An externally brooding acorn worm (Hemichordata, Enteropneusta, Torquaratoridae) from the Russian arctic

Oocyte Size Suggests the Presence of Larvae in Deep-Sea Acorn Worms Torquaratoridae (Hemichordata, Enteropneusta)

The type of development is known to correlate with the egg size in marine invertebrates. Data on the maximum oocyte size were considered for deep-sea acorn worms of the family Torquaratoridae. The family was assumed to include species with direct development, species with lecithotrophic larvae, and species with planktotrophic larvae. Mysterious giant larvae of Planctosphaera pelagica may be planktotrophic larvae of Torquaratoridae.

Male reproductive system of the deep-sea acorn worm Quatuoralisia malakhovi (Hemichordata, Enteropneusta, Torquaratoridae) from the Bering Sea

BACKGROUND

The deep-sea acorn worm Quatuoralisia malakhovi belongs to the phylum Hemichordata, class Enteropneusta, family Torquaratoridae, which was described in 2005. Owing to their epibenthic lifestyle and deep-sea habitat features, torquaratorids differ anatomically from shallow-water acorn worms; however, their morphology and fine structure are poorly studied. We have the opportunity to make three complete detailed series of histological sections of Q. malakhovi and to study the microscopic anatomy, histology and fine structure of the reproductive system of this acorn worm using scanning and transmission electron microscopy.

RESULTS

The sexes of Q. malakhovi are separate and indistinguishable externally. The lobed testes occupy the dorsal side of the genital wings and distinctly bulge into the peribranchial cavity by their mature lobes. The central part of the testis is always submerged into the genital wing and opens via a single gonad pore. The monociliary muscle cells stretch along the external wall of the testis and surround the gonad pore, probably taking part in the contraction of the testis lobes for spawning. The germinative epithelium of the testis contains spermatogenic cells at different stages of development and interstitial cells. Yolk cells are not found. Interstitial cells embrace the spermatogonia and spermatogenic columns, providing horizontal compartmentalization of the germinative epithelium, and contain numerous phagosomes with remnants of degenerating spermatogenic cells. The testis wall contains haemal lacunae, which are usually located on the side opposite the gonad pore. We describe the fine structure of spermatogonia, spermatocytes clustered in spermatogenic columns, spermatids, and spermatozoa. Spermatozoa are of the ectaquasperm type and consist of an acorn-shaped head and a flagellum 18-25 µm long. The sperm head includes a beak-shaped acrosomal part, a spherical nucleus and a midpiece containing a ring of 5 or rarely 6 mitochondria.

CONCLUSIONS

The male reproductive system and sperm structure of Q. malakhovi, a representative of the family Torquaratoridae, have a number of differences from shallow-water acorn worms; however, the spermatogenesis and sperm structure of Q. malakhovi generally follow the pattern of the other three enteropneust families, and the phylogenetic significance of these deviations should be the subject of further research.

Nemertodermatida-Endosymbionts of Deep-Sea Acorn Worms (Hemichordata, Torquaratoridae)

Worm-like endosymbionts were found in the hepatic region of the digestive tract of the deep-sea acorn worm Quatuoralisia malakhovi Ezhova et Lukinykh, 2022 (family Torquaratoridae) from the Bering Sea. The symbionts were assigned to the taxon Nemertodermatida on the basis of histological examination. Torquaratoridae are similar in feeding type to holothuroids, which have also been found to have Xenacoelomorpha endosymbionts.

Discovery of Trunk Coelomoducts in Hemichordata

Histological examination of a specimen of a deep-sea enteropneusts that belongs to a yet undescribed species (Torquaratoridae gen. sp.) revealed numerous trunk coelomoducts. They open into the genital wing coelom as a typical funnels; short ducts communicate with environment through pores located on the outer side of the genital wings. Total number of coelomoducts in a specimen is estimated at several thousand. Trunk coelomoducts have not been found earlier in any member of the phylum. We believe that the release of the male gonad products occurs through coelomoducts of Torquaratoridae gen. sp.

Biogeography and adaptations of torquaratorid acorn worms (Hemichordata: Enteropneusta) including two new species from the Canadian Arctic

The enteropneust family Torquaratoridae, discovered in 2005, has the fewest species of the four living families. It is composed of seven species that live on the cold, deep-sea floor. Torquarator bullocki Holland, Clague, Gordon, Gebruk, Pawson and Vecchione, 2005 was the first species described and collected from the Northeastern Pacific. Two new species of Torquaratoridae were collected from the Eastern Pacific and described as Tergivelum baldwinae Holland, Jones, Ellena, Ruhl and Smith, 2009 and Allapasus aurantiacus Holland, Kuhnz and Osborn, 2012. A further three species have been collected from the North Atlantic: Yoda purpurata Priede, Osborn, Gebruk, Jones, Shale, Rogacheva and Holland, 2012, Allapasus isidis Priede, Osborn, Gebruk, Jones, Shale, Rogacheva and Holland, 2012, and Tergivelum cinnabarinum Priede, Osborn, Gebruk, Jones, Shale, Rogacheva and Holland, 2012. The latest Torquaratoridae species was collected from the Russian Arctic in 2013. We add two new species to the Torquaratoridae from Baffin Bay and Viscount Melville Sound, in the eastern Arctic of Canada. Terminstomo arcticus gen. nov. and sp. nov. was collected at a depth of 505 m and is characterized by lacking a heart and a stomochord that extends from the posterior end of the proboscis through the entire length of the collar. Allapasus fuscus sp. nov. was collected from a depth of 444 m and is characterized by a typhlosole process along the dorsal midline of the collar buccal cavity.

The Global Diversity of Hemichordata

Phylum Hemichordata, composed of worm-like Enteropneusta and colonial Pterobranchia, has been reported to only contain about 100 species. However, recent studies of hemichordate phylogeny and taxonomy suggest the species number has been largely underestimated. One issue is that species must be described by experts, and historically few taxonomists have studied this group of marine invertebrates. Despite this previous lack of coverage, interest in hemichordates has piqued in the past couple of decades, as they are critical to understanding the evolution of chordates–as acorn worms likely resemble the deuterostome ancestor more closely than any other extant animal. This review provides an overview of our current knowledge of hemichordates, focusing specifically on their global biodiversity, geographic distribution, and taxonomy. Using information available in the World Register of Marine Species and published literature, we assembled a list of 130 described, extant species. The majority (83%) of these species are enteropneusts, and more taxonomic descriptions are forthcoming. Ptychoderidae contained the greatest number of species (41 species), closely followed by Harrimaniidae (40 species), of the recognized hemichordate families. Hemichordates are found throughout the world’s oceans, with the highest reported numbers by regions with marine labs and diligent taxonomic efforts (e.g. North Pacific and North Atlantic). Pterobranchs are abundant in Antarctica, but have also been found at lower latitudes. We consider this a baseline report and expect new species of Hemichordata will continue to be discovered and described as new marine habitats are characterized and explored.

Phylogenomic resolution of the hemichordate and echinoderm clade

Ambulacraria, comprising Hemichordata and Echinodermata, is closely related to Chordata, making it integral to understanding chordate origins and polarizing chordate molecular and morphological characters. Unfortunately, relationships within Hemichordata and Echinodermata have remained unresolved, compromising our ability to extrapolate findings from the most closely related molecular and developmental models outside of Chordata (e.g., the acorn worms Saccoglossus kowalevskii and Ptychodera flava and the sea urchin Strongylocentrotus purpuratus). To resolve long-standing phylogenetic issues within Ambulacraria, we sequenced transcriptomes for 14 hemichordates as well as 8 echinoderms and complemented these with existing data for a total of 33 ambulacrarian operational taxonomic units (OTUs). Examination of leaf stability values revealed rhabdopleurid pterobranchs and the enteropneust Stereobalanus canadensis were unstable in placement; therefore, analyses were also run without these taxa. Analyses of 185 genes resulted in reciprocal monophyly of Enteropneusta and Pterobranchia, placed the deep-sea family Torquaratoridae within Ptychoderidae, and confirmed the position of ophiuroid brittle stars as sister to asteroid sea stars (the Asterozoa hypothesis). These results are consistent with earlier perspectives concerning plesiomorphies of Ambulacraria, including pharyngeal gill slits, a single axocoel, and paired hydrocoels and somatocoels. The resolved ambulacrarian phylogeny will help clarify the early evolution of chordate characteristics and has implications for our understanding of major fossil groups, including graptolites and somasteroideans.

Hemichordate molecular phylogeny reveals a novel cold-water clade of harrimaniid acorn worms

Hemichordates are instrumental to understanding early deuterostome and chordate evolution, yet diversity and relationships within the group have been understudied. Recently, there has been renewed interest in hemichordate diversity and taxonomy, although current findings suggest that much hemichordate diversity remains to be discovered. Herein, we present a molecular phylogenetic study based on nuclear 18S rDNA sequence data, which includes 35 previously unsampled taxa and represents all recognized hemichordate families. We include mitochondrial 16S rDNA data from 66 enteropneust taxa and three pterobranch Rhabdopleura species, and recover colonial pterobranchs and solitary enteropneusts as reciprocally monophyletic taxa. Our phylogenetic results also reveal a previously unknown clade of at least four species of harrimaniid enteropneusts from cold waters, including Antarctica, the North Atlantic around Iceland and Norway, and the deep sea off Oregon. These small worms (1-5 mm in length), occur from 130 to 2950 m and are not closely related to other deep-sea harrimaniids, indicating that diversity of enteropneusts within the deep sea is broader than previously described in the literature. Discovery of this clade, as well as larger torquaratorids from Antarctica, strengthens hypotheses of close associations between Antarctic and deep-sea fauna.