Studies of regeneration provide insight across many scales of animal biology from the processes of cellular communication to the ecology of whole populations. Sponges are highly regenerative animals, with studies showing adults can both recover large portions of their body after predation or damage due to storms, and even reform whole individuals, via an aggregation stage, from dissociated tissues. While sponges are clearly highly regenerative, few studies actually show dissociated cells forming functional individuals. As sponges often serve as model organisms for studying the development and function of traits in metazoans, determining the universality and mechanics of their regeneration potential is important. We tested the capacity of members of seven sponge species from temperate freshwater and marine environments, from a range of taxonomic positions, and with different habits, to form functional sponges after dissociation. Development to a functional sponge progressed through a series of checkpoints: the sorting of cells and removal of debris; adhesion to a substrate and differentiation of cells; organization of cells into tissues; and regionalization of tissues. Two of the seven species tested, Spongilla lacustris and Haliclona cf. permollis, progressed through all four checkpoints, while the remaining five species progressed to various levels of development before aggregates disintegrated. Our findings highlight three important conclusions: (1) The ability of aggregates to differentiate into functional sponges is not as widespread as previously thought; (2) The species-specific ability of aggregates to develop to functional sponges appears to be an adaptive trait; and (3) The progression of development in aggregates through checkpoints, which in later development involves formation of tissues and regionalization of tissues, highlights the complexity of the sponge body plan and suggests fundamental rules in development shared across metazoans.

The budding process of the tetillid sponge Cinachyrella cavernosa was studied for one year in the low intertidal zone near Mhapan (15°55′27.48″N, 73°33′29.89″E), on the central west coast of India. The sponges showed the highest budding frequency when the average water temperature of intertidal rock pools was 32.4±0.23°C (February–March), followed by a significant decrease in budding frequency at 28.2±0.12°C (April–July), and no budding at ≤25.9±0.12°C (August–November). Stepwise multiple regression analysis of physico-chemical factors revealed temperature as the most prominent factor regulating the intensity of budding. Based on size and morphology, three stages of sponge buds were defined. The production of buds was found to be asynchronous, as adult sponges possessed buds of all three stages. Differences among these stages were examined at ultrastructural (in terms of spicules) and molecular (in terms of RNA/DNA) levels. Stage I (<0.5 mm diameter) buds showed a complete absence of microscleres (sigmaspires), whereas stage II (0.5–1 mm) and stage III (>1 mm) buds contained all spicules characteristic of the adult sponge. There was a significant increase in RNA/DNA ratio from stage I to III, suggestive of a progressive increase in physiological activity during the developmental process. Additionally, we studied post-settlement bud growth under field and laboratory conditions. Newly settled buds displayed a lower average-specific growth rate in the field, owing to variability in environmental conditions, but more rapid growth under controlled conditions in the laboratory. This study highlights the role of abiotic factors in regulating the budding process and stresses the ecological significance of budding in maintaining natural sponge populations. Our data suggest that an increased frequency of budding under stressful conditions, such as high temperature, is an advantageous adaptation for these sponges. Buds showed rapid development, as no metamorphosis is involved, and retained the genotype of the parents, yielding high reproductive outputs and survival rates.

Free-spawning species of chitons produce eggs enclosed in a coating known as the hull. In Chitonida, several studies have shown that the hull helps to direct sperm to specific areas of the egg surface, facilitating fertilization. One study has found evidence that this structure also serves to reduce the sinking rates of the eggs. To clarify how the presence of the hull modifies sinking rates in chiton eggs, here we compare sinking speeds and densities of eggs of Mopalia kennerleyi with and without the hull. Sinking rates of eggs with the hull were approximately one-third of those without it. This structure acts as a flotation device because it has a density very close to that of seawater, and it increases the effective diameter and therefore the drag on the negatively buoyant egg. Since there is limited knowledge about morphology and behavior of chiton larvae, we also analyzed changes during ontogeny in behavior, swimming speeds, and body shape of larvae of M. kennerleyi. Over time, the larvae decreased their upward swimming tendency and preferred to stay near the bottom, and their bodies became elongated and dorso-ventrally compressed. These changes may be related to preparation for settlement and metamorphosis. Further studies of these subjects are required in chitons, since movement of early stages, as eggs/embryos sinking or larvae swimming in the water column, may affect their survival.

Laboratory experiments showed that the mussel Mytilus edulis aggregated more intensely around living organisms (the bivalve Hiatella arctica and the solitary ascidian Styela rustica, which commonly co-occur with mussels in fouling communities) than around inanimate objects. When exposed to an inanimate object, mussels attached their byssal threads primarily to the substrate, close to the object, but when exposed to a living organism, they attached their byssal threads directly to the organism. The ascidian was more intensely covered with byssal threads than was the bivalve. Mussel attachment to the ascidians was apparently determined by the physical characteristics of the tunic and to a lesser extent by the excretion-secretion products released by S. rustica. This study indicates that mussels can use byssus threads as a means of entrapment of potential competitors for space. It remains unclear why mussels preferentially attached to ascidians compared to the bivalve. This can be explained either by competitive interactions, or by attractiveness of the ascidian tunic as an attachment substratum.

Members of only a few species of annelids are reported as being incapable of regeneration; of these, Myxicola infundibulum is the only example in the family Sabellidae. Interestingly, its congener Myxicola aesthetica exhibits noteworthy regenerative ability. Unambiguously identifying non-regenerating species is critical to reconstructing how regenerative abilities evolved within the phylum. However, studies designed specifically to assess the regenerative potential of M. infundibulum have never been performed. In this study, we aimed to confirm the lack of regeneration ability of M. infundibulum, reported previously for Atlantic specimens, or to determine the extent to which regeneration occurs. Our results showed that individuals from the Mediterranean Sea (Adriatic Sea) do undergo regeneration of lost body parts, although to a lesser extent than do other sabellids. Therefore, M. infundibulum should no longer be considered a non-regenerating species. At present, uncertainties regarding phylogenetic relationships of Sabellidae prevent inferences about the polarity of change in M. infundibulum. Since our findings are counter to those of previous studies which describe Atlantic specimens as non-regenerating, more extensive analysis is required to ascertain if they could actually belong to a different species than Mediterranean M. infundibulum, accounting for these differences in reported regenerative capacity.

By controlling the traction between its body and the tube wall, a tube-dwelling polychaete can move efficiently from one end of its tube to the other, brace its body during normal functions (e.g., ventilation and feeding), and anchor within its tube avoiding removal by predators. To examine the potential physical interaction between worms and the tubes they live in, scanning electron microscopy was used to reveal and quantify the morphology of worm bodies and the tubes they produce for species representing 13 families of tube-dwelling polychaetes. In the tubes of most species there were macroscopic or nearly macroscopic (∼10µm–1 mm) bumps or ridges that protruded slightly into the lumen of the tube; these could provide purchase as a worm moves or anchors. At this scale (∼10 µm-1 mm), the surfaces of the chaetal heads that interact with the tube wall were typically small enough to fit within spaces between these bumps (created by the inward projection of exogenous materials incorporated into the tube wall) or ridges (made by secretions on the interior surface of the tube). At a finer scale (0.01–10 µm), there was a second overlap in size, usually between the dentition on the surfaces of chaetae that interact with the tube walls and the texture provided by the secreted strands or microscopic inclusions of the inner linings. These linings had a surprising diversity of micro-textures. The most common micro-texture was a “fabric” of secreted threads, but there were also orderly micro-ridges, wrinkles, and rugose surfaces provided by microorganisms incorporated into the inner tube lining. Understanding the fine structures of tubes in conjunction with the morphologies of the worms that build them gives insight into how tubes are constructed and how worms live within them.

In northern North America, invasive earthworms (including the nightcrawler Lumbricus terrestris) have been dispersing from points of introduction and dramatically affecting soil structure, soil food webs, and forest floor dynamics. However, little is known about the factors influencing the local distribution of invasive earthworms south of the Wisconsinan glaciation. Earthworms were sampled at suspected sites of introduction near Mountain Lake Biological Field Station, Virginia, USA. The density of invasive earthworms decreased as distance from suspected sites of introduction increased; native earthworms displayed the opposite relationship. However, the distance that L. terrestris was found into the forest was less than expected given dispersal rates calculated from more northern invasions. We also found correlations among population densities of L. terrestris and physical–chemical properties of the soil, and differences between field and forest soils in terms of temperature, moisture, and soil chemical properties. We conducted two experiments to analyze some factors possibly responsible for the observed distribution: (1) temperature and moisture, and (2) soil type (field vs. forest) and food resources. Our results suggest that L. terrestris may not disperse as far into forested habitats of the Southern Appalachians compared to northern forests due to local physical-chemical soil characteristics.

Bioactive secondary metabolites are common components of marine animals. In many cases, symbiotic bacteria, and not the animals themselves, synthesize the compounds. Among marine animals, ascidians are good models for understanding these symbioses. Ascidians often contain potently bioactive secondary metabolites as their major extractable components. Strong evidence shows that ∼8% of the known secondary metabolites from ascidians are made by symbiotic bacteria, and indirect evidence implicates bacteria in the synthesis of many more. Far from being “secondary” to the animals, secondary metabolites are essential components of the interaction between host animals and their symbiotic bacteria. These interactions have complex underlying biology, but the chemistry is clearly ascidian species-specific. The chemical interactions are ancient in at least some cases, and they are widespread among ascidians. Ascidians maintain secondary metabolic symbioses with bacteria that are phylogenetically diverse, indicating convergent solutions to obtaining secondary metabolites and reinforcing the importance of secondary metabolism in animal survival.