Syllids
| Syllids | ||||||||||||
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| Syllidae | ||||||||||||
| Pit , 1850 |
Sylliden (Syllidae) are a family of polychaete (Polychaeta), whose representatives almost all marine colonize habitats. The size of adult specimens ranges from less than 1 mm to 90 mm. Most species live on the sea floor ( benthos ) or in the sand gap system ( mesopsammon ); They are free-swimming (pelagic) only during the breeding season.
The species of the larger Syllinae , Eusyllinae and Autolytinae live mostly benthic, colonize primarily hard soils, but also occur silty or calcareous substrates. Often seaweed , green algae (e.g. Caulerpa , Enteromorpha ), red algae or rock growth are populated with cushion algae . Living coral stocks or the canal systems of sponges (Porifera) also serve as a habitat or food base, whereby the hosts are usually populated in a species-specific manner. Also living tubes of other fixed polychaetes are visited. The very small Exogoninae , some of which are less than 1 mm in size, live mainly in the interstitial of mostly coarse-sand sediments.
Parasitism is known only from a few species (e.g. Trypanosyllis asterobia , on mussels or starfish ; Haplosyllis cephalata , ectoparasitic on other polychaetes).
A special property of some syllids (e.g. Odontosyllis phosphorea ) is their ability to produce bioluminescence , the stimulation of which is induced by moonlight during sexual maturity.
diagnosis
morphology
The body of the syllids is almost cylindrical in cross section. The anterior head lobe ( prostomium ) has paired large, forward-facing palps and dorsally usually three, articulated or non-articulated appendages (antennae). In addition to four complex eyes, there may be tiny forehead eyes (ocelles). The segment following the prostomium ( peristomium ) has paired dorsal and ventral appendages (peristomial cirrus, tentacle cirrus). The bristle segments following the peristomium have paired skin flaps that serve for locomotion ( parapodia ). These have internal supporting bristles (aciculae) and numerous bristles protruding from the parapodial lobe, which are usually very complex (composed of the bristle shaft and bristle end link) and species-specific. Dorsally, the parapodia are each equipped with a mostly longer, articulated or un articulated appendix (dorsal cirrus) and ventrally with a short un articulated appendix (ventral cirrus). The last segment ( pygidium ) is always bristle-free and also has paired appendices (anal cirrus), sometimes also a short unpaired appendix.
The three-part foregut of the syllids is unique within the polychaetes. It extends over several segments and consists of the anterior, evertable pharynx , the muscular proventricle and the esophagus behind it , which leads into the actual intestinal canal. As an endocrine organ, the proventricle controls the reproductive process of the syllids and is a characteristic unmistakable feature of this family. The pharyngeal opening can have a single tooth (“armament”), the function of which is unclear.
reproduction
At the time of sexual maturity , most of the syllids gather to reproduce on the surface of the sea. This requires morphological and physiological transformations of these otherwise benthic animals ( epitocia ), which result in a change in movement (fast continuous swimming ) and a change in behavior (positive phototaxis , reactions to sex pheromones , cessation of food intake). The entire muscular system is transformed, the muscle fibers show an increase in stored glycogen and a multiplication of mitochondria . From the outside, this transformation is mainly recognizable by the enlargement of the eyes and the extension of the antennae as well as by the appearance of numerous floating bristles (capillary bristles).
A distinction is made between two basic forms of epitocia: epigamy , in which the entire animal is transformed into an epitocial form, and schizogamy , in which the animals transform only their posterior segments into epitocial sexual forms, which are able to move in the pelagic region after being pinched off by the mother .
Modified modes of reproduction are viviparity , hermaphroditism , external gestation, lateral budding and architomy.
Epigamy
In epigamy , the entire animal transforms into an epitoke form ("heterosyllis"), with the animals' eyes increasing in size and the antennae in length. The oocytes or spermatocytes develop in almost all postproventricular body segments. The egg cells are usually fertilized externally ( ontogenesis via free-swimming trochophora larva).
Epigamy occurs in most Eusyllinae (e.g. Odontosyllis ctenostoma ) and Exogoninae (e.g. Exogone gemmifera ), but has also been described within the Autolytinae (e.g. Autolytus longeferiens ).
Schizogamy
In schizogamy , in the course of a metamorphic process that runs parallel to sexual maturation, the fully developed animals produce epitoke sex animals ( stolons ), which after detachment ( stolonization , budding ) from their parent animal (wet nurse, stick) lead a short free-swimming (pelagic) life of their own. The female and male stolons release their gametes for external fertilization to the outside ( ontogenesis via free- living trochophora larva). After fulfilling their task as sexual stages, the stolons who are unable to eat independently perish. The tribe individuals (atoke animals) that remain on the ground, on the other hand, survive the stolonization process and can regenerate and stolonize the segments that have been lost in the form of stolons.
Schizogamy is typical of the Syllinae and Autolytinae (e.g. Autolytus prolifer ), but is also said to occur within the Exogoninae.
Viviparity
Viviparia is a form of brood care in which the next generation develops up to the juvenile stage in the mother's coelom space . The juveniles are fully equipped with a two-part pharynx and several bristle-bearing segments. It is released by breaking open the mother's epidermis . There is no free- living trochophora .
Viviparie is only known from a few Syllinae (e.g. Typosyllis vivipara , Dentatisyllis mortoni ) and Exogoninae (e.g. Exogone hebes ).
Hermaphroditism
Hermaphroditism has only been proven for a few syllids. Simultaneous hermaphrodites, which form oocytes and sperm at the same time, and succedan hermaphrodites, which form oocytes and sperm at different times, are known.
Simultaneous hermaphrodites are known within the Syllinae (e.g. Typosyllis amica ) and Exogoninae (e.g. Sphaerosyllis hermaphrodita ). A succedan hermaphrodite that forms male gametes in autumn and female gametes in spring is e.g. B. Grubea protandrica (Exogoninae).
External gestation
External gestation is a form of brood care in which the embryonic development takes place on the outside of the mother, where the subsequent generation usually develops with the rear end attached to the lateral side of the segments or to the dorsal cirrus of the mother until the point of detachment. Only after completion of the juvenile development (possession of eyes, 3–5 bristle segments, pharynx) do these individuals break away from their mother. There is no free- living trochophora .
External gestation is typical of the Exogoninae (e.g. Exogone gemmifera ).
Lateral budding
One phenomenon is the lateral budding ("collateral budding") of the Syllis ramosa, which occurs in deep-sea sponges ( Hexatinellida ) . This species propagates through branching, whereby the shoots do not develop intersegmental, but instead of a dorsal cirrus of an already existing segment (“regeneration budding”) or on newly formed segments without parapodia and without cirrus (“intercalary budding”). There is no free- living trochophora .
Architomy
An unusual type of reproduction for Syllidae is architomy ("fragmentation"), in which the individuals disintegrate into units of two, three or more segments and from these new, fully developed individuals emerge (e.g. Autolytus pictus ).
distribution
The distribution area of the Syllidae extends to both hemispheres , from the Arctic to the Antarctic. In all marine areas, the Pacific , the Atlantic , the Indic , but also smaller seas surrounded by land, e.g. B. the North Sea or the Mediterranean , syllids are common. In particular, warmer tropical or subtropical regions are populated. Vertical syllids are common both in the shallow eulitoral and in the deep sea .
Brackish water or groundwater near the coast are rarely colonized by syllids. Most species prefer clean, oxygen-rich water, although light water pollution from several species is acceptable.
Taxonomy
The traditional polychaete taxonomy divides the species of the Syllidae into four subfamilies: Syllinae , Eusyllinae , Exogoninae and Autolytinae . The earliest subdivision of this taxon (still called Syllidea Grube , 1850) goes back to Langerhans (1879). He distinguished (1) Syllideae ("Syllideae palpis non coalitis"), (2) Exogoneae ("Syllideae palpis coalitis prominentibus, in pharynge recto brevi dente uno") and (3) Autolyteae ("Syllideae palpis coalitis, ventralibus; dent. Ost. Dent ").
Malaquin (1893) recognized the presence or absence of parapodial ventral cirrus as the most important characteristic for the separation of the Syllidae: Species without ventral cirrus were placed under the (1) "Autolytés", clearly a clear autapomorphy . Within the species with ventral cirrus, Malaquin (1893) retained the "Exogonés", but divided the remaining species into "Syllidés" and "Eusyllidés". Their differential diagnostic features are: (2) "Exogonés ... ont les palpes soudés sur toutes leur étendue", (3) "Eusyllidés ... ont leurs palpes soudés à la base seulement et divergents au sommet", (4) " Syllidés ... l'absence de soudure “. According to Malaquin (1893), “Syllidés” and “Eusyllidés” also differ in the form of their cirrus: “des cirres nettement moniliformes, c'est-à-dire formés d'articles” as in Syllis and “des cirres cylindriques, presentant seulement des constrictions superficielles ”as in Eusyllis .
The Latinized names of these taxa were probably first used by Fauvel (1923). Since that time the Syllidae are more numerous due to the new description, e.g. Some very closely related species have become one of the largest and most confusing taxa within the polychaete (approx. 60 genera). It turned out that the "differences between the subfamilies ... especially between the Eusyllinae and Syllinae appear to be of more practical than scientific value" (Fauchald 1977). So genera were shifted back and forth between the subfamilies.
It is very likely that the Eusyllinae comprise an artificial group of polychaetes, since the species in this subfamily are presumably not attributable to just one common ancestral species . The current taxonomy of the syllids based on these morphological features must therefore be described as unsatisfactory (San Martín 1984).
Web links
literature
- K. Fauchald: The polychaete worms. Definitions and keys to the orders, families and genera. Science Series, 28, 1977, 1-188. Los Angeles (Natural History Museum of Los Angeles County).
- P. Fauvel: Polychètes érrantes. In: Faune de France. Vol. 5., 1923, 1-488. Paris (Lechevalier).
- AE Grube: The families of the annelids. In: Arch. Naturgesch. 1850, 16: 249-364.
- P. Langerhans: The worm fauna of Madeira. Pt. 1. In: Z. wiss. Zool. 1879, 32 (4): 513-592.
- A. Malaquin: Recherches sur les Syllidiens. Morphology, anatomy, reproduction, développement. In: Mém. Soc. sci. agricult. arts. Lille 1893, 4th series, 18: 1–477.
- G. San Martín: Estudio biogeográfico, faunístico y systemático de los poliquetos de la familia sílidos (Syllidae: Polychaeta) en Baleares . Tesis Doctoral 187/84, Universidad Complutense de Madrid, Madrid, 1984, 1-529.