Phylogenetic analysis of the Malacostraca (Crustacea)

Abstract

The Malacostraca comprises about 28 000 species with a broad disparity in morphology, anatomy, embryology, behaviour and ecology. The phylogenetic relationships of the major taxa are still under debate. Is the Leptostraca the sister group of the remaining Malacostraca, or is this taxon more closely related to other Crustacea? Does the Stomatopoda or the Bathynellacea represent the most basal taxon within the remaining taxa? Is the Peracarida monophyletic or are some peracarid taxa more closely related to other ‘caridoid’ taxa? Is the Thermosbaenacea part of the Peracarida or its sister group, and how much support is there for a taxon Amphipoda + Isopoda? To answer these questions a phylogenetic analysis of the Malacostraca combining different phylogenetic approaches was undertaken. In a first step, the monophyly of the Malacostraca including the Leptostraca is shown using the ‘Hennigian approach’. A computer cladistic analysis of the Malacostraca was carried out with NONA and PEE‐WEE, based on 93 characters from morphology, anatomy and embryology. Nineteen higher malacostracan taxa are included in our analysis. Taxa whose representatives are exclusively fossils were not included. The Leptostraca was used as an operational out‐group. The present analysis supports the basal position of the Stomatopoda. Syncarida and Peracarida (including Thermosbaenacea) are supported as monophyletic, the Eucarida is not. Instead a sister‐group relationship is suggested between Euphausiacea and Peracarida (including Thermosbaenacea), with the Syncarida as the sister group to both taxa. Certain embryonic characters are interpreted as support for the monophyly of the Peracarida (without Thermosbaenacea) because convergences or reversals of these characters seem implausible. Within the Peracarida, the Mysidacea (Lophogastrida + Mysida) represents the sister group to the remaining taxa. A sister‐group relationship between Amphipoda and Isopoda is not supported.

The Proof is in the Pouch:Tealliocarisis a Peracarid

“…However, it has been recognized that maxilliped three in Dendrobranchiata, Procaridoidea, and Stenopodidea differs little from thoracopods four to eight (RICHTER & SCHOLTZ 2001), and maxillipeds two and three in Palinuridae and Polychelida are manifestly pediform (LAVALLI & SPANIER 2010). This becomes problematic in interpreting thoracopods of eumalacostracan taxa with possible decapod affinities as maxillipeds vs. ambulatory thoracopods.…”

“…CLARK (2013) conducted an analysis using a subset of the taxa included in RICHTER & SCHOLTZ (2001) C h a r a c t e r s t a t e s : Changes made from the coding of CLARK (2013), as well as coding for Notocaris and Pygocephalus are included in the complete character matrix provided in Table 1. Changes to the character coding of CLARK (2013) (25) Character 25 involves the number of epipodites without oostegites on at least some thoracopods (RICHTER & SCHOLTZ 2001). In the analysis of CLARK (2013), this character was coded as (2) more than two epipodites as arthrobranchiae and pleurobranchiae, or any other way.…”

“…In the analysis of CLARK (2013), this character was coded as (2) more than two epipodites as arthrobranchiae and pleurobranchiae, or any other way. This coding is inclusive of the branchial arrangement of Lophogastrida, and Decapoda ( RICHTER & SCHOLTZ, 2001), and seems to have been chosen by CLARK (2013) to apply to the decapod branchial arrangement, in which branchiae are generally borne as arthrobranchia, pleurobranchiae, and podobranchia, on at least some thoracic somites (CALMAN 1909). Detailed morphological studies of Tealliocaris (BRIGGS & CLARKSON 1985;CLARK 2013) have simply not demonstrated this, not surprisingly given the low preservational potential of gill branches.…”

“…The full character matrix in NEXUS format has been uploaded to Morphobank v3 website. (31) Character 31 deals with whether the coxa-thorax articulation allows promotion and remotion, as in Leptostraca, Decapoda, Anaspidacea, Thermosbaenacea, Cumacea, and Isopoda (0), or abduction and adduction, as in Mysida, Lophogastrida, Spelaeogriphacea, and Amphipoda (1) (RICHTER & SCHOLTZ 2001). Effectively evaluating this character in fossil taxa with no closely related modern analogues is problematic, and would require detailed justification based on preserved evidence of the coxa thorax articulation, especially since this character is variable within Peracarida (HESSLER 1982), and the position of the pygocephalomorphs within peracarids is far from certain.…”

“…(53, 54, 55) Characters 53-55 of RICHTER & SCHOLTZ (2001) code for the presence or absence of the nauplius eye sensu stricto, dorsal frontal organ, and ventral frontal organ. The presence or absence of these features varies within the higher malacostracans, as outlined by SCHOLTZ (2001) andELOFSSON (1963).…”