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2.2. A revision of the definitions of the early-life story stages McEdward & Janies (1993) reviewed the definitions of some important features that have been described inconsistently in the literature. Their re-definitions took into consideration the continuous nature of development and the evolutionary diversification of ontogenetic sequences. 2.2.1. Embryo In general, embryos can be defined morphologically as cleavage stages; blastulae, and gastrulae. These are particular stages in the development of all invertebrates. The blastula and gastrula are stages inherent of the Metazoa (Buss, 1987; Booner, 1988; Margulis, 1990). It is the uncertainty concerning the exact state of transition involving the start of morphogenesis and the completion of a definitive form that makes the distinction between late embryo and early larva, impossible to identify with precision. For this reason the expression “embryo” is best restricted to the stages of development which are of universal incidence (cleavage, blastula, and gastrula) and it should not be used to refer to the subsequent period of transition (McEdward & Janies, 1993). 2.2.2. Larva In spite of numerous attempts, no author has provided a specific and generally accepted definition of the term “larva”. Such a definition is very difficult to obtain especially because some authors on one hand consider the distinction between a larva and an embryo, and a larva and a juvenile on the other to be at some point arbitrary (Strathmann, 1985). The first problem when trying to define larva is in relation to recognizing boundaries between developmental stages. The second problem is that structural, ecological, and morphogenetic definitions generally do not coincide; 29
therefore a developing form may well be a larva by one definition, but not by the others. In the structural definition, a larva is a structural state or series of states that occur between the start of morphogenesis, which follows embryonic development (cleavage, blastula, and gastrula), and metamorphosis to the adult. The larva is a combination of temporary features and adaptations inherent to larval life that 1) are lost at metamorphosis, 2) are modified to generate adult functions at metamorphosis, and 3) are features of the adult that begin their development before metamorphosis occurs. In the structural definition, as long as morphogenesis continues to produce larval structures, and as long as those structures are lost (literally lost or absorbed by the rudiment) during metamorphic transformation, there will be a larva. The structural definition is in conflict with the standard ecological definition of a larva, especially when regarding brooded development in which young stages can be larval by morphological and developmental criteria, but do not live in the plankton and therefore do not provide larval dispersal (McEdward & Janies, 1993) From the ecological point of view, the developing organism is a larva only if it hatches and passes through a pelagic phase. The ecological definition considers the larva as a dispersive phase of the life history. Thus, an organism is still an embryo if it develops in a benthic egg mass or capsule, even if it possesses all the structural features of a larva. As an example, Chia (1974) defines that “Larva is a developmental stage, occupying the period from post embryonic stage to metamorphosis, and it differs from the adult in morphology, nutrition, or habitat. Post embryonic stage designates the time after emerging (hatching) from the primary egg membrane; prior to that it is considered as embryonic stage”. The difficulty with this definition from an evolutionary point of view, however, is the definition of post-embryonic stages as the 30
Page 1 and 2: University of Southampton Research
Page 3 and 4: Graduate School of the National Oce
Page 5 and 6: UNIVERSITY OF SOUTHAMPTON ABSTRACT
Page 7 and 8: Marthasterias glacialis (ECHINODERM
Page 9 and 10: Acknowledgments I know everybody sa
Page 11 and 12: Prière de l’étoile de mer Seign
Page 13 and 14: species of asteroid Patiriella para
Page 15 and 16: embryonic tolerances could determin
Page 17 and 18: dispersed over great distances and
Page 19 and 20: lecithotrophic development. The pre
Page 21 and 22: al., 1997). In a review (Vrijenhoek
Page 23 and 24: Although larval dispersal and settl
Page 25 and 26: larvae of shallow water echinoids f
Page 27 and 28: few hours, whereas other species su
Page 29: een defined as a unique sequence of
Page 33 and 34: adults” that have the definitive
Page 35 and 36: The dipleurula is the hypothetical
Page 37 and 38: among tunicate tadpoles, sponge and
Page 39 and 40: Fig.2.4. Bipinnaria larvae of the a
Page 41 and 42: In marine invertebrates there is a
Page 43 and 44: independent of habitat or mode of n
Page 45 and 46: whether juveniles derived from larv
Page 47 and 48: and the northern Mediterranean have
Page 49 and 50: Wares (2001) performed phylogenetic
Page 51 and 52: Fig. 3.2. A. Marthasterias glaciali
Page 53 and 54: 3.2.2- Temperature/pressure effects
Page 55 and 56: Pressure was applied using an Enerp
Page 57 and 58: Fig. 3.7. Bippinaria larvae of Mart
Page 59 and 60: X Data were abnormal. At 150 and 20
Page 61 and 62: X Data all the embryos were abnorma
Page 63 and 64: X Data blastulae, at 200atm 90% of
Page 65 and 66: 50atm more than 90% of embryos were
Page 67 and 68: Those data (Young et al., 1997; Ped
Page 69 and 70: Percentage of individuals swimming
Page 71 and 72: that settling invertebrate larvae d
Page 73 and 74: CHAPTER FOUR- REPRODUCTIVE FEATURES
Page 75 and 76: In 1973, Vance proposed a theoretic
Page 77 and 78: Species Volume Energy Dev Class Ref
Page 79 and 80: 4.1.2- Body Size The reproductive o
Page 81:
Species Depth of max. Depth range (
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arm, and the minor radius (r), from
Page 86 and 87:
From Eqs.1 and 3 we obtain the esti
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Data on maximum body size, maximum
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250 200 Body size (mm) 150 100 50 0
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Fig. 4.7. Hierarchical cluster anal
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(Hymenaster membranaceus Thomson).
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gamete production at low population
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varies depending on the vitellogeni
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size, high fecundity, small egg siz
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strategy described by Winemiller an
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It seems that evolutionary features
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strategy stablished by the Winemill
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which normal embryonic/larval devel
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Larval dispersal from eight populat
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These results are very important si
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possibility. The opportunistic spec
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hypothesis that the Mesozoic and ea
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Barnes, H. and H.T. Powell. 1951. T
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Chester, C.M. 1996. The effect of a
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Crisp, D.J. 1978. Genetic consequen
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David, B., A. Guille, J-P. Féral a
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Grant, A. 1983. On the evolution of
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Inaba, D. 1934. On some holothurian
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of the 6 th International Echinoder
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Lieberkind, I. 1926. Ctenodiscus au
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McEdward, L.R. and S.F. Carson. 198
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Minchin, D. 1987. Sea-water tempera
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Pawson, D.L. 1976. Some aspects of
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Pingree. R.D., B. Sinha and C.R. Gr
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Savidge, G., H.J. Lennon, and A.D.
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Shilling, F.M. and D.T. Manahan. 19
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Turner, R.L. and J.M. Lawrence. 197
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Vickery, M.S. and J.B. McClintock 2
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(Eds.): Reproduction Genetics and D
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