EVOLUTIONARY LOSS OF LARVAL FEEDING: DEVELOPMENT,FORM AND FUNCTION IN A FACULTATIVELY FEEDING LARVA,BRISASTER LATIFRONS

Species with large eggs and nonfeeding larvae have evolved many times from ancestors with smaller eggs and feeding larvae in numerous groups of aquatic invertebrates and amphibians. This change in reproductive allocation and larval form is often accompanied by dramatic changes in development. Little is known of this transformation because the intermediate form (a facultatively feeding larva) is rare. Knowledge of facultatively feeding larvae may help explain the conditions under which nonfeeding larvae evolve. Two hypotheses concerning the evolutionary loss of larval feeding are as follows: (1) large eggs evolve before modifications in larval development, and (2) the intermediate form (facultatively feeding larva) is evolutionarily short-lived. I show that larvae of a heart urchin, Brisaster latifrons, are capable of feeding but do not require food to complete larval development. Food for larvae appears to have little effect on larval growth and development. The development, form, and suspension feeding mechanism of these larvae are similar to those of obligate-feeding larvae of other echinoids. Feeding rates of Brisaster larvae are similar to cooccurring, obligate-feeding echinoid larvae but are low relative to the large size of Brisaster larvae. The comparison shows that in Brisaster large egg size, independence from larval food, and relatively low feeding rate have evolved before the heterochronies and modified developmental mechanisms common in nonfeeding echinoid larvae. If it is general, the result suggests that hypotheses concerning the origin of nonfeeding larval development should be based on ecological factors that affect natural selection for large eggs, rather than on the evolution of heterochronies and developmental novelties in particular clades. I also discuss alternative hypotheses concerning the evolutionary persistence of facultative larval feeding as a reproductive strategy. These hypotheses could be tested against a phylogenetic hypothesis.     

Nanisme écologique de deux espèces d'échinides irréguliers dans l'Hauterivien de la région de Castellane (Alpes-de-Haute-Provence)

Three species of irregular see-urchins form the settlement of a marly level (lower Hauterivian) of the Castellane area. Two of them are strongly smaller than the norm. Their stunting is prouved by ontogenic, mecanical, sedimentological and ecological arguments. The ecological grounds of stunting are discussed.     

Un modèle d'ontogenèse: La croissancede Toxaster granosus kiliani (Lambert)), échinide spatangoide du Valanginien

A complete study of the ontogeny of the irregular urchin Toxaster granosus kiliani (Lambert)) was achieved from an important lower cretaceous sample. In a first time, ontogenic variations of each character were analysed versus length chosen as size reference. This first step had shown that very important changes in shape take place during the development of the young urchin: increase in height, downward migration of the periproct, forward movement of the peristome… In a second time, a growth crisis was proved and described. Such a crisis occurs, from 12 mm to 16 mm length, as a momentary acceleration of the growth (i.e. the rate of growth increases during a short time). The reality of this crisis was demonstrated by both mathematical and biological ways and also by comparing with other actual and fossil populations. The whole of these results leads to a synthetical pattern of growth divided into three main stages which are successively a juvenile, a preadult and an adult stage. This division was proved also by a principal components analysis.     

Effets du retard dans la première alimentation sur le développement de la larve planctotrophe de l'oursin Paracentrotus lividus

The growth of the skleleton and changes in biochemical constituents (proteins, carbohydrates and lipids) were observed in embryos (days 0 and 1) and larvae (2-8 days) of the sea urchin Paracentrotus lividus that were starved (C 0) or fed low (1,000 cells of Hymenomonas elongata, C 1) or high (5,000 cells, C 5). Different batches of C 1 and C 5 larvae were fed first the 2nd, 3rd, 4th or 5th day after fertilization. Initiation of feeding 24 h after hatching slowed down the rate of increase in length of the larval appendages. A differential growth of skeletic rods (somatic and postoral rods) and variations in proteins weights shows the existence of a mixotrophic phase between endotrophic and planktotrophic phases. Best development occured in larvae which were first fed at the lower food level 24 h after hatching. On the 2nd and the 3rd day the somatic/postoral rod ratio is ≥1. This higher ration can be used to estimate the age of natural populations of larvae.     

Mechanical properties of sea-urchin lantern muscles: a comparative investigation of intact muscle groups in Paracentrotus lividus (Lam.) and Stylocidaris affinis (Phil.) (Echinodermata, Echinoidea)

The jaw apparatus, or lantern, of sea-urchins contains five pairs of retractor and protractor muscles which are responsible for lantern displacement. Using intact retractor or protractor groups, the force-length relations of these muscles were compared in two taxonomically distant species, Paracentrotus lividus and Stylocidaris affinis. The total contractile forces generated by the muscles can be resolved into vertical and horizontal components. It was found that the vertical component of the retractors is maximal at a lantern position which is significantly lower (i.e. more protruded) in Paracentrotus than in Stylocidaris. Total forces generated by the retractors were in both species maximal at or above the lantern `resting positions'. In Paracentrotus alone, the total force-displacement curves tended to be bimodal. It is hypothesized that the retractors of Paracentrotus contain two populations of muscle fibres, one adapted for jaw opening and one for lantern retraction. No significant differences in the properties of the protractors of the two species could be identified. The lantern of Paracentrotus is more mobile than that of Stylocidaris and is able to exploit a wider range of food sources. This investigation has shown that the force-length relations of the lantern muscles match their differing working conditions. Accepted: 3 November 1997     

Systematic assessment of the Atelostomata (Spatangoida and Holasteroida; irregular echinoids) based on spine microstructure

Spines of irregular echinoids occur in very high abundance in each specimen, and display distinct architecture as a result of the specialized functions of the spines; however, studies on spine microstructure in atelostomate echinoids have rarely been carried out. Accordingly, little is known about their specific morphology. This work aims to elaborate differences in the spine morphology of selected Atelostomata (Spatangoida and Holasteroida) in detail, and to discuss spine microstructure for its potential systematic value. Based on 82 atelostomate species (56 spatangoids and 26 holasteroids), we show that the perforation pattern in the internal cylinder of the spine (helicoidal versus horizontal pattern) provides a safe distinction between the Spatangoida and Holasteroida. According to this character we discuss the geological history of atelostomate echinoids, in particular their migration into the deep sea, based on well‐preserved records of fossil spines. © 2015 The Linnean Society of London     

Colonization, dispersal, and hybridization influence phylogeography of North Atlantic sea urchins (Strongylocentrotus droebachiensis)

We used frequency-based and coalescent-based phylogeographic analysis of sea urchin (Strongylocentrotus droebachiensis) mitochondrial DNA (mtDNA) sequences and previously published microsatellite data to understand the relative influence of colonization and gene flow from older (north Pacific) and younger (northeast Atlantic) sea urchin populations on genetic variation in the northwest Atlantic. We found strong evidence of survival of northwestern Atlantic populations in local Pleistocene glacial refugia: most haplotypes were the same as or closely related to Pacific haplotypes, with deep gene genealogies that reflect divergence times within the northwestern Atlantic that are much older than the last glacial maximum. We detected gene flow across the North Atlantic in the form of haplotypes shared with or recently descended from European populations. We also found evidence of significant introgression of haplotypes from a closely related species (S. pallidus). The relative magnitude of gene flow estimated by coalescent methods (and the effective population size differences among oceanic regions) depended on the genetic marker used. In general, we found very small effective population size in the northeastern Atlantic and high trans-Arctic gene flow between the Pacific and northwestern Atlantic. Both analyses suggested significant back-migration to the Pacific. However, microsatellites more strongly reflected older Pacific migration (with similar effective population sizes across the Arctic), whereas mtDNA sequences appeared to be more sensitive to recent trans- Atlantic dispersal (with larger differences in effective population size). These differences across marker types might have several biological or methodological causes, and they suggest caution in interpretation of the results from a single locus or class of markers.     

Species distributions: virtual or real – the case of Arbaciella elegans (Echinoidea: Arbaciidae)

The distribution of species is expressed by their occurrence in local faunal lists often compiled by non‐taxonomists. In the case of rare or cryptic species, this can pose a severe limitation on the validity and thus the application of the resulting biodiversity data. Here, we show an example of a shallow‐water echinoid to illustrate problematic distribution data based on misidentification. This species, Arbaciella elegans, was established on the basis of Central African material and later reported from various places in the Mediterranean and the Northern Atlantic. Morphological comparison with the type material casts considerable doubt on the validity of these records. Genetical characterization of material from the Azores clearly shows that the dark Arbaciella phenotype reported to the Mediterranean and north‐east Atlantic in fact represents juveniles of another species, namely Arbacia lixula.     

Possible effects of global environmental changes on Antarctic benthos: a synthesis across five major taxa

Because of the unique conditions that exist around the Antarctic continent, Southern Ocean (SO) ecosystems are very susceptible to the growing impact of global climate change and other anthropogenic influences. Consequently, there is an urgent need to understand how SO marine life will cope with expected future changes in the environment. Studies of Antarctic organisms have shown that individual species and higher taxa display different degrees of sensitivity to environmental shifts, making it difficult to predict overall community or ecosystem responses. This emphasizes the need for an improved understanding of the Antarctic benthic ecosystem response to global climate change using a multitaxon approach with consideration of different levels of biological organization. Here, we provide a synthesis of the ability of five important Antarctic benthic taxa (Foraminifera, Nematoda, Amphipoda, Isopoda, and Echinoidea) to cope with changes in the environment (temperature, pH, ice cover, ice scouring, food quantity, and quality) that are linked to climatic changes. Responses from individual to the taxon-specific community level to these drivers will vary with taxon but will include local species extinctions, invasions of warmer-water species, shifts in diversity, dominance, and trophic group composition, all with likely consequences for ecosystem functioning. Limitations in our current knowledge and understanding of climate change effects on the different levels are discussed.