic and anchialine cave species exhibit hypertrophy of the dorsal cirri, yet, these morphological modifications are achieved along different evolutionary pathways, i.e., elongation of the cirrophore versus style. Together, these findings suggest that a water column lifestyle elicits similar morphological adaptations, favouring bodies designed for drifting and sensing.

Concentric Inclusions in the Intestinal Epithelium of Echiura Bonellia viridis: Structure and Possible Function.

The study of the anatomy and fine structure of Echiura is of great importance for understanding the biology of these animals, which lead a secretive life and dominate in various benthic communities. The first data on the organization of the siphonal part of the midgut of female Bonellia viridis were obtained by the methods of scanning and transmission electron microscopy. Unusual concentric inclusions similar in the ultrastructure to those described in other animals, e.g., in the gut of many nematode species and in the tegument of some cestodes, were first found in the cells of the midgut. It is known that, in these animals, the concentric inclusions play an important role in the binding of chemical agents inherent in redox environments. Interestingly, the individuals of B. viridis studied were found on the surface of a substrate devoid of redox environment signs. New results indicate the presence in B. viridis and, possibly, in all spoon worms, of preadaptations to life in redox environments. New data on the structure and composition of concentric inclusions will shed light on their origin and function.

The 20-million-year old lair of an ambush-predatory worm preserved in northeast Taiwan.

The feeding behavior of the giant ambush-predator "Bobbit worm" (Eunice aphroditois) is spectacular. They hide in their burrows until they explode upwards grabbing unsuspecting prey with a snap of their powerful jaws. The still living prey are then pulled into the sediment for consumption. Although predatory polychaetes have existed since the early Paleozoic, their bodies comprise mainly soft tissue, resulting in a very incomplete fossil record, and virtually nothing is known about their burrows and behavior beneath the seafloor. Here we use morphological, sedimentological, and geochemical data from Miocene strata in northeast Taiwan to erect a new ichnogenus, Pennichnus. This trace fossil consists of an up to 2 m long, 2-3 cm in diameter, L-shaped burrow with distinct feather-like structures around the upper shaft. A comparison of Pennichnus to biological analogs strongly suggests that this new ichnogenus is associated with ambush-predatory worms that lived about 20 million years ago.

A Cambrian crown annelid reconciles phylogenomics and the fossil record.

The phylum of annelids is one of the most disparate animal phyla and encompasses ambush predators, suspension feeders and terrestrial earthworms1. The early evolution of annelids remains obscure or controversial2,3, partly owing to discordance between molecular phylogenies and fossils2,4. Annelid fossils from the Cambrian period have morphologies that indicate epibenthic lifestyles, whereas phylogenomics recovers sessile, infaunal and tubicolous taxa as an early diverging grade5. Magelonidae and Oweniidae (Palaeoannelida1) are the sister group of all other annelids but contrast with Cambrian taxa in both lifestyle and gross morphology2,6. Here we describe a new fossil polychaete (bristle worm) from the early Cambrian Canglangpu formation7 that we name Dannychaeta tucolus, which is preserved within delicate, dwelling tubes that were originally organic. The head has a well-defined spade-shaped prostomium with elongated ventrolateral palps. The body has a wide, stout thorax and elongated abdomen with biramous parapodia with parapodial lamellae. This character combination is shared with extant Magelonidae, and phylogenetic analyses recover Dannychaeta within Palaeoannelida. To our knowledge, Dannychaeta is the oldest polychaete that unambiguously belongs to crown annelids, providing a constraint on the tempo of annelid evolution and revealing unrecognized ecological and morphological diversity in ancient annelids.

Histochemical detection of free thiols in glandular cells and tissues of different marine Polychaeta.

Either through differentiated glands or specialised individual cells, the coating epithelia of soft-bodied marine invertebrates are responsible for the secretion of a broad span of peptidic substances, from protective mucins to biocides. These secretions are characterised by the presence of cysteine-rich proteins and peptides, rendering a distinct histochemical signature of secretory epithelia. Through a histochemical procedure for fluorescence microscopy in paraffin sections, we performed a comparative assessment of the distribution of thiol-rich compounds in multiple epithelia of different species of intertidal Polychaeta, which revealed distinctive patterns of distribution that closely relate to ecology, morphoanatomy and physiology. The presence of free thiols was notorious in mucocytes and enzyme-plus toxin-secreting cells. Consequently, strong signals were recorded in the mucocytes of the parapodia of Nereis splendida, the epidermis and pharynx epithelium of Mysta picta and the venom glands of Glycera alba. The findings show an investment in mucus secretion in foragers such as Nereis and Mysta, especially the latter, which is not a native burrower, as a protective response and as lubricant for locomotion. Additionally, nereidids are believed to secret integumentary toxins for defence. On the other hand, Glycera is an ambush predatorial burrower whose behaviour entirely revolves around the delivery of venom making use of its four jaws. The results showed that the detection of thiol-rich compounds in histological sections can be a tool to identify potential toxin secretion and delivery structures, with important consequences for the bioprospecting of novel bioreactives from marine invertebrates for the purpose of drug discovery.

Obturacula of Vestimentifera (Annelida, Siboglinidae) Are Homological to the Dorsal Lips of the Polychaete of the Family Sabellidae.

We have conducted comparative analysis of the structure of the dorsal lips of the polychaete Eudistylia polymorpha from the family Sabellidae and the obturacula of Oasisia alvinae (Vestimentifera). It has been concluded that the obturacula of Vestimentifera are homologs of the dorsal lips of Polychaete from the family Sabellidae. It has been suggested that the head lobe of siboglinids of the subfamily Frenulata is homologous to the fused obturacula of Vestimentifera.

Delimitation of cryptic species drastically reduces the geographical ranges of marine interstitial ghost-worms (Stygocapitella; Annelida, Sedentaria).

The recognition of cryptic species concealed in traditionally established species may reveal new biogeographical patterns and alter the understanding of how biodiversity is geographically distributed. This is particularly relevant for marine ecosystems where the incidence of cryptic species is high and where species distribution data are often challenging to collect and interpret. Here, we studied specimens of the 'cosmopolitan' interstitial meiofaunal annelid Stygocapitella subterranea Knöllner, 1934 (Parergodrilidae, Orbiniida), obtaining data from four coastlines in the Northern hemisphere. Using phylogenetic tools and several species-delimitation methods (haplotype networks, GMYC, bPTP, maximum likelihood, posterior probability and morphology) we describe eight new Stygocapitella species. With one exception, all species are present along a single coastline, ultimately challenging the idea that Stygocapitella subterranea has a cosmopolitan distribution. We found evidence for several oceanic transitions having occurred in the past as well as a recent translocation, potentially due to human activity. No diagnostic characters were found, and qualitative and quantitative morphological data do not allow an unequivocal differentiation of the identified cryptic species. This suggests that (i) neither traditional diagnostic features nor quantitative morphology suffice to recognise species boundaries in cryptic species complexes, such as the Stygocapitella species complex; and that (ii) the recognition and description of cryptic species is of seminal importance for biodiversity assessments, biogeography and evolutionary biology.

Deceleration of morphological evolution in a cryptic species complex and its link to paleontological stasis.

Morphological stasis or the absence of morphological change is a well-known phenomenon in the paleontological record, yet it is poorly integrated with neontological evidence. Recent evidence suggests that cryptic species complexes may remain morphologically identical due to morphological stasis. Here, we describe a case of long-term stasis in the Stygocapitella cryptic species complex (Parergodrilidae, Orbiniida, Annelida). Using phylogenetic methods and morphological data, we find that rates of morphological evolution in Stygocapitella are significantly slower than in closely related taxa (Nerillidae, Orbiniidae). Assessment of quantitative and qualitative morphology revealed the presence of four morphotypes with only subtle differences, whereas molecular data supports 10 reproductively isolated clades. Notably, estimates for the time of Stygocapitella species divergence range from ∼275 million years to ∼18 million years, including one case of two morphologically similar species that have diverged about 140 million years ago. These findings provide evidence for morphological deceleration and long-term morphological stasis in Stygocapitella, and that speciation is not necessarily accompanied by morphological changes. The deceleration of morphological divergence in Stygocapitella can be potentially linked to niche conservatism and tracking, coupled with the fluctuating dynamics of the interstitial environment, or genetic constraints due to progenetic evolution. Finally, we conclude that failing to integrate speciation without morphological evolution in paleontology may bias estimates of rates of speciation and morphological evolution.