Early classification of decapods based on morphology: According to the earliest classification (Latreille 1806), the decapods were subdivided into Macrura and Brachyura, depending on the condition of the abdomen. However, this simple classification has been questioned by later researchers. In 1834, a new taxon, Anomala, was established, and placed between Macrura and Brachyura by H. Milne Edwards. This new taxon included certain of Macrura and Brachyura, and was characterized by a modified abdomen and the sixth pair of limbs.
However, Edwards’ classification received differing opinions in subsequent studies. In 1880, Boas firstly suspected monophyly of Macrura and divided Decapoda into Natantia, i.e, the swimming forms, and Reptantia, i.e., the walking forms. His classification received the support of Borradaile (1907), although the opinions on the relationships of three natantian groups, namely
10 Dendrobrachiata, Caridea, and Stenopodidea, differed. Boas suggested that Dendrobrachiata is the sister group of Stenopodidea, and that Caridea is the sister group of the clade consisting of the other two, whereas Borradaile has proposed that Dendrobrachiata is the sister group of Caridea, and that Stenopodidea is the sister group of the clade consisting of the other two (Fig. 1.1 a ).
However, in the following eighty years, the monophyly of the Natantia received a lot of disagreements. Finally, according to lots of researchers (Burkenroad 1963, 1981; Felgenhauer and Abele 1983; Abele and Felgenhauer 1986; Christoffersen 1988a; Kim and Abele 1990; Abele 1991), Natantia is thought to be paraphyletic (Fig. 1.1 b, c, d).
Fig. 1.1: Various hypotheses of relationships among natant decapods. From (a) Borradaile (1907); (b) Burkenroad (1963, 1981); (c) de Saint Laurent (1980), Abele and Felgenhauer (1986), Abele (1991); (d) Felgenhauer and Abele (1983).
Unlike that of Natantia, the classification of the monophyletic Reptantia has been well received. However, the interrelationships of Reptantia are quite controversial. First, Boas (1880) separated the reptantians into six groups: Astacidea, Achelata, Polychelida, Thalassinida, Anomala, and Brachyura. Later, some researchers (e.g. Borradaile 1907; Burkenroad 1963) modified the Edwards’ classification (1834), and divided Reptantia into Palinura (Polychelida + Achelata), Astacidea, Anomura (Thalassinida + Anomala), and Brachyura. Since then, controversies about the monophyly of the Palinura and Anomura, and the relationships of these reptantian groups have never ceased (e.g., de Saint Laurent 1980; Forest and de Saint Laurent 1989; Abele 1991). During this period, the discovery of Neoglyphea inopinata (Forest and Chace 1976) is significant, because it establishes that Glypheoidea, a group that was thought to have been extinct since the Mesozoic, is still extant. Subsequently, this group has been recognized as a sister group of Palinura (Forest and de Saint Laurent 1989). Unfortunately, this finding cannot resolve the internal relaionships of the reptantians. Until then, depence on a few morphological characters such as tail length
11 (Linnaeus until Boas 1880), gill type (Huxley 1878), number of chelae (Beurlen and Glaessner 1930), and mode of locomotion (Boas 1880) had not been enough to classify the decapods. A more holistic method, integrating a wider variety of characters, was required.
In the period of simplistic morphological analyses, several important taxa were erected within Decapoda, such as Dendrobranchiata, Caridea, Stenopodidea, and Reptantia. However, the relationships of these taxa, especially the internal relationships of Reptantia, such as (1) whether the polychelids should be included in Palinura (Borradaile 1907; Burkenroad 1981; Abele 1991), (2) the systematic position of the thalassinids and whether it is a genuine monophyletic group (Gurney 1942; de Saint Laurent 1973), (3) which animals should be placed within the “Anomala”
(Borradaile 1907; Martin and Abele 1986) or should be excluded from it (Burkenroad 1963, 1981;
Kaestner 1970), and (4) whether the dromiaceans should be included in the Brachyura (e.g., Gurney 1942; Guinot 1978, 1979; de Saint Laurent 1979b, 1980a; Williamson 1988; Martin 1991;
Abele 1991; Spears et al. 1992), were quite controversial.
Classification of decapods based on morphological cladistic analyses: The first cladistic study on de capods was conducted by Martin and Abele (1986), who followed the concepts of phylogenetic systematics (Hennig 1966) and scored 54 morphological and ecological characters to analyze the phylogenetic relationships within Anomala. Their research provided a good example for later studies. Then, in 1995, Scholtz and Richter used a cladistic method to investigate the relationships within Reptantia (Fig. 1.2.A); four clades within Reptantia, namely the Eureptantia (Achelata + Macrochelata), Macrochelata (Homarida + Fractosternalia), Fractosternalia (Astacida + Thalassinida + Meiura), and Meiura (Anomala + Brachyura), were proposed, and the monophyly of three traditional groups were negated, according to their analyses. First, they disagreed with the monophyly of Astacidea (Astacida + Homarida) and placed Homarida as the sister group of fractosternalians. Second, they disagreed with the monophyly of Palinura and placed Polychelida as the sister group of eureptantians. Third, they disagreed with the monophyly of Anomura and placed Thalassinida as the sister group of meiurans. Later, these phylogenetic relationships obtained support from Schram’s (2001) study, which was computerized and integrated a number of morphological characters.
Subsequently, Dixon et al. (2003) (Fig. 1.2.B) expanded the data set of Scholtz and Richter, increased the taxonomic sampling, and improved the coding of characters to elucidate the
12 relationships among Reptantia. Finally, three new clades within Reptantia were proposed from their results: Astacura (Glypheoidea + Astacida + Homarida + Thaumastochelida), Sterropoda (Thalassinida + Eurysternalia), and Eurysternalia (Achelata + Meiura). Different from the classifications proposed by Scholtz and Richter, their data support the monophyly of Astacidea and place it as the sister group of Glyphoidea. Finally, only two parts are the same in these two topologies: Polychelida is the sister group of the rest reptantians, and Anomala is the sister group of Brachyura. Later in 2004, Schram and Dixon introduced additional fossil evidence to their data matrix and obtained the same phylogenetic tree as the previous one of Dixon et al. (2003).
Fig. 1.2: Hypotheses of phylogenetic relationships among Decapoda lineages based on morphological cladistic analyses. (A) Scholtz and Richter (1995) and Schram (2001); (B) Dixon et al. (2003) and Schram and Dixon (2004).
Although the different parts in these analyses of the decapod phylogenetic relationships are noteworthy, some agreements occur, such as the paraphyletic Natantia leading to a monophyletic Reptantia, Reptantia being composed of polychelids and eureptantians, the traditional monophyletic group Palinura (Glypheoidea + Achelata + Polychelida) and Anomura (Thalassinida + Anomala) not being monophyletic, and Anomala being the sister group of Brachyura, which together form Meiura.
Phylogenetic studies among decapods using molecular data: Molecular data provide another way to address these issues. Abele (1991) first used 18s rRNA to investigate the phylogenetic relationships within Decapoda. However, this study contained only one astacid and two brachyurans in Reptantia, which was not enough to resolve the reptant phylogenetic
13 relationships. Then, in 2004, Ahyong and O’Meally combined the 16S, 18S, and 28S rRNA sequences to study reptant phylogeny and obtained a comprehensive reptantian phylogenetic tree.
Although their results were quite similar to those of Scholtz and Richter (1995), they supported the monoplyletic Astacidea. Later, additional molecular data were used to analyze the phylogenetic relationships within Decapoda, such as the combination of 16S, 18S, and 28S rRNA genes with the histone H3 nuclear genes in the studies of Porter et al. (2005) and Bracken et al.
(2009), the combination of PEPCK with the NaK nuclear protein-coding genes in the study of Tsang et al. (2008), the combination of four rRNA genes (12S, 16S, 18S, 28S), two nuclear protein-coding genes for H3 and EF-2, with one nuclear gene (EPRS) in the study of Toon et al.
(2009). However, all of these studies produced different topologies from the known ones, and finally several different combinations of higher level relationships were suggested within Reptantia (Fig. 1.3).
Fig. 1.3: Hypotheses of phylogenetic relationships among Decapoda lineages based on molecular data.
(A) Ahyong and O’Meally (2004); (B) Porter et al. (2005); (C) Tsang et al. (2008); (D) Toon et al. (2009);
(E) Bracken et al. (2009).
The previous research on molecular data suggests that the resolution of the systematics of decapods based on a small amount of molecular data is an impossible task. Among these studies, a few agreements can be obtained regarding high level relationships, such as Dendrobranchiata representing a basal lineage within Decapoda. Most questions are still open or more complicated,
14 such as whether polychelids should be removed from Palinura, whether Meiura is the most recently derived lineage within Reptantia, the possibility that relationships exist between three shrimp-like groups. With regard to the low level relationships, the monophylytic Thalassinida is in dispute, and the interrelationships of most decapod taxa are still unclear. Therefore, more comprehensive molecular data are needed for the study of decapod phylogenetic relationships.