Subphylum CRUSTACEA Brünnich, 1772

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Subphylum CRUSTACEA Brünnich, 1772

Crustacea

Compiler and date details

14 February, 2025 - Dr Gary C. B. Poore

Crustacea Brünnich, M.T. 1772. Zoologiae fundamenta praelectionibus academicus accommodata; grunde i dyrelaeren. Hafniae et Lipsiae : Frider. Christ. Pelt 253 pp. [184].

Introduction

The Crustacea comprise some 67,000 living species of which more than 50,000 are marine. About 9400 named species have been recorded in Australia but many remain to be described.
Crustacea vary enormously, particularly through varying degrees of body regionalization and limb specialization. This differentiation of body regions and specialization of limbs along the body axis is known as tagmatisation. In crustaceans, basic body tagma are the cephalon (head) and trunk. The trunk itself may be divided into the thorax and abdomen, and in malacostracans the thorax is further divided into a pereon and pleon. This variability, especially the grade between long crustaceans with slight tagmatisation and shorter ones with extreme tagmatisation and limb differentiation has led to many debates about the origin of Crustacea. And also to controversies over phylogenetic relationships within Crustacea summarised by Ahyong (2020) and Schram & Koeneman (2021).

The monophyly of Crustacea has historically not been doubted. Recently it has been realised that insects (Hexapoda) are nested within Crustacea. The unexpected position of insects among crustaceans was discovered largely through molecular phylogenetic studies, with results initially treated with suspicion but now consistently corroborated by increasing volumes of data (Schwentner et al. 2017). Pancrustacea, Tetraconata and Crustaceomorpha have been proposed as names for the crustacean-hexapod clade. Insects can now be seen as terrestrial pancrustaceans, a perspective neatly “solving” the long- standing puzzle of why insects were unable to successfully diversify in the ocean and why crustaceans have had comparatively limited success on land. How these two groups are integrated into an acceptable formal classification is yet to be settled.

The phylogeny of Pancrustacea is a work in progress and the resulting classification is fluid. The most comprehensive phylogenetic study to date for taxon coverage and multiple molecular and morphological data sources (Oakley et al., 2013: fig. 3.1) seems to be the most reliable. Consensus is yet to be reached on pancrustacean relationships, but the most recent major studies (Regier et al., 2010; von Reumont et al., 2012; Oakley et al., 2013; Rota-Stabelli, 2013) recovered several common patterns: close proximity of Remipedia to Hexapoda, a close relationship between “Maxillopoda” and Malacostraca, and recovery of Oligostraca as usually basal in Pancrustacea. Oligostracans, rather than cephalocarids or remipedes, appear to be phenotypically nearest to the stem lin¬eage crustaceans. These phylogenies serve as a basis for the classification. AFD uses the classification adopted by WoRMS (2025). Crustacea. Accessed at: https://www.marinespecies.org/aphia.php?p=taxdetails&id=1066 on 2025-01-30 is used.

The major crustacean groupings follow differences in tagmatisation. The subphylum Crustacea is divided into three superclasses essentially on molecular rather than morphological characteristics.

1. Allotriocarida are a mixture of shield shrimps, water fleas, clam shrimps, fairy shrimps, cephalocarids, remipedes and insects that appear to have little in common. Allotriocaridans have an elongated trunk and typically possess gonopores on or near somite12 which may be or not the last segment of the trunk (Schram & Koeneman, 2021).

2. Oligostraca includes fish lice, tongue worms, mystacocarids and seed shrimps (or ostracods). Oligostraca seems impossible to diagnose (Schram & Koeneman, 2021) but have a short body with gonopores, usually on the fourth thoracomere (thoracic somite or segment), and a short abdomen (except in the vermiform mystacocarids).

3. Multicrustacea include most of the more familiar crustaceans: copepods, various sorts of barnacles, krill, prawns, various lobsters and shrimps, crabs, amphipods, isopods and related brooding groups. Included in Multicrustacea are several less familiar taxa. Multicrustacea is best diagnosed as having a 5–7–5 ground pattern, that is, four segments in the head, seven in the thorax, and about five in the abdomen.

Former classifications of the Crustacea included a major group, Maxillopoda, promoted by Dahl (1956) and later by Newman (1992). Most researchers suspected that Maxillopoda was not a natural group but accepted and used the name. See for example two catalogues which both listed all families. Martin & Davis (2001) included only barnacles in Maxillopoda and Ahyong et al. (2011) included barnacles, copepods, mystacocarids, pentastomes and tantulocarids. The Maxillopoda may still be a monophyletic group but encompasses only barnacles and copepods (Oakley et al., 2013; Ahyong, 2020). Maxillopoda is not part of the AFD classification.