Digestive System Anatomy and Feeding Mechanism of Quatuoralisia malakhovi (Hemichordata, Torquaratoridae).

The digestive system was anatomically studied in the deep-sea enteropneust Quatuoralisia mala-khovi. It was shown that lateral collar lips are twisted in such a way that they form a ciliary groove that leads to an internal channel, through which collected detritus particles are transferred to peripheral pharyngeal channels. The size of the selected particles ranges from 1-6 to 100-200 µm, which corresponds to feeding on the remains of planktonic diatoms. A fecal cord was observed to act as an anchor that holds the heavily watered jelly-like body of Torquaratoridae at the sea floor during feeding.

Cephalochordates: A window into vertebrate origins.

How vertebrates evolved from their invertebrate ancestors has long been a central topic of discussion in biology. Evolutionary developmental biology (evodevo) has provided a new tool-using gene expression patterns as phenotypic characters to infer homologies between body parts in distantly related organisms-to address this question. Combined with micro-anatomy and genomics, evodevo has provided convincing evidence that vertebrates evolved from an ancestral invertebrate chordate, in many respects resembling a modern amphioxus. The present review focuses on the role of evodevo in addressing two major questions of chordate evolution: (1) how the vertebrate brain evolved from the much simpler central nervous system (CNS) in of this ancestral chordate and (2) whether or not the head mesoderm of this ancestor was segmented.

Molecular insights into deuterostome evolution from hemichordate developmental biology.

Hemichordates, along with echinoderms and chordates, belong to the lineage of bilaterians called the deuterostomes. Their phylogenetic position as an outgroup to chordates provides an opportunity to investigate the evolutionary origins of the chordate body plan and reconstruct ancestral deuterostome characters. The body plans of the hemichordates and chordates are organizationally divergent making anatomical comparisons very challenging. The developmental underpinnings of animal body plans are often more conservative than the body plans they regulate, and offer a novel data set for making comparisons between morphologically divergent body architectures. Here I review the hemichordate developmental data generated over the past 20 years that further test hypotheses of proposed morphological affinities between the two taxa, but also compare the conserved anteroposterior, dorsoventral axial patterning programs and germ layer specification programs. These data provide an opportunity to determine which developmental programs are ancestral deuterostome or bilaterian innovations, and which ones occurred in stem chordates or vertebrates representing developmental novelties of the chordate body plan.

Is the Gill Skeleton of Acorn Worms (Enteropneusta) Similar to the Gill Skeleton of Amphioxus (Cephalochordata)?

The gill skeleton of the enteropneust Saccoglossus mereschkowskii consists of a series of tridents. The central prong of each trident bifurcates in its ventral end. The most anterior gill skeletal element has a simple horseshoe shape. Homologues of the elements of the enteropneust gill apparatus were found in the structure of the gill apparatus of Cephalochordata. The organization of the gill skeleton of Enteropneusta and Cephalochordata can be derived from the metameric horseshoe-shaped elements. The similarity of the structure of the gill skeleton of Enteropneusta and Cephalochordata contradicts a common "upside-down theory" of the origin of Chordata.

Cambrian Tentaculate Worms and the Origin of the Hemichordate Body Plan.

Hemichordate relationships remain contentious due to conflicting molecular results [1-7] and the high degree of morphological disparity between the two hemichordate classes, Enteropneusta and Pterobranchia [8-11]. Additionally, hemichordates have a poor fossil record outside of the Cambrian, with the exception of the collagenous tubes of the pterobranchs (which include graptolites). By the middle Cambrian, tube-dwelling colonial pterobranchs [12, 13] and tube-dwelling enteropneusts coexisted [14, 15], supporting the origin of the hemichordate body plan earlier in the Cambrian without clarifying the morphology of their last common ancestor. Here, we describe a new hemichordate, Gyaltsenglossus senis, based on 33 specimens from the 506-million-year-old Burgess Shale (Odaray Mountain, British Columbia). G. senis has a unique combination of soft anatomical characters found in both extant classes of hemichordates, namely a trimeric-vermiform body plan with an elongate proboscis and six feeding arms with tentacles. The trunk possesses a long through-gut and terminates with a bulbous structure potentially used for locomotion and/or as a temporary anchor. There is no evidence of a secreted tube. Our phylogenetic analyses retrieve this new taxon as a stem-group hemichordate, supporting the hypothesis that a vermiform body plan preceded both tube building and colonial ecologies. This new taxon suggests that a bimodal feeding ecology using tentacles to filter feed and a proboscis to deposit feed may be plesiomorphic in hemichordates. Finally, the presence of a muscular, post-anal attachment structure in all known Cambrian hemichordates supports this feature as an additional hemichordate plesiomorphy critical for understanding early hemichordate evolution.

A stem group echinoderm from the basal Cambrian of China and the origins of Ambulacraria.

Deuterostomes are a morphologically disparate clade, encompassing the chordates (including vertebrates), the hemichordates (the vermiform enteropneusts and the colonial tube-dwelling pterobranchs) and the echinoderms (including starfish). Although deuterostomes are considered monophyletic, the inter-relationships between the three clades remain highly contentious. Here we report, Yanjiahella biscarpa, a bilaterally symmetrical, solitary metazoan from the early Cambrian (Fortunian) of China with a characteristic echinoderm-like plated theca, a muscular stalk reminiscent of the hemichordates and a pair of feeding appendages. Our phylogenetic analysis indicates that Y. biscarpa is a stem-echinoderm and not only is this species the oldest and most basal echinoderm, but it also predates all known hemichordates, and is among the earliest deuterostomes. This taxon confirms that echinoderms acquired plating before pentaradial symmetry and that their history is rooted in bilateral forms. Yanjiahella biscarpa shares morphological similarities with both enteropneusts and echinoderms, indicating that the enteropneust body plan is ancestral within hemichordates.

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.

A new generic name, Semigothograptus, for Gothograptus? meganassa Rickards & Palmer, 2002, from the Silurian post-lundgreni Biozone recovery phase, and comparative morphology of retiolitids from the lowermost upper Homerian (upper Wenlock).

Gothograptus? meganassa Rickards & Palmer, 2002 is assigned to a new genus Semigothograptus. New, well preserved material from the dubius/nassa Biozone (upper Homerian, Silurian) of the Bartoszyce IG-1 drill core Poland is described. This provides a new phylogenetic perspective on the evolution of post-lundgreni Event retiolitines. Semigothograptus meganassa is considered to be a descendant of Gothograptus nassa, although one of the most significant differences between these forms is the position of the nema, and narrow finite tubarium ending in an appendix in G. nassa. S. meganassa possesses looping meshes of the ancora umbrella recognised in Gothograptus, Papiliograptus, and Baculograptus and shares the common characters of all stratigraphical younger retiolitines. It is recognised that the S. meganassa is known from four terrains: Avalonia, Baltica, Bohemia, and Saxo-Thuringia. Analysis of the genicular hoods of nassa type, characteristic of Gothograptus nassa, Gothograptus kozlowskii, Semigothograptus meganassa, and Neogothograptus eximinassa, demonstrates their unique, solid bandage construction.

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.

Cambrian suspension-feeding tubicolous hemichordates.

BACKGROUND: The combination of a meager fossil record of vermiform enteropneusts and their disparity with the tubicolous pterobranchs renders early hemichordate evolution conjectural. The middle Cambrian Oesia disjuncta from the Burgess Shale has been compared to annelids, tunicates and chaetognaths, but on the basis of abundant new material is now identified as a primitive hemichordate. RESULTS: Notable features include a facultative tubicolous habit, a posterior grasping structure and an extensive pharynx. These characters, along with the spirally arranged openings in the associated organic tube (previously assigned to the green alga Margaretia), confirm Oesia as a tiered suspension feeder. CONCLUSIONS: Increasing predation pressure was probably one of the main causes of a transition to the infauna. In crown group enteropneusts this was accompanied by a loss of the tube and reduction in gill bars, with a corresponding shift to deposit feeding. The posterior grasping structure may represent an ancestral precursor to the pterobranch stolon, so facilitating their colonial lifestyle. The focus on suspension feeding as a primary mode of life amongst the basal hemichordates adds further evidence to the hypothesis that suspension feeding is the ancestral state for the major clade Deuterostomia.

Phylostratigraphic profiles in zebrafish uncover chordate origins of the vertebrate brain.

An elaborated tripartite brain is considered one of the important innovations of vertebrates. Other extant chordate groups have a more basic brain organization. For instance, cephalochordates possess a relatively simple brain possibly homologous to the vertebrate forebrain and hindbrain, whereas tunicates display the tripartite organization, but without the specialized brain centers. The difference in anatomical complexity is even more pronounced if one compares chordates with other deuterostomes that have only a diffuse nerve net or alternatively a rather simple central nervous system. To gain a new perspective on the evolutionary roots of the complex vertebrate brain, we made here a phylostratigraphic analysis of gene expression patterns in the developing zebrafish (Danio rerio). The recovered adaptive landscape revealed three important periods in the evolutionary history of the zebrafish brain. The oldest period corresponds to preadaptive events in the first metazoans and the emergence of the nervous system at the metazoan-eumetazoan transition. The origin of chordates marks the next phase, where we found the overall strongest adaptive imprint in almost all analyzed brain regions. This finding supports the idea that the vertebrate brain evolved independently of the brains within the protostome lineage. Finally, at the origin of vertebrates we detected a pronounced signal coming from the dorsal telencephalon, in agreement with classical theories that consider this part of the cerebrum a genuine vertebrate innovation. Taken together, these results reveal a stepwise adaptive history of the vertebrate brain where most of its extant organization was already present in the chordate ancestor.

Tubicolous enteropneusts from the Cambrian period.

Hemichordates are a marine group that, apart from one monospecific pelagic larval form, are represented by the vermiform enteropneusts and minute colonial tube-dwelling pterobranchs. Together with echinoderms, they comprise the clade Ambulacraria. Despite their restricted diversity, hemichordates provide important insights into early deuterostome evolution, notably because of their pharyngeal gill slits. Hemichordate phylogeny has long remained problematic, not least because the nature of any transitional form that might serve to link the anatomically disparate enteropneusts and pterobranchs is conjectural. Hence, inter-relationships have also remained controversial. For example, pterobranchs have sometimes been compared to ancestral echinoderms. Molecular data identify enteropneusts as paraphyletic, and harrimaniids as the sister group of pterobranchs. Recent molecular phylogenies suggest that enteropneusts are probably basal within hemichordates, contrary to previous views, but otherwise provide little guidance as to the nature of the primitive hemichordate. In addition, the hemichordate fossil record is almost entirely restricted to peridermal skeletons of pterobranchs, notably graptolites. Owing to their low preservational potentials, fossil enteropneusts are exceedingly rare, and throw no light on either hemichordate phylogeny or the proposed harrimaniid-pterobranch transition. Here we describe an enteropneust, Spartobranchus tenuis (Walcott, 1911), from the Middle Cambrian-period (Series 3, Stage 5) Burgess Shale. It is remarkably similar to the extant harrimaniids, but differs from all known enteropneusts in that it is associated with a fibrous tube that is sometimes branched. We suggest that this is the precursor of the pterobranch periderm, and supports the hypothesis that pterobranchs are miniaturized and derived from an enteropneust-like worm. It also shows that the periderm was acquired before size reduction and acquisition of feeding tentacles, and that coloniality emerged through aggregation of individuals, perhaps similar to the Cambrian rhabdopleurid Fasciculitubus. The presence of both enteropneusts and pterobranchs in Middle Cambrian strata, suggests that hemichordates originated at the onset of the Cambrian explosion.

Morphology of a new deep-sea acorn worm (class Enteropneusta, phylum Hemichordata): a part-time demersal drifter with externalized ovaries.

Ten individuals of an enteropneust in the family Torquaratoridae were videotaped between 2,900 and 3,500 m in the Eastern Pacific--one drifting a few centimeters above the bottom, two exposed on the substrate, and seven partly burrowed, reflecting a bentho-pelagic life style. Here, we describe a captured specimen (26 cm living length) as the holotype of Allapasus aurantiacus n. gen., n. sp. The small proboscis is dome-shaped, and the collar is only slightly wider than deep; both of these body regions are more muscular than in other torquaratorids, which presumably facilitates burrowing. The proboscis complex, in contrast to that of shallow-living enteropneusts, lacks a pericardial sac and is located relatively posteriorly in the proboscis stalk. The stomochord is separated from the main course of the gut by the intervention of a small, plate-like proboscis skeleton lacking posterior horns. The most anterior region of the trunk houses the pharynx, in which the pharyngeal skeletal bars are not connected by synapticles. The postpharyngeal trunk comprises three intestinal regions: prehepatic, hepatic (with conspicuous sacculations), and posthepatic. On either side of the worm, a flap of body wall (lateral wing) runs the entire length of the trunk. The two lateral wings can wrap the body so their edges meet in the dorsal midline, although they often gape open along the pharyngeal region. The holotype is a female (presumably the species is gonochoric) with numerous ovaries located in the lateral wings along the pharyngeal region. Each larger ovary contains a single primary oocyte (up to 1,500 µm in diameter) and bulges outwards in an epidermal pouch attached to the rest of the body by a slender stalk. Such externalized ovaries are unprecedented in any animal, and nothing is yet known of their role in the reproductive biology of A. aurantiacus.

Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia.

The Middle Cambrian Pikaia gracilens (Walcott) has an iconic position as a Cambrian chordate, but until now no detailed description has been available. Here on the basis of the 114 available specimens we review its anatomy, confirm its place in the chordates and explore with varying degrees of confidence its relationships to both extant and extinct chordates and other deuterostomes. The body of Pikaia is fusiform, laterally compressed and possesses about 100 myomeres. The head is small, bilobed and bears two narrow tentacles. There is no evidence for eyes. Apart from a thin dorsal fin (without finrays) and a series of at least nine bilaterally arranged appendages with possible pharyngeal pores at the anterior end, there are no other external features. In addition to the musculature the internal anatomy includes an alimentary canal, the anterior of which forms a prominent lenticular unit that is almost invariably preserved in positive relief. The cavity is interpreted as pharyngeal, implying that the mouth itself was almost terminal. The posterior extension of the gut is unclear although the anus appears to have been terminal. The most prominent internal structure is a reflectively preserved unit, possibly hollow, termed here the dorsal organ. Although formerly interpreted as a notochord its position and size make this less likely. Its original function remains uncertain, but it could have formed a storage organ. Ventral to the dorsal organ a narrower strand of tissue is interpreted as representing the nerve chord and notochord. In addition to these structures, there is also evidence for a vascular system, including a ventral blood vessel. The position of Pikaia in the chordates is largely based on the presence of sigmoidal myomeres, and the more tentative identification of a notochord. In many other respects, Pikaia differs from the expected nature of primitive chordates, especially as revealed in amphioxus and the Cambrian record (including Cathaymyrus, Haikouichthys, Metaspriggina, Myllokunmingia, and Zhongxiniscus). Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed, we prefer to build a scenario that regards Pikaia as the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans. This hypothesis has implications for the evolution of the myomeres, notochord and gills. Finally, the wealth of material of Pikaia indicates that, although by definition there must be some sort of taphonomic imprint, the consistency of preservational details allows a reliable reconstruction of the anatomy and does not significantly erode phylogenetically relevant characters.

Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction.

The morphological study of extinct taxa allows for analysis of a diverse set of macroevolutionary hypotheses, including testing for change in the magnitude of morphological divergence, extinction selectivity on form, and the ecological context of radiations. Late Ordovician graptoloids experienced a phylogenetic bottleneck at the Hirnantian mass extinction (∼445 Ma), when a major clade of graptoloids was driven to extinction while another clade simultaneously radiated. In this study, we developed a dataset of 49 ecologically relevant characters for 183 species with which we tested two main hypotheses: (i) could the biased survival of one graptoloid clade over another have resulted from morphological selectivity alone and (ii) are the temporal patterns of morphological disparity and innovation during the recovery consistent with an interpretation as an adaptive radiation resulting from ecological release? We find that a general model of morphological selectivity has a low probability of producing the observed pattern of taxonomic selectivity. Contrary to predictions from theory on adaptive radiations and ecological speciation, changes in disparity and species richness are uncoupled. We also find that the early recovery is unexpectedly characterized by relatively low morphological disparity and innovation, despite also being an interval of elevated speciation. Because it is necessary to invoke factors other than ecology to explain the graptoloid recovery, more complex models may be needed to explain recovery dynamics after mass extinctions.

Distinct gene structure and expression pattern of Smad nuclear interacting protein 1 in amphioxus and zebrafish.

A Smad nuclear interacting protein 1 (SNIP1) homologous gene was identified in amphioxus. Phylogenic analysis showed that SNIP1 proteins from different species share a highly conserved FHA domain at the C-terminus, but their N-terminus varies remarkably. The genomic structure of SNIP1 varies in different species, especially at the 5' end. Through in situ hybridization, we studied SNIP1 expression patterns in amphioxus and zebrafish embryos. Amphioxus SNIP1 transcripts were specifically located in the notochord in larval and adult stages. In zebrafish, however, snip1 transcripts were specifically located not only in the notochord, but also in the rhombencephalic ventricle, otic vesicles and pectoral fin buds. This is the first report of SNIP1 expression pattern in early development, which clearly shows different expression patterns between invertebrates and vertebrates. Previous studies reported that it is the N-terminal domain of human and mouse SNIP1 that functions to inhibit both TGF-ß and the NF-κB pathways. Therefore, it is most likely that the modification of SNIP1 expression pattern is related to the remarkable evolution in the N-terminal sequence. In addition, the difference in SNIP1 expression pattern between amphioxus and zebrafish implies the role of SNIP1 in the vertebrate body structural innovation of brain, otic vesicles and pectoral fins.

Characterization of the immune defense related tissues, cells, and genes in amphioxus.

Amphioxus is an important animal model for phylogenetic analysis, including comparative immunology. Exploring the immune system in amphioxus contributes to our understanding of the origin and evolution of the vertebrate immune system. We investigated the amphioxus immune system using ultrastructural examination and in situ hybridization. The expression patterns of TLR1 (toll-like receptor 1), C1Q (complement component 1, q subcomponent), ECSIT (evolutionarily conserved signaling intermediate in Toll pathways), SoxC, DDAHa (Dimethylarginine dimethylaminohydrolase a), and NOS (nitric oxide synthase) show that these genes play key roles in amphioxus immunity. Our results suggest that the epidermis and alimentary canal epithelium may play important roles in immune defense, while macrophages located in the coelom and so-called lymph spaces may also be crucial immune cells.

Dynamic topology of the cephalochordate to amniote morphological transition: a self-organized system of Russian dolls.

This note presents a mechanistic explanation of the transition between the morphology of cephalochordates to that of amniotes. By a careful study of the morphogenetic movements which occur during the early stages of development of a typical amniote (a chicken embryo), we are able to show that the formation of a vertebrate body follows a sequence: first, formation of dorsal folds, then head and heart as dorsal and ventral folds, and finally another dorsal fold, which eventually builds up the chorion. This order has a physical origin linked to the velocity field of the tissue flow. These folds form at right angles to the flow direction, and the topology of the chordates flow is hyperbolic. This mechanism explains the differences between the successive bauplans, by the cumulate forward and backward movement of the flow. Eventually, the entire phenomenon can be described as a self-organized system of Russian dolls, by which the heart finds itself inside the embryo, and the embryo itself inside the chorion. In addition, the phenomenon has a mirror symmetry in the anterior and in the posterior part, thereby explaining naturally the existence of animals having a caudal heart.

Identification and expression of amphioxus AmphiSmad1/5/8 and AmphiSmad4.

Smad family proteins are identified as intracellular signal mediators of the TGF-ß superfamily. In this study, we identified two novel members of the Smad family, termed as AmphiSmad1/5/8 and AmphiSmad4, from Chinese amphioxus. Both AmphiSmad1/5/8 and AmphiSmad4 showed a typical domain structure of Smad proteins consisting of conserved MH1 and MH2 domains. Phylogenetic analysis placed AmphiSmad1/5/8 in the Smad1, 5 and 8 subgroup of the R-Smad subfamily, and AmphiSmad4 in the Co-Smad subfamily. The spatial and temporal gene expression patterns of AmphiSmad1/5/8 and AmphiSmad4 showed that they may be involved in the embryonic development of notochord, myotome and alimentary canal, and may help to establish the specification of dorsal-ventral axis of amphioxus. Moreover, AmphiSmad1/5/8 and AmphiSmad4 showed extensive distribution in all adult tissues examined, suggesting that these two genes may play important roles in the morphogenesis of a variety of tissues especially notochord and gonad.

Cilia-like structures anchor the amphioxus notochord to its sheath.

Body stiffness is important during undulatory locomotion in fish. In amphioxus, the myosepta play an important role in transmission of muscular forces to the notochord. In order to define the specific supporting role of the notochord in amphioxus during locomotion, the ultrastructure of 10 adult amphioxus specimens was analyzed using transmission electron microscopy. Numerous cilia-like structures were found on the surface of each notochordal cell at the sites of their attachment to the notochordal sheath. Ultrastructurally, these structures consisted of the characteristic arrangement of peripheral and central microtubular doublets and were anchored to the inner layer of the notochordal sheath. Immunohistochemically, a positive reaction to applied dynein and ß-tubulin antibodies characterized the area of the cilia-like structures. We propose that reduced back-and-forth movements of the cilia-like structures might contribute to the flow of the fluid content inside the notochord, thus modulating the stiffness of the amphioxus body during its undulatory locomotion.