Invertebrate Biology

These experiments are part of a larger study designed to investigate the influence of husbandry parameters on the life history of the apple snail, Marisa cornuarietis. The overall objective of the program is to identify suitable husbandry conditions for maintaining multi-generation populations of this species in the laboratory for use in ecotoxicological testing. In this article, we focus on the effects of photoperiod, temperature, and population density on adult fecundity and juvenile growth. Increasing photoperiod from 12 to 16 h of light per day had no effect on adult fecundity or egg hatching and relatively minor effects on juvenile growth and development. Rearing snails at temperatures between 22 degrees C and 28 degrees C did not influence the rates of egg production or egg clutch size. However, the rates of growth and development (of eggs and juveniles) increased with increasing temperature in this range, and when temperatures were reduced to 22 degrees C egg-hatching success was impaired. Juvenile growth and development were more sensitive to rearing density than adult fecundity traits. On the basis of the present results, we conclude that rearing individuals of M. cornuarietis at a temperature of 25 degrees C, a photoperiod of 12L:12D, and a density of <0.8 snails L(-1) (with lower densities for juvenile snails) should provide favorable husbandry conditions for maintaining multi-generation populations of this species.

The present experiments are part of a larger study designed to investigate the influence of husbandry parameters on the life history of the ramshorn snail, Marisa cornuarietis, in order to identify suitable husbandry conditions for maintaining multi-generation populations in the laboratory for use in ecotoxicological testing. In this paper we focus on the effects of a combination of food types and feeding frequencies (i.e., the frequency with which the snails were offered food) on juvenile growth and survival at different temperatures. Offspring produced in the laboratory by wild specimens of M. cornuarietis, from Puerto Rico, were used to test the effects of three types of food (lettuce, alginate with fish food, alginate with snail mix) fed at three frequencies (given ad libitum on 4/4, 2/4, or 1/4 d) on juvenile survival and growth. The 4-d feeding regimens were repeated four times, giving a total of 16 d for the experiments. The experiments were conducted at two temperatures (22° and 25°C) under a 12 h light:12 h dark photoperiod. Juvenile growth rates increased with increasing feeding frequency for all food types. The most rapid growth rates occurred in the high-frequency lettuce treatments and the slowest growth rates in the low-frequency lettuce and alginate with snail mix treatments. Juvenile snails grew faster at 25° than at 22°C, and mortality was about twice as high at the lower temperature. Growth rates were used to provide a rough estimate of time to maturity, which was determined to take about twice as long at 22° than at 25°C. The results showed that lettuce is the best food if supplied in abundance, but effects on growth are very dependent on feeding frequency and temperature. We conclude that 25°C is a more appropriate temperature for maintaining populations than 22°C, that lettuce provides a suitable food source, and that food should be supplied continuously for husbandry and toxicity testing of populations of M. cornuarietis.

Sipunculans are a phylum of non-segmented, marine worms. Although they are well characterized morphologically, relationships within the phylum and the relationship of Sipuncula to other spiralian phyla have been strongly debated. I analyzed representatives of 13 of 17 described genera using a 654-bp fragment of the mitochondrial gene, cytochrome c oxidase subunit I, to construct the first intraphylum phylogenetic hypothesis for sipunculans based on molecular sequence data. Within the phylum, tree topologies are loosely congruent with a previously published morphological analysis, except that the monotypic genus Phascolopsis occurred within the Golfingiaformes as a sister group to, or nested within, the Themistidae. Phylogenetic analyses, including 30 sequences from additional invertebrate taxa, suggest that sipunculans are most closely related to the Annelida (including Echiura). A previously proposed sipunculan-molluscan relationship is not supported. While not universally accepted, this hypothesis is consistent with other recent and past data on phylum-level relationships.

We have constructed molecular phylogenetic trees for members of the Sphaeriidae in order to test proposed generic level relationships, and to reconstruct the evolutionary pattern of parental care, in this exclusively freshwater family of heterodont bivalves. An ∼480 nucleotide fragment of the mitochondrial large ribosomal subunit (16s rDNA) was sequenced for 4 corbiculid outgroups in addition to 19 sphaeriid ingroup taxa. Ingroup species were obtained from North and South America, Europe, and Australasia and included representatives of the main sphaeriid genera. Our analyses support four primary conclusions: 1) the Sphaeriinae are robustly monophyletic with respect to Eupera platensis; 2) the genus Pisidium is paraphyletic and P. sterkianum is sister to the 17 other sphaeriine taxa in our dataset; 3) synchronous brooding is the ancestral reproductive pattern in the Sphaeriinae; 4) the sequential brooders form a clade in which Musculium taxa are monophyletic and nested among lineages of Sphueriunz. Our gene trees reveal an evolutionary progression in parental care complexity from the relatively simple pattern in the Euperinae, to the origin of brood sacs and of extraoogonial embryonic nutrition in the common ancestor of the Sphaeriinae, and ultimately to the development of sequential brooding in Sphaerium/Musculium taxa.

The sexual system of the semi-terrestrial shrimp Merguia rhizophorae is described, along with natural history observations on this unusual caridean. Individuals of M. rhizophorae in the Bocas del Toro Archipelago, Panama, were found occupying fossilized coral terraces in the upper and mid-intertidal zones, inhabiting caves and crevices, in and out of water. These fossilized coral terraces represent a new habitat for this species, which was previously reported only from mangrove swamps. Males, which made up 65% of the studied population, were smaller than females on average. No small juvenile females were observed, but transitional individuals having the characteristics of both males (gonopores) and females (ovaries) were observed in the population. These data suggest that individuals of M. rhizophorae are protandric hermaphrodites. Logistic regression indicated that the carapace length at which 50% of the individuals change sex is 4.89 mm. The abundance of shrimps at the study site was low. Shrimps were usually solitary, but occasionally observed in groups of ≤5 individuals. Shrimps were commonly observed walking while out of water, and in some cases, emerged shrimps jumped vigorously, presumably to avoid capture by the researcher or by predatory crabs. Additional studies on the reproductive biology and the behavioral ecology of members of this genus and of members of the closely related families Barbouridae and Lysmatidae will aid in understanding the evolutionary origin and the adaptive value of gender expression patterns in shrimps.

A growing body of evidence suggests that interleukin-1α (IL-1α) is present in invertebrates. Both invertebrate and human IL-1α can bind to invertebrate receptors and stimulate invertebrate immune functions. The present study shows that IL-1α increases reactive oxygen species (ROS) production by oyster immunocytes. However, physiological doses of noradrenaline (NA) exert a suppressive effect on IL-1α stimulation in vitro. The β-adrenoceptor agonist isoproterenol mimicked the effects of NA and the β-adrenoceptor antagonist propanolol blocked the NA-induced suppression of hemocyte responsiveness to IL-1α. The type IV phosphodiesterase inhibitor rolipram acted in synergy with isoproterenol to reduce hemocyte response to IL-1α and the protein kinase A inhibitor H-89 suppressed the effects of isoproterenol. These results suggest that circulating NA impairs IL-1α-stimulation of oyster hemocyte via a β-adrenoceptor/cyclic AMP/protein kinase-A signaling pathway. Considering that mollusc immunocytes secrete NA, an autocrine regulatory loop may also modulate the ability of these cells to respond to IL-1α.

The cytokine interleukin-1β (IL-1β) mediates interactions of immune and inflammatory cells in mammals. Previous reports also have linked plasma (cell-free hemolymph) levels of IL-1β in the snail Biomphalaria glabrata to resistance against Schistosoma mansoni. In the present study, fluorescent probes were used to study larval schistosome and snail hemocyte viability during in vitro encounters. Hemolymph (plasma and hemocytes) from schistosome-susceptible (M-line) and resistant (13–16-R1) B. glabrata was added to sporocysts of S. mansoni and the viability of hemocytes and parasites was assessed. Next, IL-1β was added to sporocyst-hemolymph samples, the viability of sporocysts and hemocytes determined and then compared to control assays. The number of live sporocysts present after incubation for 1 h with hemolymph from M-line snails was significantly greater than the number seen when hemolymph from 13–16-R1 snails was tested. Nearly all sporocysts survived the 1 h incubation with M-line hemolymph, and most of the hemocytes attached to sporocysts were dead. In contrast, nearly all sporocysts were dead when hemolymph from 13–16-R1 snails was tested, and most attached hemocytes were alive. Addition of IL-1β to M-line hemolymph resulted in a dramatic increase in sporocyst death. Addition of IL-1β to 13–16-R1 hemolymph produced a small but significant increase in the rate of sporocyst death. These results indicate that the concentration of IL-1β present in hemolymph from B. glabrata is directly related to the ability of this snail to kill S. mansoni sporocysts in vitro.

We examined the brain architecture in different species of Chaetognatha using immunofluorescence methods with a set of nervous system markers and confocal laser-scan microscopic analysis. These markers include antibodies against synaptic proteins, RFamide-related peptides, and tyrosinated tubulin, as well as a marker of cell nuclei. Furthermore, we present a 3D reconstruction based on histological section series. Our results expand the previous knowledge on neuroanatomy in Chaetognatha. We suggest a structural and functional subdivision of the rather complex chaetognath brain into two domains, a posterior domain that may be primarily involved in the integration of sensory input, and an anterior domain that may be involved in the control of the mouthparts and the anterior part of the digestive system. Immunolocalization of a neuropeptide suggests the presence of an identifiable group of neurons associated with the brain of all species examined here. However, our data also reveal a certain degree of interspecific variation and divergence within the Chaetognatha concerning, for example, the pattern of nerves branching off the brain and the proportional sizes of the various neuropil compartments. We compare our data to brain architecture in various other representatives of Protostomia and Deuterostomia. The chaetognath brain fits within the range of structural variation encountered in protostomian brains, and we cannot find any brain characteristics that would argue in favor of placing chaetognaths outside of the Protostomia. Rather, we see the circumoral arrangement of their cephalic nervous system as an argument that suggests protostome affinities.

Acochlidian gastropods combine several aberrant biological and morphological features. The poorly known Caribbean Tantulum elegans is one of the few opisthobranch species inhabiting a freshwater system, and the only one found in muddy interstices of a Caribbean mountain spring swamp. Morphological details of this tiny species were either unknown or not fully reliable, especially with regard to the complex central nervous and reproductive systems. We critically re-examined original paratype section series and prepared semi-thin serial sections of two additional paratypes. All organ systems were three-dimensionally reconstructed using AMIRA software. Our results show several discrepancies from the original description: the pharynx is a complex system of different muscles, but similar to that of other acochlidian species; the circulatory system shows a two-chambered heart; in the nervous system there are separate optic and rhinophoral ganglia, the latter innervating a pair of small sensory pits we assume to be Hancock's organs, and large aggregations of precerebral accessory ganglia were found. Nephropore, anus, and female gonopore open dextroventrally. To our surprise, adults of T. elegans are sequential hermaphrodites with an unusual androdiaulic reproductive system and a well-developed cephalic penial complex. In T. elegans, there is a mix of character conditions found in different genera, e.g., Pseudunela and Asperspina. The phylogenetic position of T. elegans still remains unclear.

The reproductive cycle of the entocommensal nemertean Malacobdella arrokeana inhabiting Panopea abbreviata from San José Gulf, Patagonia, Argentina, and the nemertean–host relationship were investigated. The mature nemertean population showed a sex ratio of 1:0.64, with females predominating. The number of individuals of M. arrokeana per host varied from 1–60 individuals (mean±SD=3.7±9.4 mm). Different stages of gonadic development were observed in individual nemerteans. Spawning peaks occurred during late summer and early spring. Sexually mature individuals of both sexes were present year round, possibly indicative of a continuous breeding period. Our results suggest that M. arrokeana has no cannibalistic habits and confirm its entocommensal symbiosis with P. abbreviata.

This is the first article of branchiobdellidans on blue crabs, Callinectes sapidus, in Chesapeake Bay, MD, USA. The ectosymbionts consisted of sympatric populations of Cambarincola mesochoreus and Cambarincola pamelae living on the ventral body surface and in the gill chambers of blue crabs. The occurrence of branchiobdellidans in the upper bay region was documented at ten sites during August and late September 2003. Branchiobdellidans were found on blue crabs following a rainy summer when salinity levels fell below 3 ppt. The blue crabs move into the freshwater estuaries during the summer, where they enter the habitats of crayfishes; however, the mechanism of transfer from one crustacean host to another is not known.

After 8–10 segments of posterior ventral nerve cord were ablated in Lumbriculus variegatus, touch-evoked locomotor responses were evident both in segments anterior and posterior to the ablation site. However, responses in these two regions were independent and uncoupled. During recovery, four outcomes were observed at the ablation site: (Group 1) recovery of normal functions with no growth of new segments; (Group 2) formation of a laterally protruding, multi-segmented, ectopic head; (Group 3) formation of a laterally protruding, amorphous, and multi-segmented outgrowth; and (Group 4) segmental autotomy. In Groups 1 and 2, touch-evoked swimming and body reversal were studied. In addition, sensory fields and conduction properties of giant nerve fibers were examined near the ablation site. In some Group 1 worms, clear-cut behavioral and electrical signs of recovery and reconnection were seen by 3 d after ablation. By 8 d, all worms had recovered and exhibited response patterns comparable to those of normal worms. In Group 2 worms, with an ectopic head, segments posterior to the ablation (together with those in the ectopic head), exhibited touch-evoked swimming and body reversal responses resembling those of a complete worm. Segments anterior to the ectopic head were independently capable of locomotor responses. Medial and lateral giant fiber sensory fields in worms with ectopic heads reflected a pattern expected for two worms. Thus, through apparent morphallactic reorganization, a medial giant fiber sensory field emerged which included the ectopic head and 10–15 adjacent posterior segments. In contrast, electrical recordings showed longitudinal through-conduction of giant fiber spikes, across the ablation site. Histological examination revealed that the giant nerve fibers in the ectopic head were complexly interconnected with those in the main body axis.

Here we report the effect of food concentration on the recovery from anhydrobiosis of a bdelloid rotifer, Macrotrachela quadricornifera. Cohorts were either starved, or fed high or low concentrations of food, before being dried and their subsequent recovery rates determined. The rotifers starved for 3 d before anhydrobiosis recovered in significantly higher proportion, and those fed lower food concentration recovered better than those fed higher food concentration. In addition, starvation did not decrease the recovery of other bdelloid species (Philodina roseola and Adineta sp. 1) which were either fed or starved before anhydrobiosis. These results suggest that a successful recovery from anhydrobiosis is not dependent on prior resource level supplied to the bdelloids. However, the lack of resources might not be the only factor in a successful recovery from anhydrobiosis. Observations using scanning electron microscopy of fed individuals of M. quadricornifera entering anhydrobiosis showed that some food remained in the digestive tract. Thus, we propose that the negative effect of rich food may be due to a purely mechanical effect and may be interfering with a proper folding of the rotifer body at the onset of anhydrobiosis. This contribution results from studies carried out in preparation for biological experiments scheduled on the International Space Station (ISS).

The development of the genital apparatus is described for the echinoid Paracentrotus lividus. This apparatus derives from the aboral ring, an annular structure that includes an inconspicuous coelom and, in juveniles, the germinal rachis. The germinal epithelium grows out from the germinal rachis, and the gonadal wall and coelom in early (tubular) gonads share similarities with their equivalents in the aboral ring. The original germinal rachis regresses to form a genital cord one cell wide in late juveniles. A genital cord was observed in a few field-collected adult individuals (>40 mm test diameter).

The corallimorpharian Rhodactis rhodostoma appears to be an opportunistic species capable of rapidly monopolizing patches of unoccupied shallow substrate on tropical reefs. On a fringing coral reef at Eilat, Israel, northern Red Sea, we examined patterns of abundance and clonal replication in R. rhodostoma in order to understand the modes and rates of spread of polyps across the reef flat. Polyps were abundant on the inner reef flat (maximum 1510 polyps m−2 and 69% cover), rare on the outer reef flat, and completely absent on the outer reef slope at >3 m depth. Individuals cloned throughout the year via 3 distinct modes: longitudinal fission, inverse budding, and marginal budding. Marginal budding is a replicative mode not previously described. Cloning mode varied significantly with polyp size. Approximately 9% of polyps cloned each month, leading to a clonal doubling time of about 1 year. The rate of cloning varied seasonally and depended on day length and seawater temperature, except for a brief reduction in cloning during midsummer when polyps spawned gametes. Polyps of R. rhodostoma appear to have replicated extensively following a catastrophic low-tide disturbance in 1970, and have become an alternate dominant to stony corals on parts of the reef flat.

In this study, we investigated a functional trade-off between trunk attachment and trunk-spine development in the acanthocephalan Corynosoma cetaceum. The worms live attached to the stomach and upper intestine of their cetacean definitive hosts, using the proboscis and spiny foretrunk as the main holdfast; the spiny hindtrunk can also attach by bending ventrally. When the hindtrunk bends, ventral compression generates an anterior fold (AF) and a posterior fold (PF). A morphological analysis based on 7,823 individuals collected from 10 franciscana dolphins, Pontoporia blainvillei, revealed that spines were smaller and more variable in size and occurrence in the folds than on neighboring areas; the growth of fold spines seemed to be inhibited to various degrees. Spines were more reduced in the AF than in the PF, and spines of both folds were more reduced in females than in males. Patterns of reduction appeared to be directly related to the intensity of fold compression associated with hindtrunk bending. Fold compression could induce plastic inhibition of spine growth, and/or could make fold spines maladaptive, spines being reduced by natural selection. Apparently, fold spines neither contact the substrate, nor are they exposed to the environment when the hindtrunk attaches. Therefore, fold spines could have reduced, or lost, their primary function, at least in the definitive host. The reduction and variability of spines in C. cetaceum seem to be unique among Corynosoma species.

Rotifera and Acanthocephala are generally regarded as separate phyla sharing a basal position among triploblast protostomes. This paper presents the first molecular phylogenetic examination of the relationship of Acanthocephala to all three rotifer classes, Seisonidea, Monogononta, and Bdelloidea. Inclusion of Acanthocephala within Rotifera, probably as a sister-taxon to a clade composed of Bdelloidea and Monogononta (the Eurotatoria), is strongly supported by both parsimony and distance methods, using a region of the nuclear coding gene hsp82. Previous molecular evidence for the inclusion of Acanthocephala in the Rotifera suggested that Acanthocephala is a sister-taxon of Bdelloidea, forming the clade Lemniscea. No support is found for this clade, and evidence is presented that the monogonont rotifer used in those analyses, Brachionus plicatilis, may be evolving in an anomalous manner.

The present study documents the pace of accessory gland and testes degeneration in the wasp Vespula vulgaris by means of a histological and metric approach, that has not been carried out for social wasps so far. To a certain extent, comparison is made with the degenerative processes of the mucus glands of the honeybee drone. In V. vulgaris, no generative tissue is left by the end of 9 d of age, and so degeneration is a fast process. The three different parts of the accessory glands (muscle layer, gland epithelium, and lumen) change with respect to age. The secretory cells of the epithelium reach their maximum activity during the first days of adult life, which results in a maximally filled gland lumen by 9 d. We also provide, for the first time, a histological study of testes degeneration for this species. At eclosion, well-defined cystic structures are still visible, whereas at 9 d, it is no longer possible to distinguish different cystic structures. The diameter of the testes decreases with respect to age.

Within many phylogenetic assemblages, a pattern of domination has been observed: one or a few clades have had many more speciation events or fewer extinctions than other clades in a particular assemblage. We investigated this phenomenon in the polychaete annelids. Polychaetes comprise ∼9000 described species classified in over 70 families and exhibit a great variety of life history strategies. Our goal was to test whether diverse polychaete families are characterized by species with short generation times, high reproductive output, small body size, or with planktotrophic larval development. Each of these factors has been advanced as cause for high diversity in other taxonomic assemblages. Here, we establish that the diversification pattern of polychaete families is non-random, but the data collected show no significant correlations between familial diversity and several life history traits including age at first reproduction, life span, body size, fecundity, and egg size. Pairwise comparisons of sister families do not reveal any trends between familial diversity and any of the life history traits. The great variability of life history traits within polychaete families may explain the lack of significant results; perhaps no trends are seen because polychaete life history traits cannot be generalized at the family level.

The lectin wheat-germ agglutinin (WGA) selectively binds N-acetyl-D-glucosamine. Fluorescence and electron microscopy were used to show that WGA stains the cytoplasmic granules in the granulocytes, but not the hyaline cells, of two decapods, the ridgeback prawn Sicyonia ingentis and the American lobster Homarus americanus. Using fluorescence microscopy, two intermediate stages in granulocyte maturation were observed. Cells smaller than typical small-granule hemocytes were observed with 5 or fewer granules, which in previous studies using brightfield and phase optics were probably counted as hyaline cells. Also, some granulocytes were observed containing both small and large granules, supporting the suggestion that small and large granule hemocytes represent stages in the maturation of one cell line. Granules in the single type of hemocyte in the branchiopod Artemia franciscana did not stain with WGA. The possible roles of N-acetyl-D-glucosamine in wound healing, pathogen encapsulation, and maintenance of normal crustacean connective tissues are discussed.

Light and scanning electron microscopy were used to examine protoconch form in eight species of planktotrophic heterobranch larvae, including four nudibranch species with a coiled (type 1) protoconch, two nudibranch species with an inflated (type 2) protoconch, and two cephalaspid species with a coiled protoconch. The coiled protoconchs of the cephalaspids and nudibranchs have a similar form at hatching, and shell growth up to metamorphic competence is hyperstrophic. Shell added to coiled protoconchs during the larval stage overgrows all but the left wall of the initial protoconch that exists at hatching. The entire protoconch of cephalaspids, including overgrown areas, is retained through metamorphosis. However, during later larval development in nudibranchs with a coiled protoconch, overgrown shell is completely removed by dissolution. As a result, regardless of whether nudibranch larvae hatch with an inflated or coiled protoconch type, the protoconch is a large, hollow cup at metamorphic competence. The protoconch of nudibranchs is shed at metamorphosis and absence of a post-metamorphic shell is correlated with absence of visceral coiling in this gastropod group. Internal dissolution of the coiled protoconch in nudibranchs allows the left digestive gland to uncoil prior to metamorphic shell loss. Retention of overgrown protoconch whorls in cephalaspids allows the attachment plaque of the pedal muscle to migrate onto the parietal lip of the post-metamorphic shell. Release from this constraint in nudibranchs, in which the larval pedal muscles and the entire protoconch are lost at metamorphosis, may have permitted internal protoconch dissolution and precocious uncoiling of the visceral mass, as well as evolutionary emergence of the inflated larval shell type.

This study is focused on the formation and function of sagittocysts, which are secretions typical of members of the acoel family Sagittiferidae. The needle-shaped sagittocysts are produced in specialized gland cells (sagittocytes) whose distal necks are often surrounded by muscle mantles. Contraction of the muscle mantle ejects the sagittocyst. We establish a model for the development of sagittocytes and muscle mantles out of the stem cell pool of the new acoel species Symsagittifera corsicae. We used various techniques, especially interference and phase-contrast microscopy of living specimens as well as labeling of the body-wall musculature, for species characterization. In addition to the morphological features, we provide the third complete sequence of the 18S rDNA gene in the family Sagittiferidae.

Examination of sexual reproduction in a symbiotic acoelomorph worm, Waminoa brickneri from Eilat (Red Sea), presents the first definitive evidence for maternal transmission of dinoflagellate algal symbionts in a triploblastic organism. Sexually mature worms were removed from the stony coral Plesiastrea laxa and raised in the laboratory. Eggs were detected 18 d after the collection of the worms and hatched 4 d later. Histological sections performed on sexually mature worms showed an ovary with oocytes containing two distinct types of algal endosymbionts within their ooplasm. Transmission electron microscopy corroborated the presence of algal symbionts within the developing embryos. Our findings regarding maternal transmission of symbionts shed new light on the diversity of modes of algal symbiont acquisition known in triploblastic organisms.

The subfamily Ambleminae is the most diverse subfamily of fresh-water mussels (order Unionoida), a globally diverse and ecologically prominent group of bivalves. About 250 amblemine species occur in North America; however, this diversity is highly imperiled, with the majority of species at risk. Assessing and protecting this diversity has been hampered by the uncertain systematics of this group. This study sought to provide an improved phylogenetic framework for the Ambleminae. Currently, 37 North American genera are recognized in Ambleminae. Previous phylogenetic studies of amblemines highlighted the need for more extensive sampling due to the uncertainties arising from polyphyly of many currently recognized taxa. The present study incorporated all amblemine genera occurring in North America north of the Rio Grande, with multiple species of most genera, including the type species for all but seven genera. A total of 192 new DNA sequences were obtained for three mitochondrial gene regions: COI, 16S, and ND1. In combination with published data, this produced a data matrix incorporating 357 gene sequences for 143 operational taxonomic units, representing 107 currently recognized species. Inclusion of published data provides additional taxa and a summary of present molecular evidence on amblemine phylogeny, if at the cost of increasing the amount of missing data. Parsimony and Bayesian analyses suggest that most amblemine genera, as currently defined, are polyphyletic. At higher taxonomic levels, the tribes Quadrulini, Lampsilini, and Pleurobemini were supported; the extent of Amblemini and the relationships of some genera previously assigned to that tribe remain unclear. The eastern North American amblemines appear monophyletic. Gonidea and some Eurasian taxa place as probable sister taxa for the eastern North American Ambleminae. The results also highlight problematic taxa of particular interest for further work.

Serpulidae encompasses polychaete species whose members have fused anterior ends bearing a tentacular crown, a heteronomous segmented body with a thorax and abdomen, and “chaetal inversion” between the two tagmata. The sessile filter-feeding organisms live in self-built, coiled, calcareous tubes on algae. The F-actin muscular subset of Spirorbis cf. spirorbis was stained with phalloidin and three-dimensionally reconstructed by means of cLSM, aiming to investigate (1) how the tentacular crown is organized and moved, (2) whether the internal structures, e.g., musculature, follow the thorax–abdomen inversion, and (3) whether circular muscles are present in serpulids. The third aim is by reason of recent investigations suggesting that lack of circular muscle fibers may be a common situation rather than a rare variation in polychaetes. In this manner, this article is part of a comparative evaluation of polychaete muscle systems. We found that longitudinal muscles of the body wall project into the tentacular crown, and that radioli and pinnulae possess three muscle types each, facilitating their great mobility. Operculum, collar, and a pair of unidentified organs possess distinct F-actin filaments. The trunk is mainly moved by five longitudinal muscle strands, most obvious in the abdomen: two dorsal, two ventral, and an unpaired ventromedian one, out of which the dorsal ones are the strongest. In anterior regions, the two dorsal strands form a single continuous layer; the separated strands lessen posteriorly. Solitary transverse fibers are located ventrally in the middle of each segment, stretching between longitudinal muscles and coelomic lining laterally, where they end. Peripheral and central dorsoventral muscles, two pairs per segment each, are present. Circular fibers as well as bracing muscles were not detected. The results indicate that the musculature does not follow the thorax–abdomen inversion and Serpulidae represents the 15th polychaete taxon in which circular fibers are totally missing.