Starfish

Abstract: Fisheries exploitation provides the opportunity to examine the ecosystem-scale biodiversity consequences of predator removal. We document predatory reef fish densities, coral-eating starfish densities and coral reef structure along a 13-island gradient of subsistence exploitation in Fiji. Along the fishing intensity gradient, predator densities declined by 61% and starfish densities increased by three orders of magnitude. Reefbuilding corals and coralline algae declined by 35% and were replaced by non-reef building taxa (mainly filamentous algae), as a result of starfish predation. Starfish populations exhibited thresholds and Allee-type dynamics: population growth was negative under light fishing intensities and high predator densities, and positive on islands with higher fishing intensities and low predator densities. These results suggest the depletion of functionally important consumer species by exploitation can indirectly influence coral reef ecosystem structure and function at the scale of islands.

Abstract: INVESTIGATIONS of the mechanism of starfish spawning induced by injection of a water extract of radial nerves have shown that an active polypeptide acts on the ovary and induces the production of a second substance which is, in the true sense, a meiosis inducing factor as well as a spawning inducing factor1–7. The neural substance has therefore been renamed gonad stimulating substance (GSS), and the second substance, produced in the ovary, is meiosis inducing substance (MIS)5,7. Isolated ovarian fragments immersed in seawater containing MIS discharge their eggs4,5,7, while isolated oocytes with germinal vesicles treated with MIS mature4–7. The production of MIS by GSS has been demonstrated in isolated ovarian fragments of seven species of starfish, and it seems to be non-species specific among starfishes7. MIS is heat stable (100° C, 30 min) and is not a peptide5,6.

Abstract: Evolutionary change in morphological features must depend on architectural reorganization of developmental gene regulatory networks (GRNs), just as true conservation of morphological features must imply retention of ancestral developmental GRN features. Key elements of the provisional GRN for embryonic endomesoderm development in the sea urchin are here compared with those operating in embryos of a distantly related echinoderm, a starfish. These animals diverged from their common ancestor 520-480 million years ago. Their endomesodermal fate maps are similar, except that sea urchins generate a skeletogenic cell lineage that produces a prominent skeleton lacking entirely in starfish larvae. A relevant set of regulatory genes was isolated from the starfish Asterina miniata, their expression patterns determined, and effects on the other genes of perturbing the expression of each were demonstrated. A three-gene feedback loop that is a fundamental feature of the sea urchin GRN for endoderm specification is found in almost identical form in the starfish: a detailed element of GRN architecture has been retained since the Cambrian Period in both echinoderm lineages. The significance of this retention is highlighted by the observation of numerous specific differences in the GRN connections as well. A regulatory gene used to drive skeletogenesis in the sea urchin is used entirely differently in the starfish, where it responds to endomesodermal inputs that do not affect it in the sea urchin embryo. Evolutionary changes in the GRNs since divergence are limited sharply to certain cis-regulatory elements, whereas others have persisted unaltered.

Abstract: A range of marine invertebrates were screened for prophenoloxidase (a marker protein of the prophenoloxidase activating system) in the coelomic fluid or hemolymph. The crustaceans and the ascidian, Ciona intestinalis, displayed strongest activity, with low levels found in the starfish, Asterias rubens, the sea urchin, D. antillarum, and the brachiopod, Liothyrella uva. The enzyme appeared to be absent from the isopod, Glyptonotus antarcticus. Further analyses of the blood cells of selected species revealed that prophenoloxidase tends to reside in the granular-type cells or their equivalent, and in the crustaceans and C. intestinalis is activated by lipopolysaccharides. In arthropods, prophenoloxidase is known to represent the terminal component of a complex cascade of enzymes that functions in non-self-recognition and host defence. The present study establishes that the enzyme is present in the blood cells of most, but not all, crustaceans and occurs in certain other invertebrate species, notably the urochordate, C. intestinalis.