Giulio Melone

Disentangling the morphological stasis in two rotifer species of the Brachionus plicatilis species complex

The taxonomic uncertainty surrounding cryptic species complexes has traditionally been resolved using lengthy experimental approaches, while, since the advent of PCR based techniques the number of cryptic species described in a variety of taxa is increasing steadily. Here we formally describe a new rotifer species of the Brachionus plicatilis complex: Brachionus manjavacas n.sp., disentangling what was known as a morphological stasis. Detailed morphological analyses demonstrated significant differences in body shape and size between B. manjavacas and B. plicatilis s.s., analysed by geometric morphometrics; unfortunately these statistical differences are not taxonomically reliable because of wide overlaps. Size and asymmetry of masticatory apparatus, named trophi, observed by SEM, gave similar results, with taxonomic ambiguity. Only the shape of small pieces of the trophi, named satellites, were consistently different between the species. On a strictly classical taxonomical basis it is absolutely useful to name new species on morphological bases, as we did, and to assess their status as distinct entities. Nevertheless, the two species are broadly similar; therefore, we do not suggest using the small differences in shape of satellites of trophi to identify the species for further ecological studies, but to continue discriminating them on genetic marker bases.

Rotatory apparatus in Bdelloids

All Bdelloid Rotifers have the same body plan: elongated body, ciliated apical region (rotatory apparatus or corona), telescopically retractable foot and head with pseudosegments, paired gonads, single dorsal antenna, apical rostrum, ramate mastax. Bdelloids use the rotatory apparatus for both locomotion and collecting food and therefore the shape of the corona and arrangement of the cilia, both related to the animals life style, probably are important for the fitness of the rotifers. We have analyzed the fine morphology of the corona and the rostrum from species belonging to the four families, Habrotrochidae, Philodinidae, Adinetidae and Philodinavidae, each with its own form of feeding and locomotion. In the rostrum one can distinguish a sensorial area and a ciliated area. The former is common to all bdelloids, while the latter is lacking in the Adinetidae. Three models of corona can be recognized: 1) a simple ciliated field of undifferentiated cilia (Adinetidae), 2) a well developed ciliated field with specialized cilia forming the paired trochi on the disks and the cingulum (Habrotrochidae and Philodinidae), and 3) a ciliated field with a single trochus encompassing rudimentary pedicels and cingulum (Abrochtha). We propose (1) to no longer use Digononta as a taxon, (2) to assign the class rank to Bdelloidea, (3) to distinguish three orders, grouping Philodinidae and Habrotrochidae under a single order and (4) to retain the current families.

Key to the identification of the genera of bdelloid rotifers

We propose a simple, user-friendly key joined to a pictorial key to the bdelloid genera, in the attempt to make bdelloid identification more accessible to non-bdelloid-specialists. The key is mostly based on illustrations rather than on descriptions, and is accompanied by an introduction of the main features readily observable in active bdelloids.

Phylogenetic relationships of phylum Rotifera with emphasis on the families of Bdelloidea

We investigated phylogenetic relationships of phylum Rotifera using cladistic analysis to uncover all most-parsimonious trees from a data set comprising 60 morphological characters of nine taxa: one Acanthocephala, six Rotifera, and two outgroups (Turbellaria, Gnathostomulida). Analysis of our matrix yielded a single most-parsimonious tree. From our analysis we conclude the following: (1) Class Digononta is paraphyletic; (2) it is still premature to reject rotiferan monophyly; (3) the classification hierarchy that best conforms to this morphologically based, cladistic analysis is similar to several traditional schemes. In spite of these results, it is significant that this analysis yielded a tree that is incongruent with those trees developed from molecular data or by using the principles of evolutionary taxonomy.

External Morphology and Muscle Arrangement of Brachionus urceolaris, Floscularia ringens, Hexarthra mira and Notommata glyphura (Rotifera, Monogononta)

We studied four monogonont rotifers (Brachionus urceolaris, Floscularia ringens, Hexarthra mira, Notommata glyphura) using two different techniques of microscopy: (1) the presence of filamentous actin was examined using phalloidin-fluorescent labelled specimens and a confocal laser scanning microscope (CLSM); (2) external morphology was investigated using a scanning electron microscope (SEM). B. urceolaris, F. ringens, and N. glyphura showed similar patterns of muscle distribution: a set of longitudinal muscles acting as head and foot retractors, and a set of circular muscles. However, the size and distribution of circular muscles differed among these species. H. mira differed from the other species in that it lacked circular muscles but possessed strong muscles that extended into each arm. The study showed that using both CLSM and SEM provides better resolution of the anatomy and external morphology of rotifers than using one of these techniques alone. This can facilitate better understanding of the complicated anatomy of these animals.

Connectivity and nestedness of the meta-community structure of moss dwelling bdelloid rotifers along a stream

We analyzed meta-community structure of bdelloid rotifers colonizing mosses along an 80 meter section of Rio Valnava in NW Italy. Bdelloid rotifers are small animals living associated with a substratum; colonization in bdelloids can be produced by active animals moving along the riverbed, or by passive dormant propagules, moved by wind. To detect which kind of colonization might be stronger at different spatial scales, we designed a spatially nested sampling experiment at three hierarchical levels: (1) single sample, (2) 10 communities inside each pool, (3) complete section of 10 pools. Assessing species richness and species similarity of communities, and coherence and nestedness of bdelloid meta-communities, we found that different forces may drive species composition at different spatial scales: at the largest scale, colonization of propagules may over-ride direct dispersal between pools, while at the scale of the single pool, differential movements of species give a nested structure to the meta-communities. The number of species increased as the level of analysis increased, even though this study was carried out along only a small stream section.

Shape diversity in the trophi of different species of Rotaria (Rotifera, Bdelloidea): A geometric morphometric study

Abstract Rotifers have articulated hard jaws called “trophi”;, whose shapes are considered species‐specific for monogonont rotifers but not for bdelloid rotifers. We present a geometric morphometric analysis of size and shape of bdelloid jaws, carried out on SEM pictures of trophi of nine morphospecies belonging to the genus Rotaria, using six landmarks. The study shows that trophi can also have species‐specific form in bdelloids and suggests future directions for the analysis of the relationship between shape, size and diet in modelling trophi morphology during the evolution of Rotaría.

Adhesive Papillae of Phallusia mamillata Larvae: Morphology and Innervation

Abstract The swimming larvae of most solitary ascidians belonging to the Ascidiidae family bear three anterior, simple conic adhesive papillae. They secrete adhesive substances that are used to effect transitory settlement at the beginning of the metamorphosis. The adhesive papillae of newly hatched Phallusia mamillata larvae examined by the SEM are covered by the tunic. When the larvae are about to settle, the tunic becomes fenestrated over the central part of the papilla and bulb-ended microvilli protrude through the holes. These papillae have two types of elongated cells: many peripheral cells and few larger central cells with microvilli and bundles of microtubules oriented along the major axis of the cells. We have done immunofluorescence experiments with an anti-β-tubulin monoclonal antibody (clone 2-28-33) reacting with axonal microtubules. Only the central cells of the papillae were stained and the axons appeared to arise from the proximal ends of these cells. These axons form a long nerve that reaches the brain vesicle. Branches of the same nerve appear to connect to the basal ends of the peripheral cells. By confocal laser microscopy we were able to follow the course of the papillary nerve. The two nerves connecting the dorsal papillae fuse together into a single nerve that runs posteriorly. The nerve connecting the ventral papilla runs posteriorly for a long tract before fusing with the nerve of the dorsal papillae just near the brain. The reported observations raise the hypothesis that the central cells of the adhesive papillae might be primary sensory neurons and that they may have chemosensory function.

Bdelloid Rotifers Recorded from Australia with Description of Philodinavus aussiensis n.sp.

Abstract Bdelloid rotifer research in Australia is reviewed, the current systematic status of the group is summarized. Based on a survey of floodplain and alpine sites in northern Victoria and Tasmania conducted in January-February 1999 we recorded 20 new bdelloid species for Australia. This brings the continental record to 106 species. The description of Philodinavus aussiensis n.sp. is given, with SEM images of its trophi. Trophi of three more bdelloid species are here presented.

Trophi structure in bdelloid rotifers

Bdelloids show a rather uniform morphology of jaws (trophi), named ramate. The most recognizable feature is the presence of a series of teeth forming unci plates. The unci are not uniform in size; each plate has 1–10 major median teeth. Using SEM pictures of trophi and data from the literature, we analyzed the number of major unci teeth in relation to trophi size, total number of teeth, and environmental features. Variability in the number of major unci teeth in bdelloids is not related to trophi size or to total number of unci teeth, while total number of unci teeth and trophi size seem to be related to each other: larger trophi in general have more teeth than smaller trophi. Few major teeth are more common in species living in water bodies where they possibly eat unicellular algae, while more major teeth are more common in species living outside water bodies, among mosses and lichens, where they possibly eat bacteria.