Phoronida-A small clade with a big role in understanding the evolution of lophophorates

Phoronids, together with brachiopods and bryozoans, form the animal clade Lophophorata. Modern lophophorates are quite diverse-some can biomineralize while others are soft-bodied, they could be either solitary or colonial, and they develop through various eccentric larval stages that undergo different types of metamorphoses. The diversity of this clade is further enriched by numerous extinct fossil lineages with their own distinct body plans and life histories. In this review, I discuss how data on phoronid development, genetics, and morphology can inform our understanding of lophophorate evolution. The actinotrocha larvae of phoronids is a well documented example of intercalation of the new larval body plan, which can be used to study how new life stages emerge in animals with biphasic life cycle. The genomic and embryonic data from phoronids, in concert with studies of the fossil lophophorates, allow the more precise reconstruction of the evolution of lophophorate biomineralization. Finally, the regenerative and asexual abilities of phoronids can shed new light on the evolution of coloniality in lophophorates. As evident from those examples, Phoronida occupies a central role in the discussion of the evolution of lophophorate body plans and life histories.

First data on the structure of tubes formed by phoronids

Phoronids are marine benthic animals that live in tubes in soft sediment or hard substrata; the phoronids form the tubes by digging or boring. Epidermal glands produce much of the material of the tube, which is completely imbedded in the soft sediment or hard substrata. The structure of phoronid tubes has not been previously studied in detail. In the current research, the morphology and microstructure of the tubes were studied by light microscopy, histology, and scanning electron microscopy for the following species: Phoronis ijimai, Phoronis svetlanae, Phoronis hipporcrepia, Phoronis australis, and Phoronopsis harmeri. In most of these species, the tube consists of an inner organic cylinder and an external layer. The inner organic cylinder is formed by three layers (inner, middle, and outer) of thin films. Each film is formed by fibers, whose thickness differs in different species. These fibers form a net, whose density is higher in digging phoronids than in boring phoronids. The middle layer is formed by highly compressed thin films. The outer layer is the densest portion of the inner cylinder and is associated with the external layer. The external layer is absent in some species (P. australis) but is well developed in digging phoronids. The differences in the organization of tube are consistent with the biology of each species and depend on the type of substrata and on the life style of the animal. Tube organization substantially differs between phoronids and sedentary annelids: the inner organic cylinder is much thicker in phoronid than in annelid tubes, and the fibers that form films are randomly oriented in phoronids but regularly oriented in annelids. In annelids but not in phoronids, inorganic particles in the external layer are usually surrounded and glued together by organic material. These differences may be used to distinguish phoronid tubes from annelid tubes in present-day benthic samples and also in fossil samples.

Hox gene expression during development of the phoronid Phoronopsis harmeri

Background

Phoronida is a small group of marine worm-like suspension feeders, which together with brachiopods and bryozoans form the clade Lophophorata. Although their development is well studied on the morphological level, data regarding gene expression during this process are scarce and restricted to the analysis of relatively few transcription factors. Here, we present a description of the expression patterns of Hox genes during the embryonic and larval development of the phoronid Phoronopsis harmeri.

Results

We identified sequences of eight Hox genes in the transcriptome of Ph. harmeri and determined their expression pattern during embryonic and larval development using whole mount in situ hybridization. We found that none of the Hox genes is expressed during embryonic development. Instead their expression is initiated in the later developmental stages, when the larval body is already formed. In the investigated initial larval stages the Hox genes are expressed in the non-collinear manner in the posterior body of the larvae: in the telotroch and the structures that represent rudiments of the adult worm. Additionally, we found that certain head-specific transcription factors are expressed in the oral hood, apical organ, preoral coelom, digestive system and developing larval tentacles, anterior to the Hox-expressing territories.

Conclusions

The lack of Hox gene expression during early development of Ph. harmeri indicates that the larval body develops without positional information from the Hox patterning system. Such phenomenon might be a consequence of the evolutionary intercalation of the larval form into an ancestral life cycle of phoronids. The observed Hox gene expression can also be a consequence of the actinotrocha representing a “head larva”, which is composed of the most anterior body region that is devoid of Hox gene expression. Such interpretation is further supported by the expression of head-specific transcription factors. This implies that the Hox patterning system is used for the positional information of the trunk rudiments and is, therefore, delayed to the later larval stages. We propose that a new body form was intercalated to the phoronid life cycle by precocious development of the anterior structures or by delayed development of the trunk rudiment in the ancestral phoronid larva.

Myoanatomy of the Lophophore in Adult Phoronids and the Evolution of the Phoronid Lophophore

Confocal laser scanning microscopy was used to study the myoanatomy of the lophophore of three phoronids with different types of lophophore: Phoronis ijimai, Phoronis australis, and Phoronopsis harmeri. A four-part ground plan of the lophophoral musculature was detected in all three species and was previously reported for Phoronis ovalis. The ground plan includes (i) a circular muscle, (ii) longitudinal muscles of the tentacular lamina, (iii) groups of paired distal muscles of the tentacular lamina, and (iv) frontal and abfrontal muscles of the tentacles. In P. australis, the tentacular lamina contains strong abfrontal and numerous frontal muscles. Phoronis harmeri has an inner circular muscle and arch-like muscles. Among all studied phoronids, the four-part ground plan of the lophophoral musculature is least complex in P. ijimai, which has a horseshoe-shaped lophophore. The results suggest two possible scenarios by which the morphology of the phoronid lophophore has transformed over evolutionary time. According to the first scenario, the morphology of the ancestral horseshoe-shaped lophophore became more complicated in the case of most phoronids but became simplified in the case of P. ovalis and bryozoans. According to the second scenario, the lophophore gradually transformed from a simple oval shape to a horseshoe shape and then to a spiral shape. The four-part ground plan of the lophophoral musculature is also present in bryozoans, which is consistent with the view that the lophophorates are monophyletic.

Sea stars of the genus Henricia Gray, 1840 (Echinodermata, Asteroidea) from Vostok Bay, Sea of Japan

We report seven species of the genus Henricia Gray, 1840 that were found in Vostok Bay, and two species from adjacent area, known from museum collection or seen in underwater footage. while existing literature reported no confirmed species from this area. Most of these species: H. djakonovi, H. alexeyi, H. densispina, H. hayashii, H. granulifera, H. pacifica, H. asiatica, and H. oculata robusta were reported from the Sea of Japan previously. H. nipponica, known from Japan, is reported from Russian seas for the first time. All studied taxa are re-described here using a range of morphological characters and partial 16S rRNA nucleotide sequences, life colorations of several species are reported for the first time, and an identification key is provided. Lectotype designations are fixed for studied series of species described by AM Djakonov.

Oogenesis in the viviparous phoronid, Phoronis embryolabi

The study of gametogenesis is useful for phylogenetic analysis and can also provide insight into the physiology and biology of species. This report describes oogenesis in the Phoronis embryolabi, a newly described species, which has an unusual type of development, that is, a viviparity of larvae. Phoronid oogonia are described here for the first time. Yolk formation is autoheterosynthetic. Heterosynthesis occurs in the peripheral cytoplasm via fusion of endocytosic vesicles. Simultaneously, the yolk is formed autosynthetically by rough endoplasmic reticulum in the central cytoplasm. Each developing oocyte is surrounded by the follicle of vasoperitoneal cells, whose cytoplasm is filled with glycogen particles and various inclusions. Cytoplasmic bridges connect developing oocytes and vasoperitoneal cells. These bridges and the presence of the numerous glycogen particles in the vasoperitoneal cells suggest that nutrients are transported from the follicle to oocytes. Phoronis embryolabi is just the second phoronid species in which the ultrastructure of oogenesis has been studied, and I discuss the data obtained comparing them with those in Phoronopsis harmeri. Finally, I discuss the distribution of reproductive patterns across both, molecular and morphological phylogenetic trees in Phoronida proving that parental care has evolved independently several times in this phylum.