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Speciation and Timeâšrelationships of the Nemertines to The BULLETIN OF MARINE SCIENCE, 45(2): 531-538, 1989 SPECIATION AND TIME-RELATIONSHIPS OF THE NEMERTINES TO THE ACOELOMATE METAZOAN BILATERIA Nathan W Riser ABSTRACT The discovery of a marine nemertine, lacking a proboscis apparatus, in kelp holdfasts in New Zealand induces a re-evaluation of the phylum Nemertinea and its relationship to the Bilateria, Morphological features, which can be interpreted as primitive, are analyzed and a time frame for changes in the basic morphology is suggested. Soft-bodied invertebrates have a very poor fossil record: Valentine (1987, fig. 2, 7) reports that first records of Nemertinea appear in the Carboniferous and Platyhelminthes and related Gnathostomulida in Recent (Quaternary) times. Schram (1973: 989) concluded that Archisymplectes rhothon Schram from the middle Pennsylvanian ofIllinois " ... shows remarkable similarities to nemertines, and demonstrates the ancient status of that phylum." Conway Morris (1977) considered Amiskwia sagittijormis Walcott, originally placed in Chaetognatha, difficult to place in a phylum although Owre and Bayer (1962) advocated inclusion in Nemertinea. This imprint from the middle Cambrian Burgess Shales of British Columbia, if a nemertinean, would have to be placed in the Tribe, Pelagica. Brasier (1979: 125) noted that the " ... Cambrian radiation event" possibly lasted only 10 million years and that it produced a "variety of body plan" which during the Cambrian " ... may have exceeded that at any other time in earth history." Gibson (1988) in agreement with Stiasny- Wijnhoff (1923), considered the Pelagica to be the most primitive polystiliferous nemertine taxon. However, the tribe is highly specialized for a bathypelagic existence. Amiskwia, if a nemertine, would indicate extensive involvement of the phylum in the Cambrian radiation event. Just as the Vendian (Precambrian) and Cambrian medusoids cannot be placed definitely in a taxon, the relationship of Amiskwia to other known groups remains hypo- thetical. As a nemertine, it would serve as an indication of the degree to which the acoelomate Metazoa Bilateria radiated during the Precambrian and Cambrian. Even without such evidence, we must assume that the group was part of the radiation event and as with other groups, many lines "were geologically short- lived" (Brasier, 1979: 125). Cataclysms during recorded geological times have decimated entire groups of animals and reduced others to a few relict species. As an example, the Permian/ Triassic crisis had a marked effect on the diversity of ammonoid cephalopods which, however, recovered and were then wiped out by the Cretaceous/Tertiary crisis. The effects of these crises are not as evident in the nautiloid cephalopods which underwent extensive radiation (seven orders) in the early paleozoic and then a steady decline in diversity to one genus today and the extinction of six of the orders. Riser (1985) asked the rhetorical question as to what nemertinean hierarchial units had been extinguished in those two crises. We only know the groups that are present today which survived all of the cataclysms and the selection processes that impacted upon the diversity offorms during the Cambrian radiation event. Twenty three species ofnemertineans, of which eighteen were abundant enough to permit identification, were collected by the author from holdfasts of the brown 531 532 BULLETIN OF MARINE SCIENCE, VOL. 45, NO.2, 1989 alga, Lessonia variegata at Kaikoura, New Zealand in 1986. One ofthese, Arhyn- chonemertes axi Riser, has certain primitive morphological characteristics (Riser, 1988b) which indicate that the species might be a relict of significance analogous to the coelocanth, Latimeria and the monoplacophoran, Neopilina. Distinctive Morphological Characters of Arhynchonemertes axi: 1. Nervous system internal to body-wall musculature; 2. Blood vascular system a simple loop; 3. Excretory system unilateral with dorso-lateral pore behind mouth, much coiled vessel extending posteriorly to initial portion of intestine; 4. Vitellogenic oocytes free in parenchyma; 5. Proboscis apparatus including rhynchocoel absent; 6. Mouth ventral, small, beneath brain; 7. Brain bilobed with single commissure; 8. Cerebral organs lacking; 9. Outer circular and inner longitudinal muscle layers one fiber thick, dorso-ventral muscle fibers abundant from mouth to posterior end of body; 10. Submuscular gland cells not in packets; necks extend individually through epidermis; at least three different types of secretory cells filling parenchyma be- tween body wall musculature and organ systems for the entire length of body; 11. Epidermis and pharyngeal epithelium meet at mouth without intergradation; 12. Tall, ciliated, simple columnar pharyngeal epithelium with neither epithelial nor subepithelial gland cells; 13. No morphological nor histological division offoregut into esophagus and stomach; 14. Intestine straight, without diverticula or caeca; bases of intestinal cells project into surrounding parenchyme; tunica propria not evident; 15. Monoecious, gonads along total length of intestine, testes dorso- lateral, ovaries ventro-lateral, both with preformed ducts and pores. Many of the nemertineans encountered in the holdfasts were also found in other cryptic habitats. It is assumed that A. axi may be cryptic and not restricted to the holdfast habitat, especially that of S. variegata, an austral species. The unilateral excretory system (3) with lateral nephridiopore near the mouth, and no elements of the system anterior to the pore, is unique among nemertines. The taxonomic significance of the unilateral system is unclear especially since the extensively coiled tubules oflarge cells intimately associated with the lateral blood vessels may be compensating for the lack of an excretory system on the other side of the body. The location of the nephridiopore and the absence of the system anterior to the excretory duct leading to the pore may well have phylogenetic significance beyond the species level. PHYLOGENETIC CONSIDERA nONS The simplistic, and thus easiest, interpretation of the systematic position of A. axi is that the species has lost a proboscis apparatus and therefore is specialized. It has been suggested that the species might be an herbivore, based upon the histology of the gut, and that a proboscis would not be required for food capture; that the extensive development of subepidermal glands might indicate production of a toxic or noxious mechanism negating need for a proboscis for defense; and that a proboscis as a locomotor organ would not be required by this small hard surface dwelling species. Malacobdella. a commensal on the gills of clams, has been recognized as the exception to the "rule" that nemertineans are scavengers! carnivores but its feeding mechanism is specialized and furnishes no morpholog- ical evidence that would be distinctive of an herbivore. It is a filter feeder with an apical mouth and vast pharynx which serves as a pump in feeding, and into which the proboscis opens. The small, morphologically distinctive proboscis, has geen retained and is used in the occasional entrapping of food (Gibson and Jennings, 1969). The absence of a dorsal cerebral commissure in A. axi could be RISER: RELATIONSHIPS OF THE NEMERTINEA 533 explained if the species was degenerate and had lost its proboscis apparatus, above which the dorsal commissure would pass. In tum, the absence of cerebral organs the nerves of which are normally associated with the dorsal cerebral ganglia could imply regression of the dorsal lobes of the brain associated with the loss of the proboscis apparatus, however, dorsal ganglia are present in archinemertines, pe- lagic enoplans, Malacobdella, and Carcinonemertes which lack cerebral organs. Brinkmann (1917) considered the lack of cerebral organs in Pelagica to be a secondary loss, while Stiasny- Wijnhoff (1923) maintained that they had never developed in this group. The proboscis nerves of extant nemertines arise from the ventral cerebral ganglia or the ventral commissure, thus degeneration of the dorsal ganglia would appear to have no relationship to the presence or absence of the proboscis. The simple blood vascular system (2) could be attributed to the absence of the proboscis, except that a similar vascular system is present among some archinemertines. The morphology of A. axi cannot be accounted for by the "loss" of a proboscis apparatus. The relationship of the mouth to the brain is the only anoplan characteristic. It is borderline however, since the plasticity of the precerebral region of a nem- ertinean (see Riser, 1988a: 131) can influence such a relationship and with the two organs as close to one another as in this animal, a definitive statement- anterior to or posterior to, cannot be applied. Character 1 is definitive for the class Enopla. The nervous system of Archi- and Hoplonemertinea develops in the blastocoel according to Iwata (1960: 48) and external to "the mesodermal layers of the body wall" in the paleo- and heteronemertines. The system in ar- chinemertines comes to lie in the internal longitudinal muscle layer and in ho- plonemertines it comes to lie internal to that layer. Enoplans do have submuscular epidermal glands but they are usually of one type and are arranged in packets rather than as different types of gland cells (mucous, homoserous, and granular bacillary) indiscriminately mixed and each reaching the surface independently. Some anoplans have subepidermal packet glands extending into the muscle layers, and some of these glands
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