Fine structure of the pharyngeal apparatus of the pelagosphera larva in Phascolosoma agassizii (Sipuncula) and its phylogenetic significance

Sipuncula is a small taxon of worm-like marine organisms of still uncertain phylogenetic position. Sipunculans are characterized by an unsegmented body composed of a trunk into which the anterior part, the introvert, can be withdrawn. The group has been placed at various positions within Metazoa; currently, it is either seen as sister group of a clade comprising Mollusca and Annelida or as sister to each of these. An in-group position in either Mollusca or Annelida has usually been precluded till now due to the lack of so-called annelid or molluscan “key-characters” such as segmentation and chaetae or the radula. In the development of certain taxa the trochophore stage is followed by a planktonic larva, the pelagosphera, which might exhibit phylogenetically important structures. Among these is the buccal organ, which has been considered homologous either to the ventral pharyngeal organ present in many sedentary polychaetes or to the radular apparatus of molluscs. In the present paper, the ventral pharynx of the pelagosphera larva of Phascolosoma agassizii is investigated by transmission electron microscopy. The pharynx comprises dorsolateral ciliary folds, a muscle bulb formed by transverse muscle fibres with large intercellular spaces, and an investing muscle. A tongue-like organ is lacking. These results show great structural correspondences to the ventral pharynx of polychaetes, especially to that of the flabelligerid Diplocirrus longisetosus. In contrast, there are no signs of structural similarities to the corresponding structures of molluscs. Thus evidence increases that Sipuncula are closely related to annelids; moreover, an in-group position of Sipuncula within Annelida, as suggested by recent molecular studies, is not precluded by the present data. Instead these studies find additional support. Hence the lack of segmentation and chitinous chaetae in Sipuncula would be a secondary rather than a primary situation, as has recently been shown for Echiura and Pogonophora.     

SEM studies on the early larval development of Triops cancriformis (Bosc)(Crustacea: Branchiopoda, Notostraca)

We investigated early larval development in the notostracan Triops cancriformis (Bosc, 1801–1802) raised from dried cysts under laboratory conditions. We document the five earliest stages using scanning electron microscopy. The stage I larva is a typical nauplius, lecithotropic and without trunk limbs. The stage II larva is feeding and has trunk limb precursors and a larger carapace. Stage III larvae have larger trunk limbs and a more adult shape. Stage IV larvae have well developed trunk limbs, and stage V larvae show atrophy of the antennae. We describe the ontogeny of selected features such as trunk limbs and carapace, discuss ontogeny and homologization of head appendages, follow the development of the feeding mechanism, and discuss trunk limb ontogeny.     

Larval Development and Metamorphosis in Sipuncula

In a brief review of development of the phylum Sipuncula, fourpatterns of development are recognized: (1) direct with no pelagicstage; (2) one larval stage, a lecithotrophic trochophore; (3)two larval stages, a lecithotrophic trochophore and a lecithotrophicpelagosphera; (4) two larval stages, a lecithotrophic trochophoreand a planktotrophic pelagosphera. Larval types and their metamorphosesare described, with special attention to the development andmorphology of the larval cuticle. In the majority of speciesstudied, the egg envelope is transformed into the larval cuticleat metamorphosis of the trochophore. The cuticle of many planktotrophicpelagosphera larvae is characterized by surface papillae ofdiverse form and pattern. The underlying cuticle in some speciesis composed of layers of fibers at right angles to one another.     

The nonfeeding auricularia of Holothuria mexicana (Echinodermata,Holothuroidea)

Among echinoderms, nonfeeding larvae usually are simplified in body shape, have uniform ciliation, and have lost the larval gut. A few species have nonfeeding larvae that express some remnant features of feeding larvae like ciliated bands and larval skeleton or larval arms, but typically their larval mouth never opens and their gut does not function. Still other species have retained the feeding larval form, a functional gut, and can feed, but they do not require food to metamorphose. The present note describes the development of a tropical holothurian, Holothuria mexicana, which hatches as a gastrula that is already generating coelomic structures. A translucent auricularia forms with a mouth that opens but becomes reduced soon thereafter. In form and ciliation this auricularia resembles a feeding larva, but it does not respond to food. A doliolaria forms on day 4 and the pentactula on day 6 post‐fertilization. Further study of this larva and that of its closely related congener, Holothuria floridana, is warranted.     

Genetic structure in two Phascolosoma species in the Pacific Ocean

Phascolosoma agassizii, a commonly reported species of sipunculan worm, occurs at various depths throughout the north Pacific. However, previous studies have shown that this nominal species is actually comprised of two genetically divergent lineages. Phascolosoma agassizii is isolated to the eastern Pacific, whereas a second species, here referred to Phascolosoma sp., is located in the western Pacific and represents a unique, non-sister taxon to P. agassizii. Both species exhibit the same developmental mode with a long-lived, planktotrophic, pelagosphera larval stage, which may facilitate long-distance dispersal, although developmental timing differs. Using an inter-simple sequence repeat polymerase chain reaction (ISSR-PCR) genetic fingerprinting approach, this study examines non-coding polymorphic regions of the nuclear genomes of P. agassizii from four eastern Pacific populations and of Phascolosoma sp. from six western Pacific locations. Small-scale spatial genetic variation was analysed separately for each coastline using an Analysis of Molecular Variance. The data pointed to the potential presence of fine-scale genetic structure within P. agassizii, and recovered significant genetic structure within Phascolosoma sp. from the western Pacific. This study shows that ISSR-PCR is a relatively fast and cost-effective way to study fine-scale genetic diversity in marine invertebrates, although possible analyses and interpretations of the data are subject to the same limitations as other size polymorphism-based approaches.     

Three‐dimensional reconstruction of the musculature of various life cycle stages of the cycliophoran Symbion americanus

Cycliophora is a very recently described phylum of acoelomate metazoans with a complex life cycle and a phylogenetic position that has been under debate ever since its discovery in 1995. Symbion americanus, which lives attached to the mouthparts of the American lobster, Homarus americanus, represents the second species described for the phylum. Aiming to increase the morphological knowledge about this cryptic clade, the present study describes the muscle arrangement of the feeding stage, the attached Prometheus larva with the dwarf male inside, the free living male, the Pandora larva, and the chordoid larva of S. americanus using actin staining and confocal laser scanning microscopy. 3D reconstructions of the muscular systems are presented. In the feeding stage, circular muscles compose the buccal funnel aperture. In addition, a pair of muscles runs longitudinally in the buccal funnel. A complex sphincter was found just proximally to the anus, and six longitudinal muscles run from the trunk constriction (“neck”) in basal direction. The musculature of the larval stages and the dwarf male is very complex and includes longitudinal muscles that run dorsally and ventrally. In addition, we found dorso‐ventral muscles. The male has a complex posterior muscle apparatus in the vicinity of the penis. In this stage, X‐ and V‐shaped structures were identified on the dorsal and the ventral side, respectively. Pandora and chordoid larvae possess additional circular muscles. We discuss our findings with respect to muscle elements of other metazoan groups and the chordoid larva of Symbion pandora. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

Morphogenesis and phenotypic divergence in two developmental morphs of Streblospio benedicti (Annelida,Spionidae)

Abstract. The morphology of marine invertebrate larvae is strongly correlated with egg size and larval feeding mode. Planktotrophic larvae typically have suites of morphological traits that support a planktonic, feeding life style, while lecithotrophic larvae often have larger, yolkier bodies, and in some cases, a reduced expression of larval traits. Poecilogonous species provide interesting cases for the analysis of early morphogenesis, as two morphs of larvae are produced by a single species. We compared morphogenesis in planktotrophic and lecithotrophic morphs of the poecilogonous annelid Streblospio benedicti from the trochophore stage through metamorphosis, using observations of individuals that were observed alive, with scanning electron microscopy, or in serial sections. Offspring of alternate developmental morphs of this species are well known to have divergent morphologies in terms of size, yolk content, and the presence of larval bristles. We found that some phenotypic differences between morphs occur as traits that are present in only one morph (e.g., larval bristles, bacillary cells on the prostomium and pygidium), but that much of the phenotypic divergence is based on heterochronic changes in the differentiation of shared traits (e.g., gut and coelom). Tissue and organ development are compared in both morphs in terms of their structure and ontogenetic change throughout early development and metamorphosis.     

The larval ocelli of Golfingia misakiana (Sipuncula, Golfingiidae) and of a pelagosphera of another unidentified species

 The inverse cerebral ocelli of the pelagosphera larva of Golfingia misakiana and of another unidentified larva are composed of two or three sensory cells and one supportive pigmented cell. The sensory cells bear an array of microvilli as well as a single cilium with poor undulation of its membrane; the photoreceptive organelles are regarded as the rhabdomeric type. A striking feature of these cells is the cores, which extend within the microvilli from the tip into the midregion of the cell. It is suggested that these structures are identical with the submicrovillar cisternae found in the cerebral inverse eyes of larvae of Polychaeta. The findings allow the conclusion that in the pelagosphera of the Sipuncula, contrary to the teleplanic veliger larvae of Gastropoda, a lengthy pelagic cycle is not correlated with the development of a ciliary photoreceptor. Additionally, it is assumed that the pigment cup ocelli in larvae of Sipuncula are homologous with the cerebral inverted pigment cup ocelli of larvae of Polychaeta. Accepted: 19 March 1997     

Presumptive photoreceptor structures of the trochophore of Harmothoë imbricata (Polychaeta)

The behaviour of the Harmothoë trochophore changes with age, the larva being phototropic initially and later photonegative.

The trochophore possesses two ocelli midway between the prototroch and the apex in a mid‐lateral position. They appear first at the eighth day of development and grow to be kidney‐shaped structures. There is a pigment cup derived from a single cell that encloses a rhabdomeric type of photoreceptor apparatus that is also derived from a single (or rarely two) cell.

In the late trochophore (14 days old) an organ of different origin and formation but of presumed photoreceptor type begins to develop among nerve cell bodies below the apex of the animal. This structure consists of an array of membranes developed from both cilia and microvilli. The cilia are of 9 + 2 configuration.     

The chordoid larva of Symbion pandora (Cycliophora) is a modified trochophore

Developmental and free-living stages of the chordoid larva of the cycliophoran species, Symbion pandora Funch and Kristensen 1995, were studied using light and electron microscopy. In the free-living stage of the larva, about 200 μm long, four ciliated areas are found: two anterior bands, a ventral ciliated field, and a posterior unit on the ventral side of the foot. The nervous system consists of a dorsal brain and a pair of ventral longitudinal nerves. A gut is absent. A pair of protonephridia, each with a single multiciliated terminal cell and at least one duct cell, is present. Nephridiopores are not localized. A pair of corsal ciliated organs is posterior to the brain. The homology between these and the apical organ of a trochophore larva is discussed. A distinctive longitudinal rod, the chordoid organ, consists of vacuolized cells with circular myofilaments. The organ is comparable to a similar structure in gastrotrichs. In the discussion of the phylogenetic position of Cycliophora among protostomians, important morphological observations that are described in the present study indicate that, despite some dissimilarities, the chordoid larva is a modified trochophore. © 1996 Wiley-Liss, Inc.     

Trochophore concepts: ciliary bands and the evolution of larvae in spiralian Metazoa

‘Trochophore’ is a term used in a strict sense for larvae having an opposed-band method of feeding, involving a prototroch and metatroch. Other ciliary bands such as a telotroch and neurotroch may be present. The trochophore has been proposed to represent the ancestral larval form for a group of metazoan phyla (including all members of the Spiralia). The name trochophore is also often applied to larvae that do not conform to the above definition. A cladistic analysis of spiralian taxa (with special reference to polychaete annelids), based on a suite of adult and larval characters, is used to assess several hypotheses: (1) that the trochophore (in a strict sense) is a plesiomorphic form for the Spiralia; (2) that die stricdy defined trochophore is plesiomorphic for members of the Spiralia such as the Polychaeta. The homology of each of the various separate ciliary bands of spiralian larvae, and features such as the apical tuft and protonephridia is also assessed. The results favour the conclusion that the trochophore, if defined as a feeding larval form using opposed bands, should not be regarded as an ancestral (= plesiomorphic) type for the Spiralia, or any other large taxon such as the Polychaeta or Mollusca. The evidence suggests that the various ciliary bands have differing evolutionary histories, and only the Echiura (possibly an annelid group) has members with the classical trochophore. The trochophore is re-defined as a larval form with a prototroch. This broad definition covers a wide variety of larvae, and matches the current usage more accurately than the restricted term. Features such as the neurotroch, telotroch and opposed-band feeding show convergence and reversals. The nature of the metatroch requires further investigation. The presence of a prototroch (and hence trochophore larvae) is used to identify an apomorphy-based taxon, Trochozoa, that includes the first ancestor to have evolved a prototroch and all its descendants. This minimally includes the Annelida [sensu lato), Echiura, Entoprocta, Mollusca and Sipuncula and is a less inclusive taxon than the Spiralia.     

Kauri seeds and larval somersaults: The larval trunk of the seed mining basal moth Agathiphaga vitensis (lepidoptera: Agathiphagidae)

The trunk morphology of the larvae of the kauri pine (Agathis) seed infesting moth Agathiphaga is described using conventional, polarization, and scanning electron microscopy. The pine seed chamber formed by the larva is also described and commented on. The simple larval chaetotaxy includes more of the minute posture sensing setae, proprioceptors, than expected from the lepidopteran larval ground plan. The excess of proprioceptors is suggested to be necessary for sensory input concerning the larval posture within the seed chamber. The trunk musculature includes an autapomorphic radial ventral musculature made up of unique multisegmental muscles. The combined presence of additional proprioceptors and the unique ventral musculature is proposed to be related to the larval movement within the confined space of the seed chamber, especially to a proposed somersault movement that allows the larva to orientate itself within the chamber. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Giant pelagic larvae of Phyllodocidae (Polychaeta,Annelida)

The microscopic anatomy of giant pelagic larvae of Phyllodocidae was studied using routine histological, SEM, and TEM techniques. The larvae consist of two distinct regions: a large spherical trochophore measuring up to 2 mm in diameter and a posterior, long (up to 10 mm length), narrow rudiment of the adult body with up to 120 segments. The larvae have an unusual mixture of larval and adult features, including a very complex, well-developed brain and ganglia in the ventral nerve cord, and only a single pair of protonephridia located in the hyposphere of the trochophore. A muscular pharynx is not developed. The intestinal wall, especially in the trochophore region, consists of endodermal cells containing considerable nutritive material in the form of yolk-like globular inclusions. The digestive tract of all larvae was empty. The position of the frontal sensory organ and the prototroch, the structure of the parapodia and setae, and the three pairs of tentacular cirri dictate inclusion of the larvae in the family Phyllodocidae. The relatively enormous size and unusual pattern of development of the adult body may be adaptations for a long pelagic life and rapid settlement of the species, which inhabits slopes of islands and underwater mounts located far apart. J. Morphol. 238:93–107, 1998. © 1998 Wiley-Liss, Inc.     

Origin and evolution of animal life cycles

The ‘origin of larvae’ has been widely discussed over the years, almost invariably with the tacit understanding that larvae are secondary specializations of early stages in a holobenthic life cycle. Considerations of the origin and early radiation of the metazoan phyla have led to the conclusion that the ancestral animal (= metazoan) was a holopelagic organism, and that pelago-benthic life cycles evolved when adult stages of holopelagic ancestors became benthic, thereby changing their life style, including their feeding biology. The literature on the larval development and phylogeny of animal phyla is reviewed in an attempt to infer the ancestral life cycles of the major animal groups. The quite detailed understanding of larval evolution in some echinoderms indicates that ciliary filter-feeding was ancestral within the phylum, and that planktotrophy has been lost in many clades. Similarly, recent studies of the developmental biology of ascidians have demonstrated that a larval structure, such as the tail of the tadpole larva, can easily be lost, viz. through a change in only one gene. Conversely, the evolution of complex structures, such as the ciliary bands of trochophore larvae, must involve numerous genes and numerous adaptations. The following steps of early metazoan evolution have been inferred from the review. The holopelagic ancestor, blastaea, probably consisted mainly of choanocytes, which were the feeding organs of the organism. Sponges may have evolved when blastaea-like organisms settled and became reorganized with the choanocytes in collar chambers. The eumetazoan ancestor was probably the gastraea, as suggested previously by Haeckel. It was holopelagic and digestion of captured particles took place in the archenteron. Cnidarians and ctenophores are living representatives of this type of organization. The cnidarians have become pelago-benthic with the addition of a sessile, adult polyp stage; the pelagic gastraea-like planula larva is retained in almost all major groups, but only anthozoans have feeding larvae. Within the Bilateria, two major lines of evolution can be recognized: Protostomia and Deuterostomia. In protostomes, trochophores or similar types are found in most spiralian phyla; trochophore-like ciliary bands are found in some rotifers, whereas all other aschelminths lack ciliated larvae. It seems probable that the trochophore was the larval type of the ancestral, pelago-benthic spiralian and possible that it was ancestral in all protostomes. Most of the non-chordate deuterostome phyla have ciliary filter-feeding larvae of the dipleurula type, and this strongly indicates that the ancestral deuterostome had this type of larva.     

Larval development of Myzostoma cirriferum (Myzostomida)

The larval development of Myzostoma cirriferum is described by means of SEM, TEM, and cLSM. It is similar to that of other myzostomids and includes three stages: the protrochophore, the trochophore, and the metatrochophore. The protrochophore is a ball-shaped larva present in culture from 18-48 h after egg laying. It has no internal organs and its body is made of three cell types: covering cells and ciliated cells that are external and surrounded by a cuticle, and resting cells that fill the blastocoel. The trochophore is a pear-shaped larva that develops 20-72 h after egg laying; the body includes the same three cell types as the previous stage. The metatrochophore is a pear-shaped larva that develops between 40 h and 14 days and is characterized by the presence of two bundles of four chaetae. When fully developed, the metatrochophore has a digestive system (made of a pharynx, an esophagus, and a blind digestive pouch), two pairs of protonephridia, and a nervous system composed of a supraesophageal ganglion, circumesophageal connectives, and dorsal and ventral nerves. Metamorphosis generally occurs 7 days after egg laying. At that time, the metatrochophore loses its chaetae and becomes pleated ventrally. This ultrastructural analysis suggests that chaetae and the five ventral longitudinal nerve cords of M. cirriferum metatrochophores are homologous structures to those observed in some polychaete trochophores. Coupled with recent phylogenetic analyses, where the Myzostomida are placed outside the Annelida, homologies between myzostomid and polychaete larvae support the view that a trochophore appeared early during the spiralian evolution.     

Siphonariid development: Quintessential euthyneuran larva with a mantle fold innovation (Gastropoda; Panpulmonata)

Recent phylogenetic revisions of euthyneuran gastropods (“opisthobranchs” and “pulmonates”) suggest that clades with a planktotrophic larva, the ancestral life history for euthyneurans, are more widely distributed along the trunk of the euthyneuran tree than previously realized. There is some indication that the planktotrophic larva of euthyneurans has distinctive features, but information to date has come mainly from traditional “opisthobranch” groups. Much less is known about planktotrophic “pulmonate” larvae. If planktotrophic larvae of “pulmonates” share unique traits with those of “opisthobranchs,” then a distinctive euthyneuran larval-type has been the developmental starting template for a spectacular amount of evolved morphological and ecological disparity among adult euthyneurans. We studied development of a siphonariid by preparing sections of larval and postmetamorphic stages for histological and ultrastructural analysis, together with 3D reconstructions and data from immunolabeling of the larval apical sensory organ. We also sought a developmental explanation for the unusual arrangement of shell-attached, dorso-ventral muscles relative to the mantle cavity of adult siphonariids. Adult siphonariids (“false limpets”) have a patelliform shell but their C-shaped shell muscle partially embraces a central mantle cavity, which is different from the arrangement of these components in patellogastropods (“true limpets”). It is not obvious how shell muscles extending into the foot become placed anterior to the mantle cavity during siphonariid development from a veliger larva. We found that planktotrophic larvae of Siphonaria denticulata are extremely similar to previously described, planktotrophic “opisthobranch” larvae. To emphasize this point, we update a list of distinctive characteristics of planktotrophic euthyneuran larvae, which can anchor future studies on the impressive evolvability of this larval-type. We also describe how premetamorphic and postmetamorphic morphogenesis of larval mantle fold tissue creates the unusual arrangement of shell-muscles and mantle cavity in siphonariids. This result adds to the known postmetamorphic evolutionary innovations involving mantle fold tissue among euthyneurans.     

Reproduction,development, growth,and the length of larval life of Phascolosoma turnerae,a wood‐dwelling deep‐sea sipunculan

Specimens of the deep‐sea sipunculan Phascolosoma turnerae were retrieved over a 5‐year period from fibrous collectors placed for various time intervals at a depth of 520 m in the Tongue of the Ocean, Bahamas. Sipunculans removed from the collectors were counted, weighed, and maintained in the laboratory at 14°C, where they were monitored for gametogenic activity, spawning, development, and growth. In a 2‐year study of seasonality, worms were most abundant in collectors retrieved in the spring and summer, and least abundant in the fall. Small animals (<0.01 g) were present in all seasons and represented ≥70% of the animals in winter collections. Large specimens (>0.16 g) were found from May through August, but in markedly lower frequencies than small animals. Over the entire study, spawning was observed in the laboratory from April through August. We inferred from analyses of size frequencies, growth, and spawning seasonality that settlement of the larvae occurs primarily from November through April and that oceanic larval life could be as short as 7 months and as long as 12–14 months. Cleavage of fertilized eggs, as observed from laboratory spawnings, was spiral and holoblastic, resulting in a trochophore that transformed into a typical planktotrophic pelagosphera larva at 21 d. A few larvae survived as long as 2 months in the laboratory. This is the first study of biological processes in living sipunculans from the deep sea, and one of the first studies of living deep‐sea wood dwellers.     

Epidermal ciliary ultrastructure of adult and larval sipunculids (Sipunculida)

The ultrastructure of the ciliary apparatus of multiciliated epidermal cells in larval and adult sipunculids is described and the phylogenetic implications discussed. The pelagosphera of Apionsoma misakianum has a dense cover of epidermal cilia on the head region. The cilia have a long, narrow distal part and two long ciliary rootlets, one rostrally and one vertically orientated. The adult Phascolion strombus has cilia on the nuchal organ and on the oral side of the tentacles. These cilia have a narrow distal part as in the A. misakianum larva, but the ciliary rootlets have a different structure. The first rootlet on the anterior face of the basal body is very short and small. The second, vertically orientated rootlet is long and relatively thick. The two ciliary rootlets present in the larval A. misakianum are similar to the basal metazoan type of ciliary apparatus of epidermal multiciliated cells and thus likely represent the plesiomorphic state. The minute first rootlet in the adult P. strombus is viewed as a consequence of a secondary reduction. No possible synapomorphic character with the phylogenetically troublesome Xenoturbella was found.     

孤雌生殖长角血蜱的哈氏器超微结构与发育

为阐明长角血蜱Haemaphysalis longicornis孤雌生殖种群的哈氏器结构及发育特征, 用扫描电镜对其各虫期哈氏器进行了观察, 分析了血餐对哈氏器发育的影响。结果表明： 该种群幼蜱、 若蜱和成蜱哈氏器形态结构基本相同, 均由前窝和后囊构成。幼蜱前窝感毛6根, 位于同一基盘; 若蜱和成蜱哈氏器相似, 前窝感毛7根, 其中1根孔毛位于外侧基盘, 另6根感毛位于内侧基盘。各虫期饱血后哈氏器大小均比饥饿状态下显著增大（P<0.05）。幼蜱前窝与后囊面积比值与若蜱相比无显著差异（P>0.05）, 若蜱前窝与后囊面积比值与成蜱相比差异显著（P<0.05）。各虫期哈氏器均在发育, 且血餐对哈氏器发育有重要影响。幼蜱至若蜱期哈氏器前窝与后囊的发育速度相似, 若蜱至成蜱期哈氏器前窝发育快于后囊。本研究结果在一定程度上揭示了孤雌生殖长角血蜱的哈氏器发育规律。