serotonin immunoreactivity in the nervous system of the ...their foot, secrete a tube, and begin a...

Serotonin immunoreactivity in the nervous system of the free-swimming larvaeand sessile adult females of Stephanoceros fimbriatus (Rotifera: Gnesiotrocha)

Adele Hochberga and Rick Hochberg

Department of Biological Sciences,University of Massachusetts Lowell,

Lowell, Massachusetts 01854, USA

Abstract. Unlike most rotifers (Rotifera), which are planktonic and direct developers, manygnesiotrochan rotifers (Monogononta: Gnesiotrocha) are sessile and have indirect develop-ment. Few details exist on larval metamorphosis in most gnesiotrochans, and consideringthe drastic transformation that takes place at metamorphosis—the replacement of the cili-ated corona with a new head that bears ciliated tentacles (the infundibulum)—it is perhapssurprising that there are limited data on the process. Here, we document part of this meta-morphosis by examining the presence and distribution of neurons with serotonin immuno-reactivity in the nervous system of both planktonic larvae and sessile adult females. Usingantibodies against serotonin combined with confocal laser-scanning microscopy (CLSM)and 3D reconstruction software, we mapped the immunoreactive cell bodies and neurites inboth life stages and found that relatively few changes occurred during metamorphosis. Thelarvae possessed a total of eight perikarya with serotonergic immunoreactivity (5HT-IR) inthe brain, with at least two pairs of perikarya outside the brain in the region of the corona.Cells with 5HT-IR in the brain innervated the larval corona and also sent neurites to thetrunk via the nerve cords. During metamorphosis, the corona was replaced by theinfundibulum, which emerged from the larval mouth to become the new functional head.This change led to a posterior displacement of the brain and also involved the loss of5HT-IR in the lateral brain perikarya and the gain of two perikarya with 5HT-IR in theanterior brain region. The innervation of the anterior end was retained in the adult; neuritesthat extended anteriorly to the mouth of the larva formed a distinct neural ring that encir-cled the infundibulum after metamorphosis. Significantly, there was no innervation of theinfundibular tentacles by neurites with 5HT-IR, which suggests that ciliary control is un-likely to be modulated by serotonin within the tentacles themselves.

Additional key words: indirect development, confocal, ontogeny, freshwater

It has been argued that metamorphosis is a time ofnew beginnings, when a larval stage undergoes anoften dramatic morphological change that involves asubsequent transition to a new and profoundly differ-ent environment (Young 2006). In most cases, themorphological remodeling of internal and externalanatomy is extreme, as evidenced in cases of meta-morphic transformation of the nervous system in spe-cies of ascidians (e.g., Meinertzhagen & Okamura2001), brachiopods (Altenberger & Wanninger 2010),ectoprocts (Wanninger et al. 2005), entoprocts(Fuchs & Wanninger 2008), and phoronids (Santa-gata 2002), to name a few. Most studies of

invertebrate metamorphosis are necessarily focusedon marine invertebrates, many of which go throughmaximally indirect development (Brusca & Brusca2003). Significantly, at least for the purposes of obser-vation and experimentation, these animals can be cul-tured and often produce dozens or more embryosthat can be followed through metamorphosis, makingmany species particularly tractable for studies ofdevelopment. In the case of rotifers, where indirectdevelopment is comparatively rare and only leads tothe production of a few embryos that hatch intoshort-lived larvae, few studies have made detailedobservations of metamorphosis (e.g., Kutikova 1995).

The presence of a free-living larval stage is anexceptional phenomenon among rotifers and onestrongly correlated with the sessile adult lifestyle. In

aAuthor for correspondence.E-mail: [email protected]

Invertebrate Biology 134(4): 261–270.© 2015, The American Microscopical Society, Inc.DOI: 10.1111/ivb.12102

fact, most of the approximately 2000 describedspecies of rotifers are direct developers, with onlyselect members of the Gnesiotrocha (Monogononta)undergoing indirect development (Wallace et al.2015). To date, indirect development has been docu-mented from several species of both gnesiotrochansubclades, Collothecaceae, and Flosculariaceae, butdetails on the structure of the larvae and their tran-sition (metamorphosis) to adulthood remains onlypartly known for a few species (Montgomery 1903;Fontaneto et al. 2003; Hochberg et al. 2010). Tworelatively recent studies highlight some of the detailsof larval development in members of both sub-clades. Fontaneto et al. (2003) used scanning elec-tron microscopy to study the morphology of thesessile tube-dweller Floscularia ringens (LINNAEUS

1758) and followed the larva-to-adult transition.They revealed that neonates possess many adult fea-tures at hatching (e.g., lateral antennae, dorsalhooks, sublabium, etc.), but their squat and vermi-form body shape, underdeveloped corona, apparentlack of feeding, and planktonic habitat clearly dif-ferentiates them from the adults. As these larvaedevelop in the plankton over a period of 24 h, theircorona gradually expands, their body lengthens, andafter settlement on submerged vegetation theysecrete a protective tube to house their body (Fon-taneto et al. 2003). In the collothecacean Acyclusinquietus LEIDY 1882, which lives parasitically in thecolonial rotifer Sinantherina socialis (LINNAEUS

1758), the larvae have a vermiform appearance, andwhile their time in the plankton is considerablyshorter than for other species (<24 h; unpubl. data),further development is delayed until they locatetheir new host. Once settled, the larvae attach withtheir foot, secrete a tube, and begin a drastic meta-morphosis that involves the replacement of the lar-val corona with the adult infundibulum (i.e., head)and a reconfiguration of the somatic musculaturethat supplies the head (Hochberg et al. 2010).

The infundibulum of collothecacean rotifers is aunique morphological feature and one that is apo-morphic to the clade. While the infundibulum isstructurally variable throughout the Collothecaceae,it is generally bowl-shaped with a central mouth sur-rounded by a wavy or tentaculate rim. According toobservations on Collotheca ambigua (HUDSON 1883)by Montgomery (1903), the infundibulum is derivedfrom the larval alimentary tract and not from the(external) larval corona. How the infundibulummoves from internal to external has never beenrecorded, nor has the fate of the larval corona,despite observations on several species (Kutikova1995). In species of Stephanoceros, the infundibulum

bears five ciliated tentacles that appear to emergethrough the larval mouth at settlement (unpubl.data). While the morphogenetic dynamics of thisphenomenon are largely unknown, they wouldappear to necessitate dramatic changes in both mus-cle supply and innervation of the head becausebehaviors of both the larval corona and post-meta-morphic infundibulum involve control of musclesand cilia. In this study, we follow the larva-to-adulttransition in Stephanoceros fimbriatus (GOLFUß 1820)and attempt to determine if any changes occur in thestructure of the nervous system during metamorpho-sis. We focus on immunoreactivity to serotonin inthe nervous system for two reasons: (1) serotonin is amodulator of ciliary activity in invertebrates (Weiger1997; Hay-Schmidt 2000) and (2) the large anatomi-cal differences between the ciliated, larval coronaand the ciliated, adult, tentaculate infundibulumwould appear to demand a change in innervation.To date, there are few detailed studies of the nervoussystems of any gnesiotrochan rotifer (see earlier stud-ies of Vallentin [1890], Gast [1900], & Montgomery[1903]; also Hochberg [2007] & Hochberg & Lilley[2010]), and no knowledge of how the nervous sys-tem might be affected by larval metamorphosis.

Methods

Adults of Stephanoceros fimbriatus were collectedfrom submerged grass and species of Urticularia(bladderwort) in Mascuppic Pond (42�40042.15″N,71� 24007.64″W) and Flint Pond (42� 40030.45″N,71�25032.71″W), Massachusetts during the summersof 2009–2013. Plant material containing adultanimals was placed in glass dishes containing30–50 mL of pond water and observed with a Zeissstereomicroscope. Animals were isolated on smallpieces of vegetation and placed in 5 mL embryodishes with 64 lm filtered pond water and observedfor the production of parthenogenetic eggs. Mostanimals produced 3–6 eggs over the course of 7 dwhen kept at room temperature (ca. 21–23°C) on a12 h light/dark cycle. After hatching, larvae wereobserved to swim around the dish and could beidentified by the presence of red ocelli and a bire-fringent crystal (Wallace 1993). Larvae wereremoved from the dish using a micropipette andplaced on a glass microscope slide with filtered pondwater and a coverslip. In some cases, larvae wererelaxed with the dropwise addition of 1% MgCl2 or1% neosynephrine to the slide (wicked beneath thecoverslip) until the larvae stopped swimming. Thesetwo anesthetics were the only two that worked onmore than 30% of the specimens. Other anesthetics,

Invertebrate Biologyvol. 134, no. 4, December 2015

262 Hochberg & Hochberg

including bupivacaine and cocaine, had no observ-able effects, or stimulated the larvae to contract andremain contracted prior to fixation. Photographsand digital video of all stages of development (free-swimming larvae and adults) were obtained with aSony Handycam digital camera mounted on a ZeissA1 compound microscope. Measurements of indi-vidual specimens were performed using an ocularmicrometer on mostly relaxed specimens.

Several specimens were fixed in place on micro-scope slides by placing a drop of 4% paraformalde-hyde in 0.1 mol L�1 phosphate buffer (PB, pH 7.3)next to the coverslip and then drawing it beneaththe coverslip by wicking away water from the oppo-site side of the coverslip. After a minimum of 1 hfixation, specimens were transferred via micropipetteto a small dish and rinsed with PB, followed bytransfer to a microcentrifuge tube containing PBwith 1% bovine serum albumen (PB-BSA) for 1 hon a rotator. The labeling of neural antigens by aseries of different antibodies (e.g., anti-FMRFamide,anti-GABA, anti-serotonin, and anti-tubulin) wasattempted on larval and adult specimens; however,only anti-serotonin antibodies consistently labeledneural structures. No labeling was observed whenthese antibodies were preabsorbed with serotonin,and labeling was also eliminated in control experi-ments in which the primary antibody was notincluded in the processing steps. For labeling, sero-tonin-immunoreactive neurons, relaxed specimens(n=6 larvae, n=10 adults) were processed in micro-centrifuge tubes on a rotator at 4°C. Subsequentprocessing steps were performed in the followingorder: PB plus 1% Triton X-100 (PBT) for 24 h;primary antibody (rabbit anti- serotonin, 1:2000dilution, Immunostar Inc.) in PBT for 12 h; PBTfor 12 h; secondary antibody (goat anti-rabbit con-jugated to CY3 [550 nm excitation, 570 nm emis-sion], 1:500, Life Technologies) in PBT for 12 h;and PBT for 12 h. One adult and one larva (con-tracted) were processed in an identical fashion ascontrols but incubation in primary antibody wasomitted. Two of the experimental specimens of eachstage were placed in 1 ml of NucGreen Dead 488Ready Probes Reagent (488 nm excitation, 513 nmemission, Life Technologies) for 30 min to stain cellnuclei. All specimens were mounted in Fluoro-mount-G (Southern Biotechnology Associates,Birmingham, AL, USA) on glass slides and kept at�20°C for several days prior to examination.

Neurons with serotonin immunoreactivity(5HT-IR) were observed using an Olympus FV300confocal laser-scanning microscope. A green heliumneon laser (543 nm) was used to excite the samples,

and Olympus software was used to capture theimages. Confocal z-stacks were collected and pro-cessed in Olympus software as .TIF files and .MOVvideo files. Files were processed with Volocity (Per-kin Elmer) to generate z-projections in TIF formatand to generate QTVR videos. Photoshop CS5.1(Adobe) was used to crop TIF files, increase bright-ness and/or contrast, and in four greyscale imagesto invert image brightness or colors to reveal fineneurites that did not show up well against a blackbackground. All original data files and their videosmay be requested for viewing by contacting the cor-responding author.

Results

Cultured adult females (Fig. 1A) each produced2–3 amictic eggs over the course of 1 week. Eggswere deposited within the gelatinous tube thatsurrounds the adult (Fig. 1A,B). Approximately 48–72 h later, the larvae hatched from their eggcapsules and slowly squirmed their way out of thetube, often aided by longitudinal contractions of theadult (Fig. 1B). Larvae were highly contractile andoften withdrew their foot into their trunk (Fig. 1C).After ~2–3 h, the developing infundibulum could beobserved within the larval body, close to the ante-rior end (Fig. 1C). The adult infundibulum did notemerge until the larva had settled permanently on asubstrate. No metamorphosing larvae were success-fully fixed or stained.

Larvae

Serotonin-immunoreactive (5HT-IR) cell bodiesand neurites were present in all examined larvae,although not all specimens were labeled with equalintensity or were in the appropriate orientation forvisualization. Some specimens were also partly con-tracted along the anterior-posterior axis, with thehead and/or foot partially withdrawn, thereby mak-ing neurites bend and loop in what appeared to beunnatural orientations. Serotonin immunoreactivitywas distributed throughout the central nervous sys-tem including the cerebral ganglion and paired nervecords (Fig. 2A,B). The cerebral ganglion was demar-cated by a large cluster of nuclei at the anterior endand could be distinguished from the remaining cellpopulation by the level of NucGreen staining (notshown) and its co-localization with 5HT-IR. Thebrain was bilaterally symmetrical and containedapproximately eight 5HT-IR perikarya (Figs. 2, 3).A cluster of four unipolar perikarya (BPp1-4) wascentrally positioned in the posterior side of the brain


Nervous system of Stephanoceros fimbriatus 263

(Figs. 2C–E, 3A). The two outermost perikarya(BPp1,4) were largest (~8–9 lm long96 lm wide)and had an anteriorly directed neurite that curvedbeneath the neuropil and bent ventrolaterally, even-tually projecting away from the brain. The inner-most perikarya (BPp2,3) were more spindle-shapedand smaller (5–6 lm long93–4 lm wide); each hadan anteriorly directed neurite that joined orsynapsed on the thick, pretzel-shaped, neuropil thatwas also innervated by the nerve cords (describedbelow). A pair of spindle-shaped perikarya (BPL)was present close to the lateral border on either sideof the brain (Fig. 2E). These perikarya were ~7 lmlong and appeared to be unipolar with a laterallydirected neurite, but 5HT-IR in the neurite was tooweak to follow its path. Toward the anterior end ofthe brain was a single pair of 5HT-IR perikarya.These perikarya (BPa) were large (10 lm long97–8 lm at their widest), positioned close to the midlineof the brain, and appeared to be bipolar with ananterior and posterior directed neurite (Fig. 2B–E).The posterior neurite (BPapn) projected toward the

neuropil and the anterior neurite (BPaan) bent ven-trally and terminated around the larval corona in aregion of diffuse immunoreactivity (see below).

Outside of the brain but close to its anterolateralborders was a pair of unipolar perikarya (LP), eachwith a posteriorly directed neurite (LPn) that bentmedially toward the ventral side of the brain(Figs. 2B,D, 3B); their specific site of innervationcould not be determined. Also outside of the brainin a region just posterior of the coronal cilia was adiffuse region of 5HT-IR (dIR, Fig. 2A,D). Thisdiffuse staining occurred around the ciliary cushionsof the corona and obscured visualization of most5HT-IR cells in all but one specimen. In this singlespecimen, a large pair of perikarya was observed(Fig. 2B,C). These perikarya (CP), each ~13–15 lmlong, were centrally located and appeared to be theorigins of a pair of CPn neurites, although their con-nections were never confirmed. These neurites (CPn)extended posteriorly and then bent medially to forma U-shaped arc ventral to the brain neuropil(Fig. 2B).

Fig. 1. Stephanoceros fimbriatus. A. Brightfield image of an adult female on submerged vegetation. The body is dividedinto anterior infundibulum (if), which consists of five tentacles (ift) that lead to a shallow infundibular cavity, a thicktrunk (tr), and elongate foot (ft). Most of the foot and trunk are surrounded by the secreted gel tube (gt). The approx-imate position of the brain in circled. Scale bar=150 lm. B. Brightfield image of a partly contracted female adult witha single larva (lv) that is attempting to escape the gel tube. Scale bar=150 lm. C. Brightfield image of the larva, partlycontracted. The larval corona (lc) is present as is the developing infundibulum (dif), which is formed within a cavity atthe anterior end. The approximate position of the brain (br) is circled. Scale bar=40 lm.



Immunoreactive components of the neuropil hada peculiar pretzel-like shape that lay flat in thedorsoventral plane (Figs. 2B, 3). A pair of nervecords (~1–1.5 lm wide) fed into the neuropil at itsmost anterolateral aspect (Figs. 2B,E, 3), therebyforming the anterior surface (top) of each pretzel“loop.” The thickened neurites (~1.5–2.5 lm) ofeach loop came together medially to form a central

cord that extended down the midline of the neuropil(Fig. 3A). Posteriorly, the cord split and eachbranch bent laterally again to form the posteriorsurface of the pretzel loop. These branches then pro-jected anteriorly, but instead of coalescing on theanterior surface (to form a cohesive pretzel shape),the projections crossed the anterior end and con-nected to the BPapn (see Fig. 3).

Fig. 2. Anti-serotonin immunoreactivity (5HT-IR) in the larva of Stephanoceros fimbriatus. A. Whole larva showingthe region of the brain (circled) and nerve cords (nc). Anterior end is up. Confocal stack of 41 slices (0.1 lm per slice).Scale bar=35 lm. B. Anterior end (to the left) in lateral view, inverted image processed in Volocity software. In thisview, the brain can be seen to contain the neuropil (np) with a series of four posterior brain perikarya (BPp1-4) andtwo large anterior perikarya (BPa). The nerve cords bend ventrally and extend posteriorly. A large lateral perikaryon(LP) extends an elongate neurite (LPn) into the brain. A pair of 5HT-IR perikarya (CP) are present below the coronaand close to the ventral side. Scale bar=15 lm. C. Anterior end (up) of a larva, dorsolateral view, inverted image pro-cessed in Volocity software. In this image, the anterior neurites (BPaan) of the BPa can be seen to extend toward thecorona. Scale bar=15 lm. D. Anterior end (up) of larva showing the diffuse region of 5HT-IR (dIR) that oftenobscured the CP. Confocal stack of 41 slices (0.1 lm per slice). Scale bar=20 lm. E. Enlargement of the brain showingthe individual perikarya at the posterior end (BPp1-4), one of the lateral neurites within the brain (BPL) and the thick-ening of the neuropil where the nerve cord enters the brain (arrowhead). Scale bar=7 lm.



Adults

Interpretation of staining in adults was compli-cated by the flattening of the infundibulum underthe cover slip, which compressed the anterior endinto unnatural shapes. Nonetheless, many of the5HT-IR pathways were followed at least partway,with a complete reconstruction of all elements beingbased on examination of several specimens that eachshowed only some of the details. All 5HT-IR peri-karya and neurites in the adult that were found tobe of similar size and position to those in the larvaewere considered to be the same.

The cerebral ganglion was centrally located andpositioned posterior to the dorsal infundibular ten-tacle (dt); the brain appeared as a large cluster ofcell nuclei in preparations stained with NucGreen(Fig. 4A). Both the cerebral ganglion (Fig. 4B–D)and nerve cords (Fig. 4E) displayed strong5HT-IR. Many of the 5HT-IR perikarya of thecerebral ganglion appeared to be in a similar posi-tion to those in the larvae, although in most casesthe orientation of the brain and corresponding cellpositions were affected by contraction of the speci-men. Four perikarya (BPp1-4) with 5HT-IR werecentrally located behind the neuropil within theborders of the brain (Figs. 4C, 5). The largestperikarya (BPp1,4: 8–10 lm long) were on the out-side and the smaller perikarya (BPp2,3: 5-6 lm

long) were medial in position. Anteriorly directedneurites from these perikarya appeared to follow asimilar path to those in the larvae (Figs. 4D, 5);however, the precise connections of some neurites(BPp2,3) to the neuropil could not be confirmed. Alarge pair of perikarya (BPa: 10 lm long) waspositioned just anterior of the neuropil within theborders of the brain (Figs. 4C,D, 5). In somespecimens, these perikarya appeared to be unipolarwith only short, posterior neurites directed towardthe neuropil. In other specimens, a short anteriorneurite appeared to be present, but the IR signalwas extremely weak and the pathway could not befollowed. A pair of large (30 lm long), weaklyimmunoreactive perikarya (AP, only confirmed inone specimen; Figs. 4C, 5) was present close tothe anterior side of the brain. These perikaryawere spindle-shaped and positioned with their ter-mini directed laterally. There were no obvious neu-rites extending from these cells; however, in onespecimen, the lateral termini did appear to be pro-jecting toward some neurites (CPn describedbelow) but a connection between the two was notobvious (see Fig. 5). No lateral brain perikarya(e.g., BPL) with 5HT-IR were detected in anyadult specimens.

The structure of the neuropil was not obvious inadults due to the intense immunoreactivity (fluores-cence), so the pretzel-like shape present in the larvae

Fig. 3. Color-coded schematic of the serotonin-immunoreactive elements around the anterior end of the larva ofStephanoceros fimbriatus. Individual perikarya and their associated neurites are identical colors. A. Dorsal view.B. Lateral view, anterior to the left. See text for details.



could not be confirmed in the adult (Fig. 4C,D). Apair of nerve cords (1–1.5 lm wide) projected later-ally from the neuropil (Fig. 4B,D), bent ventrally,

and extended posteriorly, eventually terminating inthe foot; no perikarya or commissures wereobserved in association with these nerve cords,although numerous varicosities were present and

Fig. 4. Nuclear staining and serotonin immunoreactivity(5HT-IR) in the adult female of Stephanoceros fimbriatus.A. Nuclear staining in the trunk of an adult, anterior isup and the infundibular tentacles are not in view. The cir-cled area shows the cluster of cells that demarcate thecerebral ganglion. Other nuclei from the epidermis andorgan systems are distributed throughout the trunk. Scalebar=60 lm. B. 5HT-IR in a partially contracted adult,inverted gray-scale image, with 5HT-IR cells and neuritesshown as white against a darkened background. The cir-cled area includes the 5HT-IR perikarya of the cerebralganglion. One nerve cord (nc) can be seen. Scalebar=80 lm. C. En-face view of the anterior end of anadult, dorsal is up. The infundibular tentacles have adashed outline. The large anterior (BPa) and posterior(BPp1-4) perikarya of the brain can be seen, as can theneurites of the CPn that make up the neural ring aroundthe infundibulum. Two large perikarya (AP) with verylow IR are present within the cerebral ganglion and artifi-cially outlined. Confocal stack of 81 slices (0.1 lm perslice). Scale bar=20 lm. D. Enlargement of the brain indorsal view, anterior is down. Confocal stack of 50 slices(0.1 lm per slice). Scale bar=13 lm. E. Enlargement ofthe nerve cords in the foot. No cell bodies or perikaryawere observed. Confocal stack of 50 slices (0.1 lm perslice). Scale bar=10 lm.

Fig. 5. Schematic of serotonin immunoreactivity (5HT-IR) in the adult female of Stephanoceros fimbriatus basedon analyses of several specimens. The perikarya and neu-rites are color coded to match their potential homologuesin the larva (Fig. 3). The position of specific anatomicallandmarks is labeled and includes the infundibular tenta-cles (dt, dorsal tentacle; la, lateral tentacles; vt, ventraltentacles), infundibular cavity (if), proventriculus (pv),mastax (mx), stomach (st), and gel tube (gt). See text fordetails.



often appeared as swellings similar in shape to cellbodies (Fig. 4E).

The infundibulum was encircled by a pair of neu-rites (CPn) that formed a ring around its periphery(Figs. 4C, 5). These neurites appeared to be exten-sions of a pair of weakly immunoreactive cell bodes(CP) close to the ventral midline (CP, Figs. 4D, 5);it was undetermined if the neurites were extensionsof these perikarya or synapsed on them. A shortpair of neurites appeared to branch off the neuralring in a region close to the position of the lateraltentacles and then reconnect with the ring a shortdistance later (arrowhead, Figs. 4C, 5). Thebranches (CPn2) were similar in morphology to theCPn but highly folded in some cases.

A single pair of perikarya (LP, Fig. 5) was pre-sent on either side of the infundibulum but not con-tained within the brain itself. These perikarya eachgave rise to a single neurite that extended mediallytoward the center of the infundibulum where theyappeared to coalesce or at least terminate close toone another.

Discussion

Our studies of metamorphosis and nervous systemarchitecture in the sessile rotifer, Stephanoceros fim-briatus, reveal for the first time the distribution ofneurons expressing serotonin-like immunoreactivity(5HT-IR) in both larvae and adults, and show thatmetamorphosis has little effect on the structure ofthese components of the nervous system. In fact,despite the drastic morphological transformationthat occurs after settlement of the larva—when thelarval corona is replaced by the adult infundibu-lum—there is little evidence to suggest that neuronalpathways are altered in any drastic way. The larvalnervous system consists of twelve distinct perikaryashowing 5HT-IR, eight of which are in the cerebralganglion and four of which are peripheral. In theadult, there are at least twelve 5HT-IR perikarya,eight of which are in the cerebral ganglion and atleast four that are peripheral. Significantly, one pairof cerebral perikarya (BPL) in the larvae could notbe detected in the adults, and a pair of large peri-karya in the adults (AP) could not be detected inthe larvae. Whether or not these cells are trulyabsent from their respective life stages or were notobserved due to low immunoreactivity remains to bedetermined.

In both the larval and adult stages, one pair of5HT-IR neurons (CP) appears to innervate a regionaround the corona and infundibulum, respectively.The cell bodies appear close to the mouth in the

larva (though the larval mouth is difficult to see)and remain in a similar position in the adult, whichis just ventral to the infundibulum approximatelymidway between the ventral tentacles. A single neu-rite projects laterally from each cell and appears toextend along the periphery of the corona in thelarva and the infundibulum in the adult (Fig. 5). Inthe adult, this neural ring appears to branch in asymmetrical fashion approximately one-third theway around the infundibulum, only to unite severalmicrometers further along the ring. In both thelarva and adult, the ring terminates in the cerebralganglion, although the precise site of innervationcould not be determined. A similar immunoreactivenerve ring, although without any perikarya or bifur-cations, is also present in other rotifers, includingspecies of Conochilus (Gnesiotrocha: Hochberg2006) and to some degree Platyias patulus (MULLER

1786) (Ploima: Kotikova et al. 2005). While the lar-val corona and adult infundibulum of S. fimbriatusare drastically different from each other, and fromthe coronae of other species, the presence of a neu-ral ring suggests a conserved pattern of innervationin Rotifera. It has been hypothesized that the neuralring might play a role in modulating ciliary activityof the corona (Kotikova et al. 2005; Hochberg2007). The position of the neural ring in the larvaeof S. fimbriatus supports this hypothesis, while itsposition in the adult is a little less convincing. Thereason for this is that there is no serotoninimmunoreactivity in the tentacles where the cilia arepresent. However, this does not rule out the possi-bility of serotonin-immunoreactive neurites in theregion of the infundibulum depolarizing the syncy-tial epithelia of the tentacles, thereby stimulating cil-iary movement. Still another possibility exists thatserotonin-immunoreactive neurites stimulate individ-ual muscle fibers that supply the tentacles; in roti-fers, muscles are known to insert on ciliary rootletsand thereby produce ciliary beating (Cl�ement &Wurdak 1991).

Other anatomical sites of serotonergic innervationin S. fimbriatus are difficult to ascertain. Accordingto Montgomery (1903), who has performed histolog-ical research on S. fimbriatus, five sets of neuronsproject from the cerebral ganglion, three of whichinnervate sense organs: one pair innervates the dor-sal sense organ; one pair innervates the lateral senseorgan; and one unpaired median neuron innervatesthe hypodermis posterior of the dorsal sense organ.Our results could not confirm these observations,but in some respects this is not surprising since sero-tonergic innervation of sensory organs has onlyrarely been documented in rotifers (e.g., Hochberg



2009; Hochberg & Lilley 2010). Our observations doindicate that at least two of the neurons detected byMontgomery (1903) are likely to correspond to apair of 5HT-IR neurites we detected in both larvaland adult specimens. He describes two thick nervesthat pass from the brain and ventral to the proven-triculus (see Fig. 5); we hypothesize that thesenerves are the nerve cords, which contain at leastone 5HT-IR neurite each extending from the cere-bral ganglion. This is a common condition through-out Rotifera, having been documented in bothBdelloidea (Leasi et al. 2009) and numerous speciesof Monogononta (Kotikova et al. 2005; Hochberg2006, 2007). Perhaps surprisingly, Montgomery(1903) does not document any cerebral nerves inner-vating the mastax, although more modern studiesusing immunohistochemical methods have revealedserotonergic innervation as well as FMRFamidergicinnervation (Kotikova et al. 2005). In our studies,there were no obvious cell bodies or neuritesexpressing 5HT-IR in the region of the mastax ofS. fimbriatus.

In conclusion, our study reveals that the larvaland adult (female) nervous systems of S. fimbriatusare broadly similar, and that metamorphosis,although dramatic in its restructuring of the head,has little obvious effect on the number or positionof neurons expressing 5HT-IR. However, somenotable changes do appear to take place after meta-morphosis, e.g., the brain becomes positioned far-ther from the anterior end, there is an apparent lossof the larval BPL, the addition of the CPn2 and APin adults, and the apparent repositioning and chang-ing innervation of the BPa in adults. The importanceof these changes remains to be determined, and wethink that only through additional studies of meta-morphosis in other indirectly developing gne-siotrochans (e.g., species of Acyclus, Cupelopagis,and Collotheca) using antibodies to serotonin andother neurotransmitters will we truly appreciate howontogeny may restructure the rotifer nervous system.Also, while it is tempting to broaden the interpreta-tions of this research to other species of Rotifera,i.e., to homologize individual cells and patterns ofneurites, we hesitate to do so until other indirectlydeveloping species have been examined. Observa-tions on direct developing species from a wide vari-ety of lifestyles (e.g., solitary and colonial, sessileand planktonic) have revealed broadly similar pat-terns that are difficult to homologize because of dif-ferences in the number and position of perikaryaand the configurations of their neurites. We thinkthat a useful approach to understanding neural evo-lution in Rotifera will be one that focuses on char-

acterizing similar patterns of immunoreactivity toneural antigens in a subset of related species (e.g.,within Stephanoceros or Collotheca) rather one thatcharacterizes patterns across large lineages (e.g., spe-cies of Ploima or Monogononta).

Acknowledgments. We thank Dr. Elizabeth Walsh (U.Texas El Paso), Dr. Robert Wallace (Ripon College), Dr.Diego Fontaneto (CNR National Research Council), andone anonymous reviewer for their comments on thismanuscript. This research was supported by the NationalScience Foundation (DEB 1257110 to RH). Any opin-ions, findings, and conclusions or recommendationsexpressed in this material are those of the authors and donot necessarily reflect the views of the National ScienceFoundation.

References

Altenberger A & Wanninger A 2010. Neuromusculardevelopment in Novocrania anomala: evidence for thepresence of serotonin and a spiralian-like apical organin lecithotrophic larvae. Evol. Dev. 12: 16–24.

Brusca RC & Brusca GJ 2003. Invertebrates, 2nd ed. Sin-auer Associates, New York. 936 pp.

Cl�ement P & Wurdak E 1991. Rotifera. In: MicroscopicAnatomy of Invertebrates, Vol. 4. Aschelminthes. Har-rison FW & Ruppert EE, eds., pp. 219–297. Wiley-Liss,New York.

Fontaneto D, Melon G, & Wallace RL 2003. Morphol-ogy of Floscularia ringens (Rotifera, Monogononta)from egg to adult. Invertebr. Biol. 122: 231–240.

Fuchs J & Wanninger A 2008. Reconstruction of the neu-romuscular system of the swimming-type larva of Lox-osomella atkinsae (Entoprocta) as inferred byfluorescence labelling and confocal microscopy. Org.Divers. Evol. 8: 325–335.

Gast R 1900. Beitr€age zur kenntniss von Apsilus vorax(Leidy). Z. Wiss. Zool. 67: 167–214.

Hay-Schmidt A 2000. The evolution of the serotonergicnervous system. Proc. R. Soc. Lond. B Biol. 267:1071–1079.

Hochberg R 2006. On the serotonergic nervous system oftwo planktonic rotifers, Conochilus coenobasis andC. dossuarius (Monogononta, Flosculariacea, Conochil-idae). Zool. Anz. 245: 53–62.

———— 2007. The topology of the nervous system ofNotommata copeus (Rotifera: Monogononta) revealedwith anti-FMRFamide, -SCPb, and -serotoninimmunohistochemistry. Invertebr. Biol. 126: 146–157.

———— 2009. Three dimensional reconstruction andneural map of the serotonergic brain of Asplanchnabrightwellii (Rotifera, Monogononta). J. Morphol. 270:430–441.

Hochberg R & Lilley G 2010. Neuromuscular organiza-tion of the freshwater colonial rotifer, Sinantherinasocialis, and its implications for understanding the evo-



lution of coloniality in Rotifera. Zoomorphology 129:153–162.

Hochberg R, O’Brien S, & Puleo A 2010. Behavior, meta-morphosis, and muscular organization of the predatoryrotifer Acyclus inquietus (Rotifera, Monogononta).Invertebr. Biol. 129: 210–219.

Kotikova EA, Raikova OI, Reuter M, & GustafssonMKS 2005. Rotifer nervous system visualized byFMRFamide and 5-HT immunohistochemistry andconfocal laser scanning microscopy. Hydrobiologia546: 239–248.

Kutikova LA 1995. Larval metamophosis in sessile roti-fers. Hydrobiologia 313/314: 133–138.

Leasi F, Pennati R, & Ricci C 2009. First description ofthe serotonergic nervous system in a bdelloid rotifer:Macrotrachela quadricornifera Milne, 1886 (Philo-dinidae). Zool. Anz. 248: 47–55.

Meinertzhagen IA & Okamura Y 2001. The larval ascidiannervous system: the chordate brain from its beginnings.Trends Neurosci. 24: 401–410.

Montgomery TH Jr 1903. On the morphology of therotatorian family Flosculariidae. Proc. Acad. Nat. Sci.Phila. 55: 363–395.

Santagata S 2002. Structure and metamorphic remodelingof the larval nervous system and musculature of Phoro-nis pallida (Phoronida). Evol. Dev. 4: 28–42.

Vallentin R 1890. Some remarks on the anatomy of Ste-phanoceros eichhornii. Annu. Mag. Nat. Hist. 6: 1–11.

Wallace RL 1993. Presence of anisotropic (birefringent)crystalline structures in embryonic and juvenile mono-gonont rotifers. Hydrobiologia 255/256: 71–76.

Wallace RL, Snell T, & Smith HA 2015. Rotifera: ecol-ogy and general biology. In: Thorp and Covich’sFreshwater Invertebrates. Thorp JH & Rogers DC,eds., pp. 225–271. Elsevier, Waltham, Massachusetts.

Wanninger A, Koop D, & Degnan B 2005. Immunohisto-chemistry and metamorphic fate of the larval nervoussystem of Triphyllozoon mucronatum (Ectoprocta:Gymnolaemata: Cheilostomata). Zoomorphology 124:161–170.

Weiger W 1997. Serotonergic modulation of behaviour: aphylogenetic overview. Biol. Rev. 72: 61–95.

Young CM 2006. A brief history and some fundamentals.Chapter 1. In: Atlas of Marine Invertebrate Larvae.Young CM, ed., pp. 1–19. Elsevier, Boston, Mas-sachusetts.



serotonin immunoreactivity in the nervous system of the ...their foot, secrete a tube, and begin a...

Documents