Systematic revision of Sabellariidae (Polychaeta) and their relationships with other polychaetes using morphological and DNA sequence data

Body traits variation of a reef building polychaete across a latitudinal gradient

Body size is considered the most important trait in ecology, and as such, helps to understand the species-environment interactions. We explored the relationship between body traits and environmental variables along a gradient range using the polychaete Phragmatopoma caudata, with well-defined and diversified morphological structures. Measurements of five traits (body length, opercular crown, branchiae, tentacles and building organ sizes) were taken at nine sites along the Southwestern Atlantic coast and their relationships to temperature, salinity, tidal range, waves height, and dissolved oxygen were assessed. Our results demonstrate that traits were influenced by the environmental gradient and temperature was the main factor that drives this variation in body traits, while the other variables showed a minor influence on this. The approach showed patterns of variation of body traits in a macroscale context, increasing the understanding of its relationships with environmental variables and eventual shifts in the distribution in the future climate scenarios.

Reeling them in: taxonomy of marine annelids used as bait by anglers in the Western Cape Province, South Africa

Background

Common names are frequently used inconsistently for marine annelid species used as bait in the peer-reviewed literature, field guides and legislative material. The taxonomy of many such species based on morphology only also ignores cryptic divergences not yet detected. Such inconsistencies hamper effective management of marine annelids, especially as fishing for recreation and subsistence is increasing. This study investigates the scale of the problem by studying the use and names of bait marine annelids in the Western Cape Province of South Africa.

Methods

Fifteen recreational and six subsistence fishers at 12 popular fishing sites in the Western Cape Province donated 194 worms which they identified by common name. Worms were assigned scientific names according to a standard identification key for polychaetes from South Africa, and mitochondrial cytochrome oxidase I (COI) amplified and sequenced.

Results

This study identified 11 nominal species known by 10 common names, in the families Siphonosomatidae, Arenicolidae, Sabellaridae, Lumbrineridae, Eunicidae, Onuphidae and Nereididae. Cryptic diversity was investigated through employing mitochondrial COI sequences and these data will facilitate future identifications among widely distributed species. Several species (Siphonosoma dayi, Abarenicola gilchristi, Scoletoma species, Marphysa corallina, Lysidice natalensis, Heptaceras quinquedens, Perinereis latipalpa) are reported as bait for the first time, and while the names blood- and moonshineworms were consistently applied to members of Arenicolidae and Onuphidae, respectively, coralworm was applied to members of Sabellaridae and Nereididae. Analysis of COI sequences supported morphological investigations that revealed the presence of two taxonomic units each for specimens initially identified as Gunnarea gaimardi and Scoletoma tetraura according to identification keys. Similarly, sequences for Scoletoma species and Lysidice natalensis generated in this study do not match those from specimens in China and India, respectively. Further research is required to resolve the species complexes detected and also to refine the use of names by fishermen over a wider geographic range.

Marine invertebrates associated with rhodoliths/maërl beds from northeast Brazil (State of Paraíba)

BACKGROUND

This study investigates the marine macroinvertebrate fauna of rhodolith beds (non-geniculated red corallinaceaous algae) in northeast Brazilian. A total of 57 species were identified, belonging to six phyla (Platyhelminthes, Annelida, Sipuncula, Mollusca, Arthropoda and Echinodermata), of which 50 are considered here as new records for the Paraíba State. Annelids (Class Polychaeta) were the most representative taxa in Miramar and Seixas Beaches, while molluscs were dominant in Maceió Beach.

NEW INFORMATION

This is the first study that includes an identification key, diagnostic features and distribution patterns worldwide and local (including new records) of the marine invertebrate fauna associated with rhodolith beds in northeast Brazil (State of Paraíba). Sampling events were performed in 2018, at low tide in the intertidal to shallow subtidal zones (1.5 and 4.0 m depth), in Miramar, Seixas and Maceió Beaches. A total of 17 species were found for the first time on Seixas Beach, as well as all identified species for Miramar and Maceió. This study tries to contribute to the knowledge of marine invertebrates in northeast Brazilian shallow habitats, providing a baseline for future environmental studies.

Quantitative evaluation of microplastics in colonies of Phragmatopoma caudata Krøyer in Mörch, 1863 (Polychaeta-Sabellariidae): Analysis in sandcastles and tissues and identification via Raman spectroscopy

The detection of microplastics in all world oceans, including the most remote, has become a major concern as this will substantially increase the possibility of interactions between these particles and the marine biota. Due to their small size, microplastics can be ingested by many marine species including invertebrates, causing physical damage. This study was the first evaluation of the occurrence and abundance of microplastics in three sample types related to the species Phragmatopoma caudata, (i.e. colony wash water, tubes and digested tissue from specimens). In total, 2118 samples of microplastics were quantified, with 1516 obtained from the wash water, 447 from the fragmented tubes and 155 from the digested tissue. Three types of microplastics were chemically identified via Raman Spectroscopy as polyethylene, polypropylene and polyethylene terephthalates. Overall, this study demonstrated that microplastics are abundantly bioavailable in the studied area and are present as filament and fragment shapes.

Fanworms: Yesterday, Today and Tomorrow

Sabellida Levinsen, 1883 is a large morphologically uniform group of sedentary annelids commonly known as fanworms. These annelids live in tubes made either of calcareous carbonate or mucus with agglutinated sediment. They share the presence of an anterior crown consisting of radioles and the division of the body into thorax and abdomen marked by a chaetal and fecal groove inversion. This study synthesises the current state of knowledge about the diversity of fanworms in the broad sense (morphological, ecological, species richness), the species occurrences in the different biogeographic regions, highlights latest surveys, provides guidelines for identification of members of each group, and describe novel methodologies for species delimitation. As some members of this group are well-known introduced pests, we address information about these species and their current invasive status. In addition, an overview of the current evolutionary hypothesis and history of the classification of members of Sabellida is presented. The main aim of this review is to highlight the knowledge gaps to stimulate research in those directions.

Ultrastructure and functional morphology of the appendages in the reef-building sedentary polychaete Sabellaria alveolata (Annelida, Sedentaria, Sabellida)

Background

The sedentary polychaete Sabellaria alveolata, the sandcastle or honeycomb worm, possesses four different kinds of appendages besides the parapodia: opercular papillae, tentacular filaments, palps, and branchiae. It exhibits a highly specialized anterior end, the operculum, formed by the prostomium, peristomium, and two anterior segments. The operculum comprises opercular papillae, tentacular filaments, and palps. Paired branchiae are present from the second thoracic chaetiger onwards on the posteriorly following segments except for the last ones. Ultrastructural data on these appendages are either scanty, incomplete, or even lacking in Sabellariidae. In order to analyze their functional morphology, to bridge the data gap, and providing data for future phylogenetic and evolutionary analyses, we investigated the appendages of S. alveolata by applying light microscopy, confocal laser scanning microscopy, scanning, and transmission electron microscopy.

Results

In S. alveolata the entire body is covered by a thin cuticle characterized by the absence of layers of parallel collagen fibers with no differentiation between the various body regions including the branchiae. The opercular papillae bear numerous tufts of receptor cells and lack motile cilia. The tentacular filaments show a distinctive pattern of motile cilia. Their most conspicuous morphological feature is a cell-free cartilaginous endoskeletal structure enclosed by ECM. Besides musculature the filaments include a single coelomic cavity but blood vessels are absent. The palps are ciliated and possess two coelomic cavities and a single blind-ending internal blood vessel. Besides external ciliation and receptor cells, the coelomate branchiae are highly vascularized and equipped with numerous blood spaces extending deep between the epidermal cells resulting in low diffusion distances.

Conclusions

All appendages, including the branchiae, bear receptor cells and, as such, are sensory. The opercular papillae resemble typical parapodial cirri. In contrast, the tentacular filaments have a triple function: sensing, collecting and transporting particles. A similarity to branchiae can be excluded. The palps are typical grooved palps. A revised classification of polychaete branchiae is suggested; thereby, the branchiae of S. alveolata belong to the most common type comprising coelom, musculature, and blood vessels. The results indicate that diffusion distances between blood and environment have been underestimated in many cases.

Developmental studies provide new insights into the evolution of sense organs in Sabellariidae (Annelida)

BACKGROUND

Sabellarids, also known as honeycomb or sandcastle worms, when building their tubes, produce chemical signals (free fatty acids) that are responsible for larval settlement and the formation of three-dimensional aggregations. The larval palps and the dorsal hump (becoming the median organ in adults) are presumed to participate in such a substrate selection during settlement. Notably, the sabellariid median organ is an apparently unique organ among annelids that has been attributed with a sensory function and perhaps with some affinities to the nuchal organs of other polychaetes. Nevertheless, detailed investigations of this prominent character complex including ultrastructural examinations are lacking so far.

RESULTS

Our comprehensive investigations provide data about the anterior sensory organs in Sabellariidae and inform about their transformation during pelagic larval development. We used a comparative approach including immunostaining with subsequent confocal laser scanning microscopy (clsm), histological sections as well as electron microscopy in a range of larval and adult stages of two sabellariid species. We find that the neuronal innervation as well as the ultrastructure of the sabellariid ciliary structures along the median organ are highly comparable with that of nuchal organs known from other polychaetes. Furthermore, the myoinhibitory protein (MIP) - a protein known to be also involved into chemo-sensation - was detected in the region of the larval median organ. Moreover, we reveal the presence of an unusual type of photoreceptor as part of the median organ in Idanthyrsus australiensis with a corrugated sensory membrane ultrastructure unlike those observed in the segmental ocelli of other polychaetes.

CONCLUSIONS

We are describing for the first time the nuchal organ-like structures in different developmental stages of two species of Sabellariidae. The external morphology, neuronal innervation, developmental fate and ultrastructure of the newly-discovered median organ-based ciliary pits are comparable with the characteristics known for annelid nuchal organs and therefore indicate a homology of both sensory complexes. The presence of myoinhibitory peptide (MIP) in the respective region supports such a hypothesis and exhibits the possibility of an involvement of the entire sabellariid median organ complex, and in particular the prominent ciliated pits, in chemo-sensation.

Adhesive gland transcriptomics uncovers a diversity of genes involved in glue formation in marine tube-building polychaetes

Tube-building sabellariid polychaetes are hermatypic organisms capable of forming vast reefs in highly turbulent marine habitats. Sabellariid worms assemble their tube by gluing together siliceous and calcareous clastic particles using a polyelectrolytic biocement. Here, we performed transcriptomic analyses to investigate the genes that are differentially expressed in the parathorax region, which contains the adhesive gland and tissues, from the rest of the body. We found a large number of candidate genes to be involved in the composition and formation of biocement in two species: Sabellaria alveolata and Phragmatopoma caudata. Our results indicate that the glue is likely to be composed by a large diversity of cement-related proteins, including Poly(S), GY-rich, H-repeat and miscellaneous categories. However, sequences divergence and differences in expression profiles between S. alveolata and P. caudata of cement-related proteins may reflect adaptation to the type of substratum used to build their tube, and/or to their habitat (temperate vs tropical, amplitude of pH, salinity …). Related to the L-DOPA metabolic pathways and linked with the genes that were differentially expressed in the parathorax region, we found that tyrosinase and peroxidase gene families may have undergone independent expansion in the two Sabellariidae species investigated. Our data also reinforce the importance of protein modifications in cement formation. Altogether these new genomic resources help to identify novel transcripts encoding for cement-related proteins, but also important enzymes putatively involved in the chemistry of the adhesion process, such as kinases, and may correspond to new targets to develop biomimetic approaches.

STATEMENTS OF SIGNIFICANCE

The diversity of bioadhesives elaborated by marine invertebrates is a tremendous source of inspiration to develop biomimetic approaches for biomedical and technical applications. Recent studies on the adhesion system of mussel, barnacle and sea star had highlighted the usefulness of high-throughput RNA sequencing in accelerating the development of biomimetic adhesives. Adhesion in sandcastle worms, which involves catechol and phosphate chemistries, polyelectrolyte complexes, supramolecular architectures, and a coacervation process, is a useful model to develop multipurpose wet adhesives. Using transcriptomic tools, we have explored the diversity of genes encoding for structural and catalytic proteins involved in cement formation of two sandcastle worm species, Sabellaria alveolata and Phragmatopoma caudata. The important genomic resource generated should help to design novel "blue" adhesives.

Here, There and Everywhere: The Radiolar Eyes of Fan Worms (Annelida, Sabellidae)

Fan worms (Annelida: Sabellidae) possess some of the strangest eyes in nature. Their eponymous fans are composed of two sets of radiolar tentacles that project from the head up out of the worm's protective tube into the water column. Primarily used for respiration and feeding, these radioles are also often involved in photoreception. They display a surprising diversity of eyes of varying levels of sophistication, ranging from scattered single ocelli to compound eyes with up to hundreds of facets. These photoreceptors could represent a relatively recent evolutionary development to cope with a sessile, tube-dwelling lifestyle, and the primary cerebral eyes (haplessly positioned within the tube most of the time) amount to little more than minute pigment cups with scant visual potential. The radiolar eyes on the other hand, appear to function as visual burglar alarms for detecting looming predators and eliciting a startle response for the worm to rapidly retreat within its fortified tube. Despite sometimes resembling arthropod compound eyes, the radiolar photoreceptors have many canonically vertebrate-like physiological characteristics. Considering the unusual and apparently recently evolved nature of the fan worm radiolar photoreceptors, these animals are an excellent case for examining the emergence of novel visual systems, the development of rudimentary visually guided behaviors, and the function of distributed sensory systems. Here, we review over 100 years of investigations into the anatomical diversity of sabellid radiolar photoreceptors and eyes in an evolutionary and functional context. We provide new information on radiolar eye structure in several species of fan worms, and we attempt to organize the various eye types and ocellar structures into meaningful hierarchies. We discuss the developmental, evolutionary, and functional significance of the radiolar eyes and highlight areas of future interest in deciphering their unique nature.

Structure, function and cell dynamics during chaetogenesis of abdominal uncini in Sabellaria alveolata (Sabellariidae, Annelida)

BACKGROUND

Dynamic apical microvilli of a single cell, called the chaetoblast, inside an ectodermal invagination form the template of annelid chaetae. Changes in the pattern of microvilli are frozen in time by release of chitin, such that the structure of the definitive chaeta reflects its formation. Cellular interactions during chaetogenesis also influence the structure of the chaeta. Analysing chaetogenesis allows for testing hypotheses on the homology of certain chaetal types. We used this approach to test whether the unusual uncini in Sabellaria alveolata are homologous to apparently similar uncini in other annelid taxa.

RESULTS

Our study reveals unexpected details of sabellariid uncini, which mechanically reinforce the neuropodia enabling their use as paddles. The final structure of the chaeta is caused by pulses of microvilli formation and dynamic interaction between the chaetoblast and adjoining follicle cells. Cell dynamics during chaetogenesis of the uncini in Sabellaria alveolata exceeds by far that reported in previous studies on the formation of this type of chaetae.

CONCLUSION

Despite the superficial similarity of uncini in sabellariids and other annelids, differences in structure and details of formation do not support the homology of this type of chaetae. Chaetogenesis of sabellariid uncini involves unexpected microvilli and cell dynamics, and provides evidence that interactions between cells play a larger role in chaetogenesis than previously expected. In addition to their function as anchors, uncini in Sabellaridae stabilize the paddle-shaped notopodia, as each uncinus possesses a long, thin rod that extends deeply into the notopodium. The rods of all uncini in a single row form a bundle inside the notopodium that additionally serves as a muscle attachment site and thus have a similar function to the inner chaeta (acicula) of errant polychaetes (Aciculata).

Variação sazonal no recrutamento de Phragmatopoma caudata (Polychaeta, Sabellariidae) na costa sudeste do Brasil: biometria e validação de metodologia para categorização de classes etárias

Este estudo explora a maturação de gametas e biometria de Phragmatopoma caudata Krøyer in Mörch, 1863 para endossar uma metodologia e oferecer uma técnica adequada para estudos que objetivam avaliar a ecologia populacional. A análise de correlação de Pearson confirmou a relação positiva (r = 0,90, P <0,0001) entre o comprimento do corpo e o comprimento da coroa opercular. Indivíduos com opercular crown < 0,9 mm podem ser considerados como juvenil devido à ausência de gametas. Portanto, utilizando-se o método aprovado para separar as classes de tamanho, a população dos recifes de P. caudata no Parque Estadual Xixová-Japuí (PEJX) na Baía de Santos, Estado de São Paulo, foi examinada durante dois anos, com o objetivo de analisar a densidade populacional e o padrão sazonal da classe juvenil. Em período de elevadas taxas de juvenis, a densidade populacional atingiu 128.115 ind./m², porém, a média foi 65.090±22.033 ind./m². As análises estatísticas (Kruskal-Wallis H = 18,475, p < 0,01) revelaram existir variação significativa na composição juvenil entre as estações chuvosa e seca. Apesar da presença de juvenis em meses de seca, as estações chuvosas contemplaram 92,1% dos juvenis amostrados. O padrão de juvenis observado pode estar relacionado com fatores biológicos (e.g. gametogênese e ciclo de vida) e abióticos (e.g. suprimento alimentar e correntes marinhas). Estes resultados destacam a necessidade de programas de monitoramento de longo prazo que integrem elementos ecológicos e abióticos, a fim de obter uma compreensão mais completa da ecologia desse poliqueta e ajudar a gerenciar a biodiversidade marinha do PEJX.