Larval development of Japanese 'conchostracans': part 2, larval development of Caenestheriella gifuensis (Crustacea, Branchiopoda, Spinicaudata, Cyzicidae), with notes on homologies and evolution of certain naupliar appendages within the Branchiopoda

Transitions in functional morphology from "large branchiopods" to Cladocera: Video and confocal microscopic studies of Cyclestheria hislopi (Cyclestherida) and Sida crystallina (Cladocera: Ctenopoda)

Great diversity is found in morphology and functionality of arthropod appendages, both along the body axis of individual animals and between different life-cycle stages. Despite many branchiopod crustaceans being well known for displaying a relatively simple arrangement of many serially post-maxillary appendages (trunk limbs), this taxon also shows an often unappreciated large variation in appendage morphology. Diplostracan branchiopods exhibit generally a division of labor into locomotory antennae and feeding/filtratory post-maxillary appendages (trunk limbs). We here study the functionality and morphology of the swimming antennae and feeding appendages in clam shrimps and cladocerans and analyze the findings in an evolutionary context (e.g., possible progenetic origin of Cladocera). We focus on Cyclestheria hislopi (Cyclestherida), sister species to Cladocera and exhibiting many "large" branchiopod characters (e.g., many serially similar appendages), and Sida crystallina (Cladocera, Ctenopoda), which likely exhibits plesiomorphic cladoceran traits (e.g., six pairs of serially similar appendages). We combine (semi-)high-speed recordings of behavior with confocal laser scanning microscopy analyses of musculature to infer functionality and homologies of locomotory and filtratory appendages in the two groups. Our morphological study shows that the musculature in all trunk limbs (irrespective of limb size) of both C. hislopi and S. crystallina comprises overall similar muscle groups in largely corresponding arrangements. Some differences between C. hislopi and S. crystallina, such as fewer trunk limbs and antennal segments in the latter, may reflect a progenetic origin of Cladocera. Other differences seem related to the appearance of a specialized type of swimming and feeding in Cladocera, where the anterior locomotory system (antennae) and the posterior feeding system (trunk limbs) have become fully separated functionally from each other. This separation is likely one explanation for the omnipresence of cladocerans, which have conquered both freshwater and marine free water masses and a number of other habitats.

Phylogeny and Biogeography of Spinicaudata (Crustacea: Branchiopoda)

Spinicaudata (spiny clam shrimp) is a taxon of Branchiopoda occurring since the Devonian and today it occurs nearly globally in temporary water bodies. We present the most species-rich phylogenetic analyses of this taxon based on four molecular loci: COI, 16S rRNA, EF1α and 28S rRNA. Our results support previous findings that Cyzicidae sensu lato is paraphyletic. To render Cyzicidae monophyletic we establish a fourth extant spinicaudatan family to accommodate Eocyzicus. Within Cyzicidae, none of the genera Cyzicus, Caenestheria or Caenestheriella are monophyletic, and the morphological characters used to define these genera (condyle length and rostrum shape) are not associated with well-delimited clades within Cyzicidae. There is insufficient resolution to elucidate the relationships within Leptestheriidae. However, there is sufficient evidence to show that the leptestheriid genera Eoleptestheria and Leptestheria are non-monophyletic, and there is no support for the genus Leptestheriella. Molecular clock analyses suggest that the wide geographic distribution of many spinicaudatan taxa across multiple continents is largely based on vicariance associated with the break-up of Pangea and Gondwana. Trans-oceanic dispersal has occurred in some taxa (e.g., Eulimnadia and within Leptestheriidae) but has been relatively rare. Our results highlight the need to revise the taxonomy of Cyzicidae and Leptestheriidae and provide evidence that the global spinicaudatan diversity may be underestimated due to the presence of numerous cryptic species. We establish Eocyzicidae fam. nov. to accommodate the genus Eocyzicus. Consequently, Cyzicidae comprises only two genera -Cyzicus and Ozestheria. Ozestheria occurs also in Africa and Asia and Ozestheria pilosa new comb. is assigned to this genus.

How reversible is development? Contrast between developmentally plastic gain and loss of segments in barnacle feeding legs

Segmented organisms and structures have fascinated biologists since William Bateson first described homeotic transformation and recognized the fundamental evolutionary significance of segmental organization. On evolutionary time scales, segments may be lost or gained during major morphological transitions. But how segment loss compares to gain on developmental time scales remains mysterious. Here, we examine the ease of reverse development (opposite to normal growth) by comparing developmentally plastic leg segment loss versus gain in individual barnacles transplanted between different water flow conditions. Plastic segment addition occurred rapidly (one to two molts) and exclusively near the limb base. In contrast, developmentally plastic segment loss-the first observation in any arthropod-took much longer (>10 molts) and, remarkably, occurred throughout the leg (23% of losses occurred mid-limb). Segment loss was not a simple reversal of segment addition. Intersegmental membranes fused first, followed by elimination of duplicate tendons and gradual shortening (but not loss) of duplicate setae. Setal loss, in particular, may impose a severe developmental constraint on arthropod segment fusion. This asymmetric developmental potential (time lag of phenotypic response)-plastic segment addition (amplified normal development) is faster and more orderly than segment loss (reverse development)-adds a new dimension to models of developmental plasticity because the cost of making a developmental mistake in one direction will be greater than in the other.

Naupliar and Metanaupliar Development of Thysanoessa raschii (Malacostraca, Euphausiacea) from Godthåbsfjord, Greenland, with a Reinstatement of the Ancestral Status of the Free-Living Nauplius in Malacostracan Evolution

The presence of a characteristic crustacean larval type, the nauplius, in many crustacean taxa has often been considered one of the few uniting characters of the Crustacea. Within Malacostraca, the largest crustacean group, nauplii are only present in two taxa, Euphauciacea (krill) and Decapoda Dendrobranchiata. The presence of nauplii in these two taxa has traditionally been considered a retained primitive characteristic, but free-living nauplii have also been suggested to have reappeared a couple of times from direct developing ancestors during malacostracan evolution. Based on a re-study of Thysanoessa raschii (Euphausiacea) using preserved material collected in Greenland, we readdress this important controversy in crustacean evolution, and, in the process, redescribe the naupliar and metanaupliar development of T. raschii. In contrast to most previous studies of euphausiid development, we recognize three (not two) naupliar (= ortho-naupliar) stages (N1-N3) followed by a metanauplius (MN). While there are many morphological changes between nauplius 1 and 2 (e.g., appearance of long caudal setae), the changes between nauplius 2 and 3 are few but distinct. They involve the size of some caudal spines (largest in N3) and the setation of the antennal endopod (an extra seta in N3). A wider comparison between free-living nauplii of both Malacostraca and non-Malacostraca revealed similarities between nauplii in many taxa both at the general level (e.g., the gradual development and number of appendages) and at the more detailed level (e.g., unclear segmentation of naupliar appendages, caudal setation, presence of frontal filaments). We recognize these similarities as homologies and therefore suggest that free-living nauplii were part of the ancestral malacostracan type of development. The derived morphology (e.g., lack of feeding structures, no fully formed gut, high content of yolk) of both euphausiid and dendrobranchiate nauplii is evidently related to their non-feeding (lecithotrophic) status.

Larval development of Japanese "conchostracans": Part 3, larval development of Lynceus biformis (crustacea, branchiopoda, laevicaudata) based on scanning electron microscopy and fluorescence microscopy

For comparison with the remarkable larvae of the laevicaudatan (clam shrimp) Lynceus brachyurus, a basic description of the larval sequence of another laevicaudatan branchiopod, the Japanese Lynceus biformis, is provided. Four larval stages have been identified, ranging in size from 258 to 560 μm in length. The first stage has no flattened dorsal shield, in contrast to the three following stages, in which such a shield is present. During development, the only significant changes to the naupliar appendages occur in the antenna at the molt from stage 1 to 2, with the addition of a fourth apical seta to the endopod and a change in the form of the naupliar process, used for food manipulation, from a long, unbranched, pointed spine to a bifid structure. In addition, buds of trunk limbs (five pairs) first appear externally in stage 4 but can be recognized through the cuticle in the previous stage. The larval sequence and larval morphology of L. biformis differ from those of L. brachyurus in at least two respects. L. brachyurus has a dorsal shield in the earliest known stages, but such a shield is lacking in the first stage of L. biformis. Another difference is that L. brachyurus has a huge, flattened, kidney-shaped labrum, whereas that of L. biformis is smaller and bears four robust, denticulate spines on the distal margin. Based on out-group comparison, the morphology of L. biformis, at least in these respects, is likely to represent the ancestral morphology. Despite the partly peculiar morphology of the larvae of Lynceus species, they share many similarities with other branchiopod larvae, at least two of which, the naupliar swimming/feeding apparatus and the mode of development of the trunk limbs, could be considered synapomorphies for the Branchiopoda.

Nervous system development in Spinicaudata and Cyclestherida (Crustacea, Branchiopoda)--comparing two different modes of indirect development by using an event pairing approach

Cladocera are the ecologically most important group within the Branchiopoda. They are unquestionably branchiopods but their evolutionary origin remains unclear. One favored explanation of their origin is that they evolved from a reproductive larva of a clam shrimp-like ancestor. To reveal a transformation and identify (potential) changes in chronology (heterochrony), we investigated and compared the development of representatives of two clam shrimp taxa, one of the Spinicaudata (Leptestheria dahalacensis) and one of the Cyclestherida (Cyclestheria hislopi), the sister group of Cladocera. Both taxa develop indirectly although the exact modes are quite different. The development of the nervous system, labeled and analyzed using immunohistochemical techniques and confocal microscopy, and that of the external morphology, scanned with an electron microscope, was investigated. L. dahalacensis hatch as a free-swimming nauplius and the nervous system and external morphology develop gradually. In C. hislopi, on the other hand, several internal and external structures develop before the hatching of a nonswimming embryo-like larva which is still carried in a dorsal brood pouch. The development in L. dahalacensis is directed from anterior to posterior, whereas in C. hislopi a more synchronous anterior and posterior differentiation is present. A comparison of both developmental sequences gives us the first indications of the evolutionary transformation which the Cladocera may have undergone from a clam shrimp-like ancestor.

Segmental mismatch in crustacean appendages: the naupliar antennal exopod of Artemia (Crustacea, Branchiopoda, Anostraca)

Based on traditional techniques and confocal laser scanning microscopy for external morphology, and immunohistochemistry for the muscular system, we describe here the segmental features of the antennal exopod of Artemia nauplii. Two kinds of serial elements are present, i.e. setae (with cuticular folds at their base) and ringlets (serially arranged sclerites separated by joint-like cuticular folds not extending to form complete rings around the appendage). The two series are usually not in register. The cuticular folds of the setae and of the ringlets are also sites of intermediate insertions of the three exopod muscles: as the two tegumentary structures are discordant in periodicity, this is also mirrored in the pattern of muscle insertions on the two sides of the appendage. Similar cases of segmental mismatch are known for the trunk of several arthropods, but segmental mismatch along the appendages has received very little attention. The occurrence of segmental mismatch in the naupliar appendages of both extant and fossil crustaceans is reviewed and it is suggested here to be a primitive feature of the exopods of both second antennae and mandibles. Problems in the interpretation of morphological evidence are discussed, also in relation to development and evolution of segmentation of naupliar appendages.