241
are sequential. Additional to the above discussion on egg
size and numbers, the available data on simultaneous oocyte
development can be considered as further support for scenarios illustrating the main trends in the hypothetical transition between the two patterns.
3.1.2.3 Shorter Duration of Larval Development
The change in the mode of oogenesis and the transition to a
new larval type resulted in considerable modifi cation of
embryonic development. There were also corresponding
changes in genome activity (Wray and Raff 1991 ; Raff 1996 ).
A mathematical model describing reproduction in marine
invertebrates (Vance 1973 ) establishes a correlation between
productivity (increasing with decreasing egg size) and mortality (depending on the life span of the larva). The model is
based on the assumption that egg enlargement results in (1)
an increase in the length of the prefeeding period, and (2) a
reduction in the feeding period. Speaking generally, egg size
(i.e. amount of nutrients stored in the oocyte) can infl uence
the larval life span by affecting its duration (see also above).
Though Vance’s model does not take into account numerous
factors that may infl uence development rate (for instance
temperature; see Hoegh- Guldberg and Pearse 1995 ), it is nevertheless a plausible refl ection of the situation observed in
nature (Strathmann 1977 , 1985 ; Havenhand 1995 ; Marshall
and Bolton 2007 ). To note, the mathematical model by
Havenhand ( 1993 ) indicates that reduction of the larval development period provides a selective advantage.
Does the increase in the size of oocytes indeed infl uence
the duration of larval development? Researchers are divided
on this point. On the one hand, a considerable body of evidence indicates a correlation between larval type and the
duration of the development period from egg to juvenile –
the life span of planktotrophic larvae to competency is typically longer than lecithotrophic ones that usually develop
from larger eggs (Todd and Doyle 1981 ; Emlet et al. 1987 ;
Wray and Raff 1991 ; Havenhand 1993 ; Hoegh-Guldberg and
Pearse 1995 ; Raff 1996 ). So it is generally thought that the
larger the eggs, the shorter the development. The idea behind
this is that the nutritive reserves contained in the oocyte fuel
the acceleration of development and metamorphosis
(Villinski et al. 2002 ) through higher physiological rates and
heterochronies (Raff 1996 ). This dependence has been
described, for instance, for sea urchins, and it is often quite
well expressed even if we compare species with planktotrophic larvae developing from eggs of different size (Sinervo
and McEdward 1988 ; Wray and Raff 1991 ; Hoegh-Guldberg
and Pearse 1995 ). When comparing development time from
fertilization till metamorphosis in two species of Clypeaster
(Echinoidea) with planktotrophic and facultativeplanktotrophic larvae correspondingly, it was 9 days less for
the species with the larger eggs and facultative planktotrophy
(Emlet 1986 ). Experiments with isolated blastomeres of two
other species from the genus Strongylocentrotus demonstrated a negative correlation between blastomere diameter
and the rate of development at early embryogenesis stages:
the smaller the initial blastomere, the slower the development rate (after a certain size has been achieved, the rate of
development is restored) (Sinervo and McEdward 1988 ).
On the other hand, an analysis by Underwood ( 1974 )
demonstrated the absence of any such correlation in prosobranch molluscs, some insects and birds. Ghiselin ( 1987 ),
too, in his review cited data from Spight ( 1975 ) about the
decreasing development rates with increasing size of oocytes
in gastropods, i.e. the tendency appears to be just the opposite (see also Emlet et al. 1987 ; Havenhand 1993 ; HoeghGuldberg and Pearse 1995 ; Marshall and Bolton 2007 ). For
instance, the development of the planktotrophic larva of the
sea star Porania antarctica is completed two weeks faster
than the lecithotrophic larva of Porania sp. At the same time,
the diameter of oocytes in these two co-occurring species is
the same (Bosch 1989 ). Strathmann ( 1977 ), too, reported
both variants from different groups of marine invertebrates.
After comparing the data in the literature, HoeghGuldberg and Pearse ( 1995 ) came to the conclusion that the
key factor determining the rate (and duration) of development of echinoderm larvae is water temperature [To note,
Clarke ( 1982 , 1992 ) considered this factor to be unimportant
for the development rate of invertebrates in polar waters].
The comparison made by the two above-mentioned authors
showed that, despite the slower development rates of planktotrophic larvae (given the same temperature) as compared
with lecithotrophic ones, a correlation between oocyte diameter and the duration of development is not at all obvious.
Against the background of a distinct dependence between
the larger size of oocytes and the shortened duration of
development, numerous contradictory examples stand out –
among echinoderms there are both species with small
oocytes and rapidly developing planktotrophic larvae and
species with large oocytes and slowly developing lecithotrophic larvae. So, as with the correlation between oocyte size
and larval type (see above), it is probable that the dependence under discussion does exist but is not as distinct as
generally thought.
As for bryozoans, the life span of cyphonautes larvae
(which are formed from microlecithal eggs) varies from presumably a few days (Dudley 1973 ) to 2 months (Marcus
1926b ; Kluge 1975 ) in different species. Indeed, larvae of
the malacostegine Membranipora membranacea reportedly
live 4 weeks in the sea, and survived up to 8 weeks in the
laboratory (Yoshioka 1982 ). Cadman and Ryland ( 1996 ),
having compared the dates of the reproductive peak in the
ctenostome broadcaster Alcyonidium mytili and the peak of
occurrence of its cyphonautes larvae in the plankton, concluded that the life span of these larvae should be 4–6 weeks.
Planktonic larval duration is not known for Electra , although
3.1 Modifi cation of Oogenesis and Its Evolutionary Consequences
are sequential. Additional to the above discussion on egg
size and numbers, the available data on simultaneous oocyte
development can be considered as further support for scenarios illustrating the main trends in the hypothetical transition between the two patterns.
3.1.2.3 Shorter Duration of Larval Development
The change in the mode of oogenesis and the transition to a
new larval type resulted in considerable modifi cation of
embryonic development. There were also corresponding
changes in genome activity (Wray and Raff 1991 ; Raff 1996 ).
A mathematical model describing reproduction in marine
invertebrates (Vance 1973 ) establishes a correlation between
productivity (increasing with decreasing egg size) and mortality (depending on the life span of the larva). The model is
based on the assumption that egg enlargement results in (1)
an increase in the length of the prefeeding period, and (2) a
reduction in the feeding period. Speaking generally, egg size
(i.e. amount of nutrients stored in the oocyte) can infl uence
the larval life span by affecting its duration (see also above).
Though Vance’s model does not take into account numerous
factors that may infl uence development rate (for instance
temperature; see Hoegh- Guldberg and Pearse 1995 ), it is nevertheless a plausible refl ection of the situation observed in
nature (Strathmann 1977 , 1985 ; Havenhand 1995 ; Marshall
and Bolton 2007 ). To note, the mathematical model by
Havenhand ( 1993 ) indicates that reduction of the larval development period provides a selective advantage.
Does the increase in the size of oocytes indeed infl uence
the duration of larval development? Researchers are divided
on this point. On the one hand, a considerable body of evidence indicates a correlation between larval type and the
duration of the development period from egg to juvenile –
the life span of planktotrophic larvae to competency is typically longer than lecithotrophic ones that usually develop
from larger eggs (Todd and Doyle 1981 ; Emlet et al. 1987 ;
Wray and Raff 1991 ; Havenhand 1993 ; Hoegh-Guldberg and
Pearse 1995 ; Raff 1996 ). So it is generally thought that the
larger the eggs, the shorter the development. The idea behind
this is that the nutritive reserves contained in the oocyte fuel
the acceleration of development and metamorphosis
(Villinski et al. 2002 ) through higher physiological rates and
heterochronies (Raff 1996 ). This dependence has been
described, for instance, for sea urchins, and it is often quite
well expressed even if we compare species with planktotrophic larvae developing from eggs of different size (Sinervo
and McEdward 1988 ; Wray and Raff 1991 ; Hoegh-Guldberg
and Pearse 1995 ). When comparing development time from
fertilization till metamorphosis in two species of Clypeaster
(Echinoidea) with planktotrophic and facultativeplanktotrophic larvae correspondingly, it was 9 days less for
the species with the larger eggs and facultative planktotrophy
(Emlet 1986 ). Experiments with isolated blastomeres of two
other species from the genus Strongylocentrotus demonstrated a negative correlation between blastomere diameter
and the rate of development at early embryogenesis stages:
the smaller the initial blastomere, the slower the development rate (after a certain size has been achieved, the rate of
development is restored) (Sinervo and McEdward 1988 ).
On the other hand, an analysis by Underwood ( 1974 )
demonstrated the absence of any such correlation in prosobranch molluscs, some insects and birds. Ghiselin ( 1987 ),
too, in his review cited data from Spight ( 1975 ) about the
decreasing development rates with increasing size of oocytes
in gastropods, i.e. the tendency appears to be just the opposite (see also Emlet et al. 1987 ; Havenhand 1993 ; HoeghGuldberg and Pearse 1995 ; Marshall and Bolton 2007 ). For
instance, the development of the planktotrophic larva of the
sea star Porania antarctica is completed two weeks faster
than the lecithotrophic larva of Porania sp. At the same time,
the diameter of oocytes in these two co-occurring species is
the same (Bosch 1989 ). Strathmann ( 1977 ), too, reported
both variants from different groups of marine invertebrates.
After comparing the data in the literature, HoeghGuldberg and Pearse ( 1995 ) came to the conclusion that the
key factor determining the rate (and duration) of development of echinoderm larvae is water temperature [To note,
Clarke ( 1982 , 1992 ) considered this factor to be unimportant
for the development rate of invertebrates in polar waters].
The comparison made by the two above-mentioned authors
showed that, despite the slower development rates of planktotrophic larvae (given the same temperature) as compared
with lecithotrophic ones, a correlation between oocyte diameter and the duration of development is not at all obvious.
Against the background of a distinct dependence between
the larger size of oocytes and the shortened duration of
development, numerous contradictory examples stand out –
among echinoderms there are both species with small
oocytes and rapidly developing planktotrophic larvae and
species with large oocytes and slowly developing lecithotrophic larvae. So, as with the correlation between oocyte size
and larval type (see above), it is probable that the dependence under discussion does exist but is not as distinct as
generally thought.
As for bryozoans, the life span of cyphonautes larvae
(which are formed from microlecithal eggs) varies from presumably a few days (Dudley 1973 ) to 2 months (Marcus
1926b ; Kluge 1975 ) in different species. Indeed, larvae of
the malacostegine Membranipora membranacea reportedly
live 4 weeks in the sea, and survived up to 8 weeks in the
laboratory (Yoshioka 1982 ). Cadman and Ryland ( 1996 ),
having compared the dates of the reproductive peak in the
ctenostome broadcaster Alcyonidium mytili and the peak of
occurrence of its cyphonautes larvae in the plankton, concluded that the life span of these larvae should be 4–6 weeks.
Planktonic larval duration is not known for Electra , although
3.1 Modifi cation of Oogenesis and Its Evolutionary Consequences