BIOL2009 LT5 Crustacea

Subphylum Crustacea

 67 000 described living species of Crustacea
 Crustaceans are commonly the dominant organisms in aquatic subterranean
ecosystems, and new species of these stygobionts continue to be discovered as
new caves are explored, also dominate ephermeral pool habitats, where many
undescribed species are likely to occur
 Crustaceans are the most widespread, diverse and abundant animals inhabiting
the oceans – biomass of Antarctic krill (Euphausia superba), has been estimated
to be at 500 million tons

Phylogeny

 Crustacean phylogeny is far from fully resolved: while there is general
agreement in some areas, many fundamental mysteries remain unsolved
 Molecular studies have done little to resolve problems – though analysis of multi-
gene sequences may be more useful

Several problems remain:

 What is the most primitive living crustacean group?
 What are the relationships of the basal crustacean clades?
 What are the relationships of the maxillopodans, and how do the Ostracoda fit
into the clade?
 What are the major decapod lineages and how are they related to one another?
 What group of crustaceans are represented by the “y-larvae” (the Facetotecta)

 Debates on crustacean phylogeny commonly centre on two competing views
regarding the nature of the primitive, or ancestral, crustacean body plan
- One view: one crustacean had leaflike (phyllopodous) thoracic legs used for
swimming and suspension feeding seen in cehalocarids, leptostracans,
branchiopods
- Other view: first crustacean had nonphyllopodous, simple, paddle-like legs used
for swimming but not feeding, feeding undertaken by cephalic appendages
- Weight of evidence: both phylogenetic analyses and fossil data seems to favour
phyllopodous limbs as the primitive condition
- Recent developmental studies following expression of Distal-less and other
developmental genes suggest that early embryogeny of limbs is very
similaramong crustaceans (eg. Trunk limbs always emerge as ventral, subdivided
limbs)
- The monophyletic nature of the class Malacostraca was rarely questioned –
within this group there are two principal groups: Leptostraca + Eumalacostraca

, - Branchipoda (class) difficult to define on basis of unique synapomorphies as it
shows morphological variation – some branchiopods secondarily lost the
carapace, others have secondarily lost most or all of the abdominal appendages
- Still the phylogeny of Crustacea is not fully understood – like arthropods in
general, crustaceans exhibit high levels of evolutionary parallelism and
convergence, many apparent reversals of character states
- Genetic flexibility: due in part to the nature of the segmented body, serially
homologous appendages, flexibility of developmental genes provide enormous
opportunity
- Any conceivable cladogram of phylogeny will require acceptance of considerable
homoplasy

Origin and early evolution of the Pancrustacea

Earliest Crustaceans

The Crustacean Bauplan

 Crustacean diversity demands
emphasis before generalisations of
their bauplan – success of
crustaceans closely tied to
modifications of the jointed
exoskeleton and appendages
Exploitation of evolutionary
flexibility that has allowed an
extensive range of modification
of these body parts for a great
variety of functions
 Most basic crustaceam body plan is a head (cephalon) followed by a long body
(trunk) with many similar appendages, as seen in the primitive class Remipedia
In other crustacean classes, various degrees of tagmosis occur, and the
cephalon is typically followed by a trunk divided into a thorax and abdomen
 All crustaceans possess at least primitively, a cephalic shield or a carapace Cephalic
shield: formed from the fusion of the dorsal head tergites to form a solid cuticular
plate – characteristic of Remipedia,, Cephalocarida, also occur in some maxillopodans
and malacostracans
Carapace: More expansive structure, comprising the head shield and a large fold
of the exoskeleton that probably arises (primitively) from maxillary somite
 Most differences among major groups, and basis for much of their
classification, arise from variations in the number of somites in the thoraz and
abdomen, the form of their appendages, size and shape of the carapace

,  Monophyly and uniformity within the subphylum is demonstrated particularly by
the elements of the cephalon (particularly two pairs of antennae) and presence
of a Nauplius larva
Head (except for a few cases of secondary reduction): All crustacean heads
have 5 pairs of appendages. From anterior to posterior: attenula (1 st antennae),
antennae, mandibles, maxillula (1st maxilla, modified as mouthparts), maxillae
Eyes: some possess simple, but most possess well developed compound eyes
either sessile or stalked
Nauplius (characteristic larval stage): bears a median simple (naupliar eye) and 3
pairs of sectose, functional appendages destined to become the atennules,
antennae and mandibles – in many groups (eg. Pericarida) the free-living Nauplius
larva is absent or suppressed, development is fully direct/mixed
Nauplius stage is passed through a series of molts, during which segments and
appendages are gradually added

Major Crustacean Lineages




Branchiopoda+Maxillopoda+Ostracoda+Remipedia+Cephalocarida = Entomostraca

Class Malacostraca (unique/special features)

 40 200 species divided into subclasses – Phyllocarida (typically viewed as
representing the primitive malacostracan condition) and Eumalacostraca (bauplan
characterised by 5-8-6 (plus telson) arrangement of body segments – recognised
in the early 1900s by W.T. Calman
 Stomatopods
 Decapods
 Cumacean
 Isopod

Class Branchiopoda

 900 species described – orders notostraca, cladocera, conchostraca, anostraca