maerski (128 microns in size)

Lección 25.- Los Rotíferos. Definición y sinopsis sistemática. Anatomía externa e interna. Reproducción y ciclos vitales. Micrognatozoos y Ciclióforos. Los Acantocéfalos.

El MICROGNATOZOOS (Limnognathia maerski, KRISTENSEN y FUNCH 2000 es un microscopico (110 m) descubierto en Groenlandia, para el cual se ha propuesto la creación del grupo de los Micrognathozoa (como un nuevo taxón dentro de los ), que ha sido relacionado con rotíferos y con gnatostomúlidos por su complejo aparato mandibular y sus cilios sensoriales anteriores.

NIELSEN (2001) considera que Limnognathia probablemente deba de situarse dentro de los Gnathifera, junto a rotíferos, gnatostomúlidos y quetognatos. AX (2000, 2003), considera prematuro establecer las afinidades sobre la base de los datos disponibles.

SORENSEN et al. (2000), tras un análisis cladista de los metazoos basado en 66 caracteres morfológicos, concluyen que Limnognathia se ubican dentro de los protóstomos espirales, dentro del grupo de los Gnathifera, como adelfotaxón del clado (( + ) + Seisonida)).

Further structures in the jaw apparatus of Limnognathia maerski (Micrognathozoa), with notes on the phylogeny of the Gnathifera.

Sorensen MV.

Invertebrate Department, Zoological Museum, University of Copenhagen, Denmark. [email protected]

The jaws of Limnognathia maerski, Micrognathozoa, were investigated with light- and scanning electron microscopy. The study yielded several new structures and sclerites, including the ventral part of main jaw, the pharyngeal lamellae, the manus, the dorsal and ventral fibularium teeth, and a reinterpretation of the fibularium compartmentalization. Furthermore, it was shown that several jaw elements are composed of densely packed rods. Comparison with Rotifera and Gnathostomulida suggested that the micrognathozoan main jaw is homologous with the incus and the articularium and that the pseudophalangids (the ventral jaws) and their associated sclerites correspond to the rotifer mallei. These results imply that Micrognathozoa is more closely related to Rotifera than to Gnathostomulida. Copyright 2002 Wiley-Liss, Inc En todo caso, con respecto a los ciclióforos y micrognatozoos, se advertirá a los alumnos que las afinidades sugeridas hasta ahora para estos dos animales deben de ser tomadas con gran precaución.

FIGURA 152. Limnognathia maerski (KRISTENSEN y FUNCH, 2000 Micrognathozoa: a new with complicated jaws like those of Rotifera and Gnathostomulida. Kristensen RM, Funch P. Department of Zoology, Zoological Museum, University of Copenhagen, Copenhagen O, Denmark. [email protected]

A new microscopic aschelminth-like animal, Limnognathia maerski nov. gen. et sp., is described from a cold spring at Disko Island, West , and assigned to Micrognathozoa nov. class. It has a complex of jaws in its pharynx, and the ultrastructure of the main jaws is similar to that of the jaws of advanced scleroperalian . However, other jaw elements appear also to have characteristics of the trophi of Rotifera. Jaw-like structures are found in other taxa as well-for instance, in proboscises of kalyptorhynch platyhelminths, in dorvilleid and aplacophoran mollusks-but studies of their ultrastructure show that none of these jaws is homologous with jaws found in Gnathostomulida, Rotifera, and Micrognathozoa. The latter three groups have recently been joined into the monophylum Gnathifera Ahlrichs, 1995, an interpretation supported by the presence of jaw elements with cuticular rods with osmiophilic cores in all three groups. Such tubular structures are found in the fulcrum of all Rotifera and in several cuticular sclerites of both Gnathostomulida and Micrognathozoa. The gross morphology of the pharyngeal apparatus is similar in the three groups. It consists of a ventral pharyngeal bulb and a dorsal pharyngeal lumen. The absence of pharyngeal ciliation cannot be used as an autapomorphy in the ground pattern of the Gnathifera because the Micrognathozoa has the plesiomorphic alternative with a ciliated pharyngeal epithelium. The body of Limnognathia maerski nov. gen. et sp. consists of a head, thorax, and abdomen. The dorsal and lateral epidermis have plates formed by an intracellular matrix, as in Rotifera and Acanthocephala; however, the epidermis is not syncytial. The ventral epidermis lacks internal plates, but has a cuticular oral plate without ciliary structures. Two ventral rows of multiciliated cells form a locomotory organ. These ciliated cells resemble the ciliophores present in some interstitial . An adhesive ciliated pad is located ventrally close to a caudal plate. As in many marine interstitial -e.g., gnathostomulids, gastrotrichs, and polychaetes-a special form of tactile bristles or sensoria is found on the body. Two pairs of protonephridia with unicellular terminal cells are found in the trunk; this unicellular condition may be the plesiomorphic condition in . Only specimens with the female have been found, indicating that all adult animals are parthenogenetic females. We suggest that 1) jaws of Gnathostomulida, Rotifera, and the new taxon, Micrognathozoa, are homologous structures; 2) Rotifera (including Acanthocephala) and the new group might be sister groups, while Gnathostomulida could be the sister-group to this assemblage; and 3) the similarities to certain gastrotrichs and interstitial polychaetes are convergent. Copyright 2000 Wiley-Liss, Inc. Limnognathia maerski (128 microns in size)

It also has many other unique bodily structures which earned it its own class, "Micrognathozoa." It has three distinct segments, including a head, abdomen, and an accordion-like thorax, which it can extend to become flexible while moving. The animal averages only one-tenth of a millimeter long, or about the width of a very fine blood vessel, making it one of the smallest known. 1. Limnognathia maerski between the in the cold spring at Isunngua, Disko Island. 2. Using its ventral ciliation it may either swim (see swimming animal on picture)

or glide slowly on the leafs, looking for food. 3. Its main diet is , blue-green algae and diatoms. 4. During the search for food, the head is moved slowly and rhythmic from side to side while the cilia on the forehead beat the food particles towards the mouth. 5. When a food item reaches the mouth it is quickly grabbed by the ventral jaws. 6. L. maerski appears to prefer cold temperatures. The spring at Isunngua is most years frozen from October to May and during the short summer the average temperature is about 8oC. In the summer period it has to eat and reproduce before the water freezes again. 7. Adult individuals cannot survive the Limnognathia maerski (128 microns in size) winter, thus they have to produce thick- shelled resting eggs that can tolerate freezing.

CILIACIÓN 1. Besides the stiff sensory cilia, L. maerski has different groups of motile cilia. The ciliated areas comprise the ventral part of the anterior and lateral head regions and the ventral part of the thorax and abdomen. 2. The cilia in the forehead is arranged in horseshoe- shaped bands and are capable of creating a current that leads food particles towards the mouth. 3. The cilia on the lateral sides of the head and on the ventral side of the trunk is arranged in a completely different pattern. 4. The cilia are arranged in groups, and in each group the cilia arise from a single cell and beat in unison. 5. This kind of ciliary arrangement is called ciliophores and it is only known from two groups of microscopic polychaetes, the Diurodrilidae and the Neotenotrocha. 6. Four pairs of ciliophores are present on the lateral sides of the head, and on the ventral side of the trunk the ciliophores are arranged in a double row. The ciliophores are used for locomotion

Limnognathia maerski (128 microns in size) MANDÍBULA OBSERVADA AL MICROSCOPIO ELECTRÓNICO

1. Scanning electron microscopy (SEM) is an extremely useful tool to obtain information about the 3-dimensional organization of an animal or a prepared organ. 2. The object is placed on a small aluminium stub inside the microscope and is subsequently bombarded with electrons that are reflected so they produce a picture of the object on a monitor. 3. While the object is inside the microscope it can be rotated and tilted so it is possible to observe it from different angles. 4. Despite their minute size the jaws of L. maerski can be prepared for SEM by placing the animal in a small drop of dilute commercial chlorine. 5. After a few minutes all soft tissues will be dissolved leaving only the hard jaw parts. When the animal is dissolved the chlorine is removed with a very fine pipette and the jaws are carefully rinsed with distilled water. 6. The SEM-investigations of the jaws in L. maerski have been a very valuable method to understand the complex morphology. Limnognathia maerski

SEM-photographs showing the jaws from four different specimens of Limnognathia maerski. Fig. A shows the jaws in dorsal view, B in lateral view, and C and D in ventral view Limnognathia maerski

When working with complicated morphological structures it may be useful to make generalized schematic images of the investigated organs. The simplest kind of presentation is the line drawings, however these may be insufficient when working with very complex organs. Thus, to give a more comprehensible presentation of the jaws they were reconstructed in a 3-dimensional illustration. The reconstructions were made in the computer program Rhinoceros, which is primarily made for industrial design. However, it also turned out to be very useful to illustrate complex organic structures. The 3-D illustrations were based on extensive SEM studies of prepared jaws, and afterwards each jaw element was rebuild in Rhinoceros using relatively simple geometric figures. When finished it is possible to tilt and rotate the structures so they can be viewed from all possible angles Limnognathia maerski Limnognathia maerski

Sensory organs and nervous system Limnognathia maerski

1. A large cerebral ganglion that occupies most of the frontal part of the head forms the brain in Limnognathia maerski. A pair of ventral nerve cords extends from the brain towards the animals’ tail. 2. This kind of nervous system is found several minor worm-like animals. 3. The whole body of L. maerski has scattered sensory bristles composed of one or up to three adjoined cilia. 4. This kind of sensory structures resemble the sensoria found in the Gnathostomulida (jaw worms). However, in L. maerski all cilia in the bristles that are composed of more than one appear to arrive from one single cell (multicilliarity), whereas the cilia in the gnathostomulid sensoria always arrive from one cell each (monociliarity). Thus, it is not likely that the sensory structures in the two groups are derived from a common form. Reproduction The reproductional cycle in L. maerski is only scarcely known. Even though the type locality on Disko Island has been sampled every summer since 1994 and the collected animals afterwards have been cultured in the laboratory, only specimens with female organs have been recorded. Based on this it has been concluded that L. maerski most of the time reproduce by parthenogenesis, which means that the females produce offspring without initial fertilization from a male. In other words, the maternal individuals are able to clone themselves.

However, there are indices that males could be present in a short period. We know that L. maerski is capable of producing two kinds of eggs. A thick-shelled, ornamented type (see the pictures) and a more thin-shelled, smooth type. The thin-shelled type is a summer egg that quickly will hatch, whereas the thick-shelled Limnognathia maerski (128 microns in size) type probably is a winter egg that are able to withstand freezing in the cold season. A similar reproductional cycle, with two kinds of eggs, is known from the group Rotifera (wheel animals) and in this group we know that the thick-shelled eggs only are produced after male fertilization. Hence, it has been proposed that L. maerski, like , is dioecious but only with males in a very short period.

However, this theory was recently revised. The newest investigations have shown that very young individuals might have male organs, thus we now believe that L. maerski is a protandrous hermaphrodite, which means that the individuals hatch as males and then become females when they grow older.

Limnognathia maerski (128 microns in size)

A freshwater animal discovered in the frigid springs of Disko Island, West Greenland, puts even the great white shark to shame. With 32 moving parts, the jaw of this microscopic invertebrate, named Limnognathia maerski by it's Danish founders, not only exceeds sharks in its complexity, but any other known invertebrate. While feeding, the animal extends jaw parts from its mouth to grasp the substrate, like two small hands. A freshwater animal discovered in the frigid springs of Disko Island, West Greenland, puts even the great white shark to shame. With 32 moving parts, the jaw of this microscopic invertebrate, named Limnognathia maerski by it's Danish founders, not only exceeds sharks in its complexity, but any other known invertebrate. While feeding, the animal extends jaw parts from its mouth to grasp the substrate, like two small hands.

The discovery adds to a growing number of forms found in extreme environments, such The Isunngua spring where the as scalding hot deep new organism was found. sea thermal vents, where conditions were thought too harsh to harbor life. Limnognathia maerski Cycliophora: un nuevo filo animal en los apéndices bucales de la cigala Nephrops norvegicus (Linnaeus, 1758) Why the excitement about the discovery of a new animal species and phylum? It is not that difficult to find a new species. Place an experienced botanist and zoologist in a poorly investigated area of the world such as virgin rainforest and leave them there for a while to study the fauna and flora. It is quite likely that a tiny beetle, fly or such as a moss may be found which is unidentifiable and is declared a new species. In fact it is believed that millions of species remain undiscovered So why did the discovery of S.pandora excite not just the scientific community but attract media attention world-wide? For two main reasons. Firstly, the organism was not discovered in the depths of a rain forest or an oceanic trench, but on the bristles surrounding the mouth of the Norway lobster Nephrops norvegicus which can be caught in the Kattegat Straits, a busy shipping lane between Sweden and Denmark. You've probably eaten the Norway lobster - as scampi and chips! Secondly, when a new animal species is discovered, no matter how unusual, it can normally be classified into a known group of creatures with the same body plan or phylum. Although there are 1.5 million plus known species in the world, they can all be classified into 35 or so phyla. These include the (eg the such as man), molluscs (snails) and (jointed limbed e.g. insects). However, S.pandora was so unusual that it could not be classified into any of the existing phyla, and a new one was suggested called Cycliophora. Los CICLIOFOROS

Symbion pandora es un animal microscópico (350 m), que ha sido descubierto en las piezas bucales de la cigala, Nephrops norvegicus, donde vive como comensal; presenta un complejo ciclo de vida metagenético, en el que se alternan la reproducción sexual y asexual.

1.Ha sido relacionados con Rotíferos • por su boca rodeada de cilios • por la presencia de machos enanos • y también sobre la base de secuencias moleculares (WINNEPENNINCKX et al., 1998) 2. Ha sido relacionado con Endoproctos y con Ectoproctos. • Por su tubo digestivo en forma de U • Por la formación de yemas internas se han sugerido también afinidades con endoproctos y con ectoproctos (FUNCH y KRISTENSEN, 1995, 1997) • D’HONT (1997) ha sugerido que podrían tener su origen en larvas neoténicas de endoproctos.

3.NIELSEN (2001) evita pronunciarse con respecto a la posición filogenética de mientras no se disponga de más datos sobre su morfología, ontogenia, biología, etc. AX (2000, 2003), considera prematuro establecer las afinidades sobre la base de los datos disponibles.

4.SORENSEN et al. (2000), tras un análisis cladista de los metazoos basado en 66 caracteres morfológicos, concluyen que tanto Symbion se ubica dentro de los protóstomos espirales, Symbion como adelfotaxón de los endoproctos.

Key: 1 - adhesive disc attached to bristle surrounding lobster's mouth 2 - dwarf male 3 - anus 4 - ring of cilia around mouth funnel

Why was it called Symbion pandora? The observation of a feeding stage which contains both an inner bud and a Pandora larva with a miniature feeding stage inside, gave rise to the pandora species name i.e. a reference to Pandora's box of Greek mythology. Symbion is derived from the Greek syn '(together) with' and bios 'living', which refers to the close association the organism has with it's host. The phylum name Cycliophora is a Greek derivative referring to the circular mouth ring

Cycliophora: un nuevo filo animal en los apéndices bucales de la cigala Nephrops norvegicus (Linnaeus, 1758)

Carmen Salas Casanova Hay actualmente en la Tierra unos tres millones de organismos descritos, aunque se cree que el número real de especies vivientes puede ser muy superior (entre 10 y 20 millones según diferentes versiones), sin contar con los que se han extinguido, que posiblemente superan en número a los actuales. El estudio de toda esta diversidad del mundo vivo y su ordenación dentro de una clasificación, en función de las relaciones existentes entre ellos, constituye el objetivo de la Sistemática, una de las disciplinas biológicas. Dentro de cada reino, el filo constituye la categoría superior y en el caso del reino Animal hay 35 filos establecidos. Cada filo representa una línea distinta de la evolución animal, con unos caracteres fundamentales (aunque a primera vista parezcan sutiles) que se reflejan en un plan básico corporal, por ejemplo los Moluscos o los Cordados. La descripción de un nuevo filo supone todo un reto para los taxónomos ya que implica el hallazgo de seres vivos en los que su plan básico corporal sea completamente diferente al de todos los otros filos conocidos y por tanto, su biología también presente características propias. Durante el presente siglo se han descrito 3 filos nuevos para la ciencia: Pogonóforos, habitantes de las profundidades marinas, Gnatostomúlidos y Loricíferos, de pequeño tamaño y componentes de la llamada fauna intersticial marina. Recientemente, P. Funch y R.M. Kristensen, dos investigadores daneses de la Universidad de Copenhagen acaban de describir un nuevo filo de diminutos organismos -menos de 1 mm de longitud-, epizoicos y que viven en agregación, al que han denominado Cycliophora, derivado de los términos griegos “cyclion”= pequeña rueda y “phoros”= llevar , en alusión a una corona de cilios que rodea la boca de dichos organismos [Nature, 378:711, (1995)]. Como señala S. Conway Morris [Nature, 378: 661, (1995)] lo que más sorprende del hallazgo, aparte de la especial anatomia y complejo ciclo biológico, es el peculiar hábitat de la nueva especie Symbion pandora Funch & Kristensen, 1995 (Fig.1), los apéndices bucales de la cigala (Nephrops norvegicus) que todos conocemos y que posiblemente hemos comido con ciclióforos. El nuevo animal es un metazoo marino, triblástico, de simetría bilateral y acelomado. La nueva especie presenta tres tipos de individuos: asexuados, llamados por los autores “estados alimentarios”, son de mayor tamaño y llevan adheridos los machos, el segundo tipo de individuo, de pequeño tamaño. Las hembras constituyen el tercer tipo y son de tamaño intermedio. El cuerpo de los individuos ASEXUADOS o estados alimentarios es ovoide, envuelto por una cutícula. Presentan un embudo bucal rodeado por una corona de cilios en la parte superior del tronco. En la parte inferior hay un corto pedúnculo o tallo que termina en un disco adhesivo, con el que se fija a las mandíbulas de la cigala. Tanto el pedúnculo como el disco basal son de naturaleza cuticular. El animal es transparente y en su interior se aprecia un tubo digestivo en forma de U, con el ano situado en las proximidades de la base del embudo bucal. En la parte basal del tronco aparecen yemas internas que corresponden a embriones en diferentes estados de desarrollo. Los machos carecen de digestivo, y presentan la cavidad interna ocupada por espermátidas, por una estructura tubular, considerada como un pene, y por mesénquima. Las hembras carecen tambien de digestivo y en su interior presentan un gran ovocito, su forma es distinta, ovoidea con una corona de cilios anterior, que orientan ventralmente para fijarse por ella a la boca de la cigala. Si la morfología de estos animales es curiosa, más sorprendente resulta su ciclo biológico, extremadamente complejo (Figura 2), con una fase asexual y otra sexual, esta última parece estar relacionada con el proceso de muda de la cigala. Durante la FASE ASEXUAL, cuando los individuos asexuados alcanzan una cierta “madurez” aparecen yemas internas que se transforman en unas larvas, llamadas “pandora” capaces de generar nuevos individuos asexuados. Cuando la cigala está próxima a terminar la muda,comienza la FASE SEXUAL. Algunas formas asexuadas forman en su interior yemas que se trasforman en machos, los cuales salen y se fijan a individuos asexuados que tengan yemas hembras en su interior, para facilitar la fecundación. No se conoce el mecanismo de fecundación, pero las hembras salen del individuo asexuado con el oocito ya fecundado y se fija en las mandíbulas de la cigala una vez finalizada la muda de ésta. Del zigoto sale una larva, llamada “cordoide”, que es lecitotrófica (no necesita comer) y que se va fijar en un nuevo hospedador - una nueva cigala -, donde sufre la metamorfosis para dar un nuevo individuo asexuado y comenzar de nuevo el ciclo. RELACIONES FILOGENÉTICAS de los Cycliophoros

Con respecto a las posibles relaciones filogenéticas del nuevo filo, según Funch y Kristensen los caracteres morfológicos los relacionan con los protóstomos, ejemplo, la epidermis multiciliada similar a la de Rotíferos y larvas de tipo espiralia (filos con segmentación espiral en su desarrollo embrionario), la cutícula similar a la de los Gastrotricos y algunos Nemátodos; la presencia de protonefridios en la larva “cordoide”, entre otros. También el ciclo biológico, los relacionan con los protóstomos. Por otro lado la formación de yemas internas aparece en Ectoproctos, y según un estudio reciente de Manylov [Acta Zool., 76: 1 (1995)] también en Gastrotricos, como mecanismo de regeneración. Por otro lado, la larva “cordoide” tiene una ligera semejanza con la de Endoproctos, en la cual la metamorfosis se hace mediante yemas internas [Mariscal, 1975, .In :Reproduction of marine invertebrates (Eds. A.C. Giese & J.S. Pearse), vol.2, pp.1-41, Academic Press]. Parece pués, que el nuevo filo deberia de encuadrarse dentro de los denominados Asquelmintos o pseudocelomados. Posiblemente, los Cycliophora podrían ser formas paedomórficas (que han retenido caracteres juveniles ancestrales en los descendientes), probablemente de larvas de endoproctos, que han madurado precozmente (progénesis), lo que de ser así sería un caso curioso, ya que los endoproctos son a su vez considerados formas paedomórficas de larvas tipo trocófora, también mediante progénesis [Olive,P.J.W., 1985, Covariability of reproductive traits in marine invertebrates: implications for the phylogeny of the lower invertebrates. In : The origins and relationships of lower invertebrates (Eds. S. Conway Morris; J.D. George, R. Gibson and H.M. Platt). The Systematic Association Special, Vol.28: 42- 59.] Carmen Salas Casanova es Profesora Titular de Biología Animal Symbion pandora

Symbion pandora ad - disco adhesivo an - ano as - rama ascendente del tubo digestivo cc - cilios compuestos ce - epidermis ciliada co - cuello de - rama descendente del tubo digestivo ep - epidermis ga - ganglio nervioso gl - células de relleno ib - yema interna mc - células mioepiteliales mr - anillo bucal ne - anillo nervioso p1 - pene 1 p2 - pene 2 sc - células digestivas sp - esperma s1 - esfínter 1 s2 - esfínter 2

FIGURA 153. Symbion pandora (FUNCH y KRISTENSEN, 1995 The structure and life cycle of Symbion pandora The organism has a very complex life cycle, with a number of well-defined sessile and free swimming stages with different morphologies. The largest and most illustrated phase is the feeding stage. The key features of this stage are as follows. - typically 350 µm long - attached by an adhesive disc to the lips of the lobster - feeds using a mouth surrounded by a ring of cilia - excretes via an anus next to the mouth ring - the feeding stage continually produces inner buds which replace the feeding structures - both asexual and sexual reproduction can occur View a beautiful annotated diagram of Pandora's feeding stage showing it's cellular structure. Pandora's sex life is so peculiar that scientists at the Natural History Museum, UK were left puzzled! A museum scientist was quoted as saying 'it is unlike anything we have seen before'. There was a debate whether Pandora had one penis or two, a fact eagerly announced by the popular press! Pandora's sexual and asexual reproductive cycle is explained and illustrated in Reference 1. The essential features of the sexual cycles are as follows. As well as sessile (stationary) stages there are three free-swimming stages i.e.. - a larva containing a new feeding stage - dwarf males (size typically 84 um) which settle on feeding stages - females which attach to the lobster, and subsequently degenerate producing larva which disperse. All the free-swimming stages do not feed and are short-lived. Sexual reproduction is initiated when the lobster is near the end of its moult cycle. At this stage sexually mature feeding stages are found attached to the lobster's lips. The asexual aspect of the life cycle explains why large populations of feeding stages with no mature sexual stages have been found on lobsters. Cycliophora, in which S.pandora has been placed, is thought to have affinities to the phyla Entoprocta and Ectoprocta. The Ectoprocta are also known as the bryozoans or moss animals. Bryozoans are microscopic invertebrates that can form beautiful colonies which are often attached to rocks and in both fresh and seawater. The bryozoans reproduce asexually by a budding process similar to that of Pandora. Key ad - adhesive disc an - anus as - ascending branch of the digestive system cc - compound cilia ce - ciliated epidermis co - constriction (or 'neck') de - descending branch of the digestive system ep - epidermis ga - ganglion gl - gut lining cell ib - inner bud mc - myoepithelial cell mr - mouth ring ne - nerve p1 - penis 1 p2 - penis 2 sc - stomach cells sp - sperm s1 - sphincter 1 s2 - sphincter 2