Origin of the Pilidium Larva in Nemertean Worms

My main focus is evolution of larval development in nemerteans. The unique planktonic pilidium larva appears to have evolved in a single clade of nemerteans (the Pilidiophora) and possesses a novel body plan and development derived from the more direct developmental program of basal nemerteans. The pilidiophoran juvenile develops inside the pilidium larva via a series of ectodermal invaginations called the imaginal disks and “hatches out” of the pilidium during drastic metamorphosis. The body plan of the pilidiophoran juvenile is comparable to the larval/juvenile body plan of non-pilidiophoran nemerteans, while the body plan of the pilidium larva has no homology within or outside the phylum. In collaboration with Dr. Mikhail Matz (University of Texas at Austin) I address this question utilizing a variety of molecular techniques. Taking advantage of decoupled development of larval and juvenile characteristics in pilidiophoran nemerteans, we are trying to identify transcripts differentially expressed during development of the larval body plan (pilidium- specific) Micrura alaskensis vs. those expressed during development of the juvenile (juvenile-specific). The future plan is to extend the comparison to nemertean species that have non-pilidial development.

Comparative Development of Nervous System in Nemerteans

Another part of my research concerns the development of larval and juvenile nervous systems in nemerteans. While the nervous system of the pilidium larva has been described (Lacally and West, 1985; Hay-Schmidt 1990), development of the nervous system in the pilidiophoran juvenile, palaeo- or hoplonemerteans is almost entirely unknown.  I utilize a combination of immunolabeling, confocal microscopy and TEM to study neuromuscular development of several intertidal species of nemerteans common on the Pacific Coast of North Amrica, e.g. the pilidiophoran Micrura alaskensis, palaeonemertean Carinoma mutabilis and hoplonemertean Paranemertes peregrina.

Hidden Trochophore in Palaeonemerteans

Nemerteans belong to the protostome clade the Trochozoa, which includes annelids, mollusks, sipunculids, echiurids and entoprocts (= kamptozoans). Typical trochozoans possess the trochophore larva characterized by the prototroch, a pre-oral belt of specialized ciliated cells derived from the founder cells called the trochoblasts. Nemerteans remain one of the understudied trochozoan phyla and until now it had been difficult to compare nemertean larvae to other trochozoan larvae. Members of the nemertean clade Pilidiophora develop via a highly specialized planktonic pilidium larva, which undergoes catastrophic metamorphosis, while members of the sister clade to the Pilidiophora - the monophyletic Hoplonemertea, and the basal paraphyletic palaeonemerteans have simple planuliform larvae, which develop into juvenile without drastic change of the body plan. Neither pilidium no uniformly ciliated planuliform larvae resemble a typical trochophore. Because pilidium larvae posesses a differentially ciliated band it had been compared to the prototroch of other trochozoans, however, the cell-lineage analysis using intracellular fluorescent markers demonstrated that both conserved (trochoblast-derived) and novel cell lineages contribute to the ciliated band in pilidium (Henry and Martindale, 1998)

        Several species with planuliform larvae possess a transitory larval ectoderm, which had been homologized with pilidium, with the implication that pilidial-type development is ancestral for nemerteans (Maslakova and Malakhov, 1999). Our research on development of a palaeonemertean Carinoma tremaphoros, which possesses a planuliform planktonic larva, revealed that what initially appeared as larval ectoderm in this species is in fact a preoral belt of 40 cleavage arrested cells, reminiscent of the prototroch of other trochozoans. In collaboration with Dr. Mark Martindale (Kewalo Marine Lab, University of Hawaii) and my Ph. D. advisor, Dr. Jon Norenburg (National Museum of Natural History, Smithsonian Institution), I used intracellular labeling markers and confocal microscopy to demonstrate that prototroch in C. tremaphoros is derived from the same cell lineage as prototrochs of other Trochozoans (Maslakova et al. 2004a,b). Uniform ciliation of the larval body (or lack of differentiated ciliation of the trochal lineages) obscures presence of the prototroch in C. tremaphoros (hence the hidden trochophore), however, trochoblast specialization is clearly manifested in their permanent cleavage arrest and ultimate degenerative fate. This refutes the homology between the pilidium and palaeonemertean larval ectoderm and suggests that pilidial development is a derived condition in nemerteans. The fact that pilidium larva appears to be an evolutionary novel body plan evolved in only one clade of nemerteans (Pilidiophora) has an important implication for comparisons with other larval types. For example, it suggest that the similarity in structure and innervation of ciliated band in the pilidium larva and Muellers larva of polyclad flatworms is a result of covergent evolution and, as such, can not be used to infer close relationships between nemerteans and flatworms. Discovery of the vestigial prototroch and, therefore, a trochophore larva in nemerteans strengthens the argument for close relationship to other trochophore-bearing phyla.

Transitory larval epidermis in Hoplonemertean larvae

Our recent work on development of the hoplonemertean Paranemertes peregrina showed that the lecithotrophic larva of this species possesses a transitory larval epidermis which is very different from the prototroch of the palaeonemertean hidden trochophore. Further research is necessary to determine what structures this larval epidermis may be homologous to in other nemerteans and, more broadly, other spiralians. A possibility remains that the transitory larval epidermis found in hoplonemerteans corresponds to the tissues lost during pilidial metamorphosis and that catastrophic metamorphosis is ancestral for the clade comprising hoplonemerteans and pilidiophorans.

Systematics and Evolution of Smiling Worms (Fam. Prosorhochmidae)

My dissertation research focused on the systematics and evolution of the family Prosorhochmidae (Geonemertes, Pantinonemertes, Prosadenoporus and Prosorhochmus), a peculiar group of monostiliferous hoplonemerteans occupying a variety of habitats from middle-upper intertidal to semi-terrestrial and arboreal - quite unusual for the almost exclusively marine phylum. Prosorhochmids possess a series of rare and unique among nemerteans morphological and life history characteristics such as viviparity, accompanied by hermaphroditism, extensive excretory system and a highly specialized tubular frontal organ through which the well-developed mucus glands discharge.  Typical Prosorhochmids are easily identified by the presence of the so-called prosorhochmid smile – a horizontal epidermal fold under which the frontal organ opens. Along with the viviparous and oviparous species with direct development prosorhochmids include oviparous species with planktonic larvae.  Such diversity of habitats and life histories makes them an interesting object for the evolutionary studies of morphological and life history characters.  Are unique characters of prosorhochmid morphology, such as modified nephridial system, viviparity and hermaphroditism – an adaptation to terrestrial habitat or an evolutionary coincidence?  Did terrestrial species evolve from the intertidal prosorhochmids via semi-terrestrial route or vice versa?  Indeed, a biologically sound answer to these questions requires knowledge of the evolutionary history of the group. The specimen-based revision of the morphological characters previously used in prosorhochmid systematics demonstrated that many characters have been misinterpreted or can not be used in cladistic analysis because the variation cannot be accurately quantified or range of intraspecific variation of quantitative characters is unknown.  A phylogeny based strictly on the morphological characters is largely unresolved, because so few characters can be reliably scored for use in cladistic analysis and because of fairly large amount of homoplasy (character conflict) in the data set.  Because nemerteans represent a morphology-poor taxon, molecular characters are essential for the analysis to add resolution and provide a test of homology for the few available morphological characters.  Combined analysis of the morphological and molecular data suggests that terrestrial species may have evolved directly from the marine ancestors and that reversals to sea are possible withing the groups occupying semi-terrestrial and terrestrial habitats. Additionally it suggests that nemertean nephridial systems can be fairly plastic resulting in convergent adaptive morphologies, which in turn may bias morphological phylogeny because so few morphological characters are available in a first place.  Concentrated change tests support correlated evolution of some characters of modified nephridial systems, such as thin-walled nephridial canals and extreme multiplication of the nephridiopores with transition to the osmotically stressful habitats (i.e. semi-terrestrial and terrestrial), however there is no   sufficient evidence to either cofirm or refute the hypothesis of adaptation.  Combined analysis also provides incidental evidence for the multiple independent origins of hermaphroditism in association with 1) terrestrial habitats and 2) viviparity in marine free-living nemerteans, although the correlation is not statistically significant.

        Taxonomic implications of this study include descriptions of two new species from Belize and Florida: Prosorhochmus belizeanus sp. nov. Maslakova and Norenburg (in press) and Prosadenoporus floridensis sp. nov. Maslakova and Norenburg (in press), redescription of Amphiporus nelsoni and its relocation to the Prosorhochmus (Maslakova et al., 2005), resurrection of the previously synonymized species Prosorhochmus korotneffi and its placement into a new genus Arhochmus gen. nov. Maslakova and Norenburg (in press). We demonstrated that the morphological differences between Pantinonemertes and Prosadenoporus are elusive, and synonymized the two genera (Maslakova and Norenburg, in press).


Last updated 13 May 2008