Nemertea – Ribbon Worms Martin Thiel & Jon Norenburg
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Nemertea – Ribbon Worms Martin Thiel & Jon Norenburg 369 Nemertea – Ribbon Worms Martin Thiel & Jon Norenburg General Introduction Morphology and Biology Development can be direct (in the Hoplonemertea) Nemerteans, also called ribbon worms, are slender or indirect. Direct development can be via a non- Phylum Nemertea Phylum worms without external body appendages (Figs. 1&2). feeding planktonic larva or an encapsulated larva with They range in length from a few millimetres to several a benthic crawl-away juvenile. Indirect development meters, while their body diameter rarely exceeds a few is typical for the Pilidiophora (which includes the old mm, but can be up to 20 mm. One nemertean species, order Heteronemertea) and usually involves a feeding Lineus longissimus (Fig. 3), has been recorded in the planktonic larval stage (Pilidium) of variable duration book of Guinness 1995 as the longest living animal on (days to weeks), but this may be modified as a non- earth. In this species, individuals of more than 30 m body feeding benthic or planktonic development (e.g., Iwata length with a body diameter of a few mm have been or Desor’s larva). recorded. However, the common nemerteans that one All nemerteans have an eversible proboscis, which typically finds are several cm up to 50 cm in length. Most they employ in prey capture and sometimes for escape. nemerteans are marine but there are also a few freshwater Some species actively forage for their more sedentary and fully terrestrial species. With the exception of a few prey, while others hide in strategic positions, e.g. in hermaphroditic species, nemerteans are gonochoric. mussel or algal patches, waiting for their mobile prey Fig. 1 370 Epidermis Subepidermal musculature Dermal Glands Dermal connective tissue Rhynchocoel Mid-dorsal blood vessel Intestine Lateral blood vessel Lateral nerve cord Longitudinal musculature Circular musculature Fig. 2 species. Due to the lack of any external appendages, which comprise the Monostilifera (with one functional nemerteans move either by cilia (in the smaller species) stylet) and the Polystilifera (with several functional stylets). or peristaltic contractions of the body musculature Based on their results, these authors abandoned the (in the larger species). Consequently, nemerteans are Anopla but created the Pilidiophora, which includes all relatively slow-moving organisms, comparable in speeds of the Heteronemertea and some former Paleonemertea. to snails, sea stars and sea urchins. Upon encountering a The Pilidiophora and the Hoplonemertea form the prey organism, however, a nemertean can rapidly evert new clade Neonemertea, which is separated from the its proboscis and immobilize prey with highly potent remaining taxa previously belonging to the class Anopla, toxins. Nemerteans of one major group (Hoplonemertea) namely the Paleonemertea, which apparently is not possess a stylet in the proboscis, which they use to monophyletic. puncture the hard exoskeleton of prey, mainly small crustaceans. Nemerteans without such a stylet prey on Collection and Identification other worms, mainly polychaetes, but also on bivalves. Nemerteans should be collected alive, because when A few nemerteans are obligate symbionts, living either they are preserved without appropriate treatment their in association with marine decapod crustaceans (mainly posterior identification may be severely compromised. crabs) and preying on their developing embryos or living When fixed in formalin or ethanol, most species contract in the mantle cavity of bivalves and feeding on particles or fragment, evert the proboscis and moreover lose brought in by the host. their colour. Thus, in preserved samples one often finds nemerteans, but these are usually impossible to identify. Phylogeny Large species from intertidal or shallow subtidal hard Based on morphological characteristics, two classes bottoms can be found by turning over rocks and small of nemerteans had been previously distinguished: the boulders. Upon encountering a nemertean, it should be Enopla with one or more stylets on the proboscis and the carefully transferred into a sampling jar with the aid of a styletless Anopla. Recent molecular analyses confirmed spatula, soft forceps or a paint brush. Smaller species (less that the Enopla are monophyletic but challenged the than 5 cm in body length) can best be obtained alive by monophyly of the Anopla. Thollesson & Norenburg transferring benthic substrata (gravel, coral rubble, mussel (2003) distinguished the Enopla (=Hoplonemertea), patches, kelp holdfasts) to the laboratory (in large buckets 371 or trays) and maintaining them in standing seawater for a to prepare these delicate worms for histology, they must few hours. The agitation during sample transfer usually be very carefully preserved (after anesthetizing them) in induces the nemerteans to abandon their hiding places, particular liquids and then embedded in paraffin wax. which is when they can be easily found, often crawling The embedded worms are then sectioned and stained for along the wall of the buckets or trays. They can then be better visualization of the relevant internal organs. placed in a petri-dish to observe them under a dissecting These complicated procedures represent an important microscope. Careful observation of the living specimen obstacle to species identification and it is mainly for this allows identifying important body characteristics that reason that for many regions of the world we have no often cannot be seen after preservation, such as body thorough inventory of nemertean diversity. In contrast, it colour, cerebral furrows, number and pattern of eyes, is relatively easy to match unknown nemerteans to those movement patterns and general body shape. described from other regions of the world, which has led to The identification of nemerteans is further complicated many questionable species distributions. The lack of reliable Phylum Nemertea Phylum due to their lack of external body characteristics. species identifications often also impedes studies of their Consequently, their identification is almost entirely biology and ecological role. Nevertheless, when found based on histology, where the number and thickness alive, some of the more common nemertean species can of muscle layers, position of the cerebral ganglia and be easily identified based on their body size and primarily the nerve cords relative to body-wall musculature, on their colouration. While poorly known to the general structure of the rhynchocoel and the proboscis, and public and even to many marine biologists, nemerteans are other characters are of importance (Figs. 1&2). In order ubiquitous predators in most marine habitats. Nemerteans of the Chilean Fjord Region In this chapter four of the best known nemertean Classification species from southern Chile are shown and described. Class Neonemertea Thollesson & Norenburg, 2003 Common names are given by the editors to enhance Order Enopla (=Hoplonemertea) Schultze, 1852 local non-scientific species knowledge. For Chile about Suborder Monostilifera Brinkmann, 1917 45 nemertean species have been recorded (R. Gibson, Family Prosorhochmidae Bürger, 1895 personal comment) and several of these are found in Prosorhochmus nelsoni (Sanchez, 1973) the fjord region of southern Chile. Their identification is Order Pilidiophora (previously Anopla) complicated and next to nothing is known about their Thollesson & Norenburg, 2003 biology. The first records of the Nemertean fauna from Suborder Heteronemertea Bürger, 1892 Chile come from Bürger, Isler and Coe (for references Family Baseodiscidae Bürger, 1904 see Friedrich, 1970). These records were synthesized Baseodiscus aureus (Bürger, 1896) by Friedrich (1970), who also examined the nemerteans Family Lineidae McIntosh, 1873–74 collected in 1948–1949 during the Lund University Lineus atrocaerulus (Schmarda, 1859) Chile Expedition (unfortunately most of these samples Parborlasia corrugatus (McIntosh, 1876) were preserved without particular procedures, making a correct identification improbable). During the 1980s, Order Enopla (=Hoplonemertea) Dr. Malva Sanchez added several new records to this As in all other hoplonemerteans, esophagus opens internally list. She and her co-workers also described several new from the rhynchodaeum (Fig. 1) — consequently there is species, including Prosorhochmus nelsoni (originally no external mouth opening in P. nelsoni (and in most other described as Amphiporus nelsoni) and Procephalothrix enoplans). 2-layered body-wall musculature with outer circular hermaphroditicus, both also known from southern Chile. muscle layer and interior longitudinal muscle layer (Fig. 2). The complete species list presented in Appendix 2 is Lateral nerve cords medial to body-wall muscle layers. mainly based on the records by Friedrich (1970). Most of these records need to be interpreted with great caution Genus Prosorhochmus and require reconfirmation or re-descriptions. Externally visible characters are an anterior frontal furrow (transverse across the snout), giving the appearance of a 372 smile; 2 pairs of eyes, and lateral, dorso-ventrally oriented, Genus Baseodiscus cephalic furrows. Rhynchocoel extends throughout body Externally visible characters are presence of ventral mouth, length, proboscis armed with 1 central stylet and 2 pouches and pair of lateral, dorso-ventrally oriented, cephalic of accessory stylets. Dermis and body-wall musculature furrows mark back of so-called head region, with pore to moderately thick. Frontal