Muddy sediments are elastic solids through which morphologically diverse animals extend burrows by fracture. Muddy sediments inhabited by burrowing infauna vary considerably in mechanical properties, however, and at high enough porosities, muds can be fluidized. In this study, we examined burrowing behaviors and mechanisms of burrow extension for three morphologically diverse polychaete species inhabiting soft muddy sediments. Worms burrowed in gelatin, a transparent analog for muddy sediments, and in natural sediments in a novel viewing box enabling visualization of behaviors and sediment responses. Individuals of Scalibregma inflatum and Sternaspis scutata can extend burrows by fracture, but both also extended burrows by plastic deformation and by combinations of fracture and plastic deformation. Mechanical responses of sediments corresponded to different burrowing behaviors in Scalibregma; direct peristalsis was used to extend burrows by fracture or a combination of plastic deformation and fracture, whereas a retrograde expansive peristaltic wave extended burrows by plastic deformation. Burrowing speeds differed between behaviors and sediment mechanical responses, with slower burrowing associated with plastic deformation. Sternaspis exhibited less variability in behavior and burrowing speed but did extend burrows by different mechanisms consistent with observations of Scalibregma. Individuals of Ophelina acuminata did not extend burrows by fracture; rather individuals plastically deformed sediments similarly to individuals of the related Armandia brevis. Our results extend the range of natural sediments in which burrowing by fracture has been observed, but the dependence of burrow extension mechanism on species, burrowing behavior, and burrowing speed highlights the need for better understanding of mechanical responses of sediments to burrowers.

Ernst Mayr suggested that understanding the features of organisms involves the study of what he called “proximate” and “ultimate” causes. Proximate causes of characters occur during the life of the organism; ultimate causes occur prior to the life of the organism, as part of the evolutionary history leading to organisms in the present. Mayr also pointed out that descriptive biology is important because it provides the basis for pursuing understanding by way of proximate and ultimate causes. Systematics encompasses each of these components. We routinely describe organisms (not taxa), and often infer proximate hypotheses (e.g., ontogenetic, functional, and physiological causes) to explain particular characters. Characters are also explained by ultimate causes in the form of intraspecific, specific, and phylogenetic hypotheses. Success at attaining ultimate understanding is distinctly limited, however, since these hypotheses are rarely tested. The consequence is that we often use specific and phylogenetic hypotheses to move toward the study of descriptive aspects and proximate understanding, where increases in causal understanding are more successful. An example of interactions between descriptive, proximate, and ultimate understanding are presented from research on the sabelliform groups Fabriciidae and Sabellidae. Members of both families have what is interpreted as two peristomial rings, anterior and posterior. A distinct mid-ventral patch of cilia occurs on the anterior peristomial ring among Fabriciidae, and the posterior peristomial ring among Sabellidae. Phylogenetic hypotheses suggest these different cilia patches are synapomorphies for the respective families. Since these phylogenetic hypotheses cannot be tested, it will be more productive to investigate whether or not these patches of cilia are homologous, which might be established through ontogenetic and histological studies of proximate causes: are these cilia derived from the larval prototroch or metatroch? Are anterior and posterior peristomial rings really peristomial? Studies directed at these proximate causes offer valuable increased understanding not possible for ultimate causes in systematics.

Phylogeographic studies are useful in reconstructing the history of species invasions, and in some instances can elucidate cryptic diversity of invading taxa. This can help in predicting or managing the spread of invasive species. Among terrestrial invasive species in North America, earthworms can have profound ecological effects. We are familiar with the centuries-old invasions of European earthworms (Lumbricidae) and their impacts on nutrient cycling in soils. More recent invasions by Asian earthworms of the family Megascolecidae are less fully understood. We used data for two mitochondrial gene fragments, cytochrome oxidase I (COI) and 16S rRNA, to examine the relationships among populations of Asian earthworms in the megascolecid genus Amynthas in the northeast United States. Recent reports have indicated that one species in particular, Amynthas agrestis, is having detrimental effects in mixed forest ecosystems, and we were interested in understanding the invasion history for this species. We were surprised to discover three divergent mitochondrial lineages of Amynthas occurring sympatrically in upstate New York. Given the gap between intra- and inter-lineage sequence divergences, we propose that these three lineages represent cryptic species of Amynthas, one of which is A. agrestis. For each of the three lineages of Amynthas, we observed shared haplotypes across broad geographic distances. This may reflect common origins for populations in each lineage, either by direct routes from native ranges or through post-introduction spread by natural dispersal or human-mediated transport within North America. Management efforts focused on horticultural imports from Asia, commercial nurseries within the USA, and on prohibition of bait disposal may help to reduce the further invasion success of Amynthas.

The genus Marphysa is widely collected for bait by recreational fishermen in Australia, and yet the number of species remains unresolved. A new species is described together with additional information on a previously described species. This study reveals that while species of Marphysa superficially resemble one another, detailed studies on the distribution of different types of chaetae along the body, together with molecular studies, uncover species which may co-occur. Correct identification is critical for management of these largely intertidal species. This study also highlights the increasing awareness of the presence of pseudo-cryptic species in polychaetes.

The ecosystem engineer onuphid polychaete Diopatra biscayensis has a continuous population in the Bay of Biscay from the Cantabria coast in Spain to southern Brittany in France. A group of disjunct populations also are found in the English Channel, separated from the Biscay population by more than 400 coastal kilometers. It remains unclear whether D. biscayensis is native to the Bay of Biscay; it is also debated whether the disjunct populations in the English Channel are relics of a formerly continuous population, or the product of recent introductions through aquaculture. Here, we use climate hindcasts to explore hypotheses about the D. biscayensis historical distribution in Europe. If D. biscayensis is native, its range would have been restricted to southern Iberia and the Mediterranean during the Last Glacial Maximum (21,000 BP). However, the species is completely absent from both regions today, further supporting its interpretation as a non-native species. If it was historically present in Europe, the climate hindcasts are congruent with range contraction in the Last Glacial Maximum (21,000 BP), expansion in the Mid-Holocene Warm Period (6000 BP), and contraction again in the past 1000 years (850–1850), prior to the first reports of D. biscayensis on the Spanish and French Atlantic coasts. However, the simulations do not support there being climatic refugia along the English Channel coast that would account for the existence of relic populations. Taken together, the evidence suggests that D. biscayensis has been introduced to the Bay of Biscay, and that disjunct populations in the English Channel are the result of recent transport through human activities, perhaps aquaculture.

The objective of this study was to examine the regeneration capacity of the spionid polychaete Marenzelleria viridis from Long Island, New York. In the field, ∼7% of the worms exhibited regeneration of the anterior end. In the laboratory, worms were ablated at the 10th–50th chaetiger and their regeneration documented. Anterior morphogenesis was similar to that previously reported for spionids, with wound healing, blastema formation, differentiation of segments, and formation of feeding and sensory structures (mouth, palps, nuchal organs) occurring within 14 d. Unlike in some spionids, the segments do not appear to all form simultaneously from the blastema; rather, external differentiation of segments was observed from posterior to anterior on the regenerate. The number of segments replaced was equal to the number ablated for up to 10 segments. A maximum of 17 segments were replaced when 20–30 chaetigers were ablated, and the number replaced decreased to 14 when 40–50 chaetigers were ablated. Survival and normal growth of the worms decreased with more chaetigers ablated; a significantly higher number of worms died or grew abnormally with ≥30 chaetigers ablated, compared to worms with ≤20 chaetigers ablated. Members of M. viridis could be valuable model organisms in the study of the cellular mechanisms involved in regeneration, and further research on regeneration in the field should be completed.

Pygospio elegans is an opportunistic, wide-spread spionid polychaete that reproduces asexually via fragmentation and can produce benthic and pelagic larvae, hence combining different developmental modes in one species. We documented the density, size distribution, and reproductive activity of P. elegans at four sites in the Danish Isefjord-Roskilde Fjord estuary complex, where all modes of reproduction were reported. We compared population dynamics of this species to environmental parameters such as salinity, temperature, and sediment characteristics (grain size, sorting, porosity, water content, organic content, C/N). We observed that new cohorts—resulting either from sexual or asexual reproduction—appeared in spring and fall, and old ones disappeared in late summer and winter. Sexual reproduction occurred from September until May, and although their timing was variable, there were two reproductive peaks at three sites. At those sites, we also observed a switch in larval developmental mode. Asexual reproduction peaked in April. While the seasonal dynamics can be related to temperature to a large extent, the differences in population dynamics among sites also correlated with sediment structure and salinity. Populations from sites with coarse and heterogeneous sediment had high levels of sexual reproduction. At the site with lower salinity, intermediate and benthic larvae were present during winter in contrast to pelagic larvae found at the other sites. However, we could not identify one clear environmental factor determining the mode of development. At present, it remains unclear to what degree genetic background contributes to mode of development. Hence, whether the differences in developmental mode are the result of genetically different cohorts will be further investigated.

Variations in DNA ploidy have been observed in Lumbriculus, a freshwater annelid, as well as in other clitellates. Interpretation and application of experimental results using these animals may be impacted as ploidy levels affect the protein expression, reproductive behavior, and response to stressors. Ploidy is typically determined by chromosome spreads, a time-consuming and inefficient method. We adapted flow cytometry protocols used on vertebrates and plants to determine the ploidy levels in different populations of Lumbriculus, including a laboratory strain (Environmental Protection Agency), a commercial strain (Aquatic Foods), and worms collected from natural habitats. To isolate nuclei, worms were homogenized, filtered to remove cell debris, and centrifuged through Optiprep^TM density gradients. Nuclei were recovered, treated with RNAse, and stained with propidium iodide. Flow cytometry of the labeled nuclei showed that Lumbriculus from natural habitats in Minnesota and Iowa were diploid, with an estimated genome size of 2.7 pg. Populations from natural habitats in California and Oregon were highly polyploid, as were the laboratory and commercial strains. Chromosome spreads verified the high ploidy levels indicated by flow cytometry results, but also suggested that flow cytometry may be underestimating the DNA content levels. Staining of nuclei with diamidino-2-phenylindole indicated that this may be due to high levels of heterochromatin in nuclei from polyploid forms of Lumbriculus. To further compare the populations, proteins in worm homogenates were subjected to isoelectrofocusing gel electrophoresis. Distinct protein profiles were seen; one was shared in common by the diploid worms, the other was characteristic of polyploid populations. Diploid worms could also be distinguished from polyploid worms based on differences in hemoglobin linker proteins. The results further support taxonomic classification of the diploid and polyploid forms of Lumbriculus as distinct species.

Regeneration, the ability to replace lost body structures, and agametic asexual reproduction, such as fission and budding, are post-embryonic developmental capabilities widely distributed yet highly variable across animals. Regeneration capabilities vary dramatically both within and across phyla, but the evolution of regeneration ability has rarely been reconstructed in an explicitly phylogenetic context. Agametic reproduction appears strongly associated with high regenerative abilities, and there are also extensive developmental similarities between these two processes, suggesting that the two are evolutionarily related. However, the directionality leading to this relationship remains unclear: while it has been proposed that regeneration precedes asexual reproduction, the reverse hypothesis has also been put forward. Here, we use phylogenetically explicit methods to reconstruct broad patterns of regeneration evolution and formally test these hypotheses about the evolution of fission in the phylum Annelida (segmented worms). We compiled from the literature a large dataset of information on anterior regeneration, posterior regeneration, and fission abilities for 401 species and mapped this information onto a phylogenetic tree based on recent molecular studies. We used Markovian maximum likelihood and Bayesian MCMC methods to evaluate different models for the evolution of regeneration and fission and to estimate the likelihood of each of these traits being present at each node of the tree. Our results strongly support anterior and posterior regeneration ability being present at the basal node of the annelid tree and being lost 18 and 5 times, respectively, but never regained. By contrast, the ability to fission is reconstructed as being absent at the basal node and being gained at least 19 times, with several possible losses. Models assuming independent evolution of regeneration and fission yield significantly lower likelihoods. Our findings suggest that anterior and posterior regeneration are ancestral for Annelida and are consistent with the hypothesis that regenerative ability is required to evolve fission.

Capitella capitata was traditionally used as a biological indicator species due to its ubiquitous distribution and high densities in disturbed and polluted marine and estuarine sediments. Based on allozyme and developmental studies, it is now clear that C. capitata is a species complex consisting of multiple distinct lineages worldwide, including the recently described C. teleta, a model species for spiralian development. The coast of the northern Gulf of Mexico, with its numerous bays and estuaries and frequently occurring natural and anthropogenic disturbances, provides an appropriate region for such studies. We sequenced a fragment of the mitochondrial cytochrome c oxidase subunit I gene for individuals of C. cf. capitata and C. cf. aciculata (distinguished by acicular spines on the first two chaetigers) collected from Texas and Florida coasts and analyzed them in conjunction with data available in GenBank. Our results indicate the presence of a Gulf of Mexico clade that is distinct from populations in Canada and the Indo-Pacific. Populations in the northern Gulf of Mexico are structured geographically, with support for Texas and Florida clades, and there do not seem to be clear boundaries between C. cf. capitata and C. cf. aciculata. This is corroborated by the fact that multiple specimens were morphologically intermediate between the two species. In future studies, we aim to clarify whether the intermediate morphologies represent ontogenetic stages, neutral morphological variation, phenotypic plasticity, or sexual dimorphism in a single species or whether several lineages with incomplete reproductive barriers are present.

Sensory organs in Annelida are very diverse and may be useful for assessments of morphological adaptation and character evolution. We used several methods to provide new insights into processes underlying the evolutionary radiation of anterior sensory organs in Sabellariidae. The presence and morphological diversity of the median organ (MO) found in the group was reviewed in order to test its phylogenetic significance and possible relationships to the distribution and ecological traits of the lineages. To test the intraspecific phenotypic plasticity of the MO, molecular analyses were conducted that focused on mitochondrial and nuclear genes from populations of Idanthyrsus australiensis exhibiting variation in the morphology of the MO. We used an integrative microscopical study of the ontogeny of Sabellaria alveolata to describe the anterior sensory structures present in the larvae and the morphological changes occurring before, during, and after settlement. In larval stages, the palps and the dorsal hump (DH) exhibit distinct innervation. The larval DH organ, which is likely to play a major role in chemoreception for settlement, is interpreted as being the incipient form of the adult MO. These results suggest that annelid sensory organs including the MO may be useful for phylogenetic and developmental investigations.