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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Octolasium

Octolasium Oerley, 1885 (ref. ID; 3692) or Orley, 1855 (ref. ID; 6916)

Family Lumbricidae (ref. ID; 1928, 6033, 7651)

ref. ID; 6916

The main diagnostic criteria for this genus are the wide arrangement of the setae posterior to the clitellum, epilobous prostomium, first dorsal pore between 9/10 and 12/13, calciferous gland with pouches opening vertically into the equator of the oesophagus in 10, aligned and obvious nephidriopores and elongated ocarina-shaped bladders, four pairs of seminal vesicles (9, 10, 11 and 12), two pairs of spermathecal pores in 9/10 and 10/11, and multilobate typhlosole (Bouche 1972; Gates 1975; Fender 1985; Sims and Gerard 1999; Csuzdi and Zicsi 2003). In the European Atlantic area the genus is represented by two widespread species, O. cyaneum (Savigny, 1826) and O. lacteum Orley, 1881, which differ in size, position of the clitellum and tubercula pubertatis and their ploidy. Because high polyploidy is only tolerated by means of unisexual reproduction (Gregory and Mable 2005), both species are considered to be parthenogenetic, where chromosome number is maintained through premeiotic doubling of the chromosome number is maintained through premeiotic doubling of the chromosome number during the last oogonial mitosis (meiotic thelytoky) (Omodeo 1952, 1955; Terhivou and Saura 1993; Gregory and Mable 2005). However, sexual reproduction has been attributed to the diploid populations of O. lacteum collected in Switzerland and the British Isles (Muldal 1952; Sims and Gerard 1999), although Casellato (1987) opined that this must be a sub-triploid race. The specimens studied by Muldal (1952) apparently contained a fair amount of sperm in comparison to O. cyaneum, but in both species the spermatheca were always empty. The formation of sperm and spermatophores in these two parthenogenetic earthworms was interpreted by Terhivou and Saura (1993) as a case of pseudogamy, possibly reminiscent of their bisexual past or having an unknown function. Interestingly, two haploid chromosome numbers have been given to the genus Octolasion. Muldal (1952) counted 38 and ca. 190 mitotic chromosomes in his O. lacteum and O. cyaneum specimens, respectively, and from this, assumed that n was 19 (the highest in the family). However, Omodeo (1952) found either 54 mitotic chromosomes (3n) or 72 (4n) in his material from Italy and Greece, which led him to conclude that the correct haploid number was 18 and to consider Muldal's lacteum as a mutant triploid which lost one haploid set of chromosomes. Unfortunately, Omodeo (1952) was unable to check the chromosome counts of Muldal's decaploid cyaneum as a result to difficulties associated with the cytological preparation of the preserved material deposited at the Museum of Zurich. From this, it is clear that a re-examination of the ploidy of this genus using fresh material from their type localities is urgently needed. Our phylogenetic trees show that these two species form a well-supported clade (100% bootstrap support (BS) for 16S and COI), in agreement with previous observations for specimens collected in Finland and Romania (Pop et al. 2008), and confirm their taxonomic status. One important issue which also deserves consideration here is the nomenclatural problems associated with the species O. lacteum (Orley, 1881) according to Csuzdi and Zicsi (2003). Over parts of the world the epithet lacteum was widely used until Gates (1972) restored the name O. tyrtaeum which is now widely accepted in most of the English based literature. However, the description of Enterion tyrtaeum Savigny, 1826 contains a high number of errors which led Csuzdi and Zicsi (2003) to re-establish the name O. lacteum as a valid taxon. Added complications arise from the fact that whereas Easton (1983) considered O. lacteum and O. tyrtaeum as valid names. Bouche (1972) ignored O. tyrtaeum and recognised two subspecies of O. lacteum in France; O. lacteum lacteum Orley, 1885 and O. lacteum gracile Orley, 1885, with the latter being distinguished from the typical form by the presence of papillae on segment 22 and a longer clitellum and tubercula pubertatis. As a consequence of this confusing situation, in the literature available they are commonly regarded as synonyms either of O. lacteum (Csuzdi and Zicsi, 2003) or O. tyrtaeum (Sims and Gerard, 1999), respectively. The most recent work by Blakemore (2007, 2008) has, however, re-established the priority of tyrtaeum and two valid species, O. lacteum lacteum (Orley, 1881) and O. tyrtaeum (Savigny, 1826), are listed. Our phylogenetic trees partly confirm this idea as our O. lacteum appears to be more closely related to O. cyaneum than to O. tyrtaeum (100% and 96% bootstrap support (BS) for 16S and COI, respectively). This observation is also in arrangement with their different ecological behaviour and thus, whereas the two barely pigmented worms (O. cyaneum and O. lacteum lacteum) fit well within the typical endogeic ecological group, O. tyrtaeum is usually darker in colour (brownish) and seems to prefer wetter environments, so it has been characterised as a 'hygrophilous endogeic' sensu Bouche (1972). In addition, recent studies using mitochondrial COII sequences (Heethoff et al. 2004) have identified two morphologically and genetically different lineages in O. tyrtaeum: (1) the larger individuals (10-14 cm) being more homogeneous in their genetic sequences, which could indicate a recent origin or better colonisation capabilities and (2) the smaller individuals (5-8 cm) showing more differences suggesting that it could be an older lineage or a slow disperser. (ref. ID; 6916)
  1. Octolasium cyaneum (Savigny, 1826) (ref. ID; 6050) reported year? (ref. ID; 1928, 7651) reported author and year? (ref. ID; 6033)
  2. Octolasium complanatum (Dug.) (ref. ID; 7651)
  3. Octolasium lacteum Orley, 1881 (ref. ID; 6916), (Oerley) (ref. ID; 1928, 7651) or (Rosa, 1884) (ref. ID; 3692) reported author and year? (ref. ID; 6033)
  4. Octolasium lissaense (Michaelsen) (ref. ID; 7651)
  5. Octolasium lissaensis (ref. ID; 6033)
  6. Octolasium transpadanum (Rosa) (ref. ID; 7651)
  7. Octolasium tyrtaeum (ref. ID; 6916)

Octolasium cyaneum (Savigny, 1826) (ref. ID; 6050) reported year? (ref. ID; 1928, 7651) reported author and year? (ref. ID; 6033)

Descriptions

This species is probably not so rare as the scarcity of records would indicate. It has usually been confused with O. lacteum, to which it is closely allied. (ref. ID; 1928)