Systematics or Phylogenetics
Order Sacoglossa are mostly specialist herbivores feeding on cytoplasm of marine plant, especially, algae. The hypotheses on the coevolution of the Sacoglossa and their food plants is proposed (Jensen 1976; Kay 1968; Deefreese 1987) due to the facts that all shelled sacoglossans feed only on one algal genus and more than 500 species are restricted to siphonalean green algae (Jensen 1997; Greene 1970). According to Jensen 1996, sacoglossa can be defined as monophyletic group, however, classification within the lower group is highly unstable (Marcus 1982; Boss 1982; Gascoigne 1976). Current classifications are base on external characteristics, central nervous system, and reproductive anatomy to some extent (Marcus 1982, Jensen 1997). Many literatures have proposed different phylogenetics trees base on different theory.
1. Shell-less Sacoglossa have diphyletic origin
According to figure 1, Sacoglossa with reduced shells are includeding with the non-shell sacoglossa. Moreover, Oxynoidea is included with the families with parapodia, and Lobigeridae with the families bearing cerata. The classification were based primarily on the nervous system organization. Both Oxynoidea and Lobigeridae have a short visceral loop (Jensen 1999).
Figure 1: Cladogram based on phylogenetic theory of Boettger (1963), modified from Jensen 1996.
2. Both shelled families and non-shlled families form a monophyletic clade.
According to Schmekel & Portmann (1982) theory, the three shelled Sacoglossa are represented as a monophyletic group. However, the characters used to group this three suborders together are all plesiomorphies. Moreover, they also group the shell-less sacoglossa together as a monophyletics groups (figure 2 left). The main character that united the shelled Sacoglossa together is the presence of shell and mantle complex (Jensen 1997). Therefore, they can be considered as plesiomorphic. Hence, a number of classification problem is obvious and is addressed by many literatures (Marcus 1982; Thompson 1988).
3. Shelled sacoglossa form a paraphyletics grade, which lead to monophyletic origin of non-shelled clade
As mentions in chemical evolution, it is possible that shell less Sacoglossan derived from shalled form. With the chemical that they can obtain from algae allow them to survive without shell and rely more on chemical defense (Guido Cimino 1998).
Figure 2: Cladogram highlighting the hypothesis that Both shelled families and non-shlled families form a monophyletic clade (left), and Shelled sacoglossa form a paraphyletics grade, which lead to monophyletic origin of non-shelled clade (right). Volvatellidae (1); Juliidae (2); Oxynoidae (3); Boselliidae (4); Placobranchidae (= Elysiidae) (5); Polybranchiidae (6); Hermaeidae (7); Limapontiidae (= Stiligeridae) (8). Modified from Jensen 1999.
Genus Thuridilla Phylogeny:
According to Marshall 1999, T. bayeri often mistaken as T.splendens as they are closely related (figure 3).This is because they very much look a like normally with a blue spot. The differences between the two species is the slightly enlarged pharyngeal punch of T.splendens and the yellow spots on the parapodia (Gosliner 1995). Understanding this phylogenetic tree is very difficult because of little information known about each species. Literatures addressing Thuridilla phylogeny is also rare. The organisms also appear in various colour form. Therefore, anatomy of each is important to understand this phylogenetic tree, however, the study in this field is still lacking.
Figure 3: Phylogeny of Thuridilla ref: Gosliner, T.M. (1995) The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of ther phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Sciences, 49(1) : 1-54.
Figure 4: Neighbour network diagram from SplitTree4 modified from Wagele & Handeler, 2007 (Left). Phylogeny of T.bayeri and it's colour morph (right) based on information from Jensen (1992) and Wagele & Handeler (2007).
Thuridilla bayeri and other two colour morph (T.ratna and T.gracilis) are differentiated mainly by their colouration (Gosliner 1995). Moreover, they can also be distinguished by the slight pharyngeal pouch size different. (Jensen 1992). With software SplitsTree4, the neighbour network show affinites of T.gracilis and T. ratna, where as, T. bayeri appears to be separate (figure 4 left).
More study still need to be done to cover the phylogeny of Sacoglossa. It is because the information on individual family and genus is still lacking, construction of phylogenic trees is harder. Only few studies have been done to address this field of studies, one reason might be because some of the animals are rare, and variation between individual species is high. Hence, it is hard to classify them based on external morphology or basic systems.
Taxon classification:
Taxonomic classification of Thuridilla bayeri based on Jensen 1992. However, superfamily and family can be refer differently. According to EOL (2011); WoRMS (2011), superfamily and family for T. bayeri are Plakobranchoidea, and Plakobranchidea, respectively.
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