Transcript 5 Reproduction, Dispersal, and Migration

5 Reproduction and Sexuality in
the Marine Environment
©Jeffrey S. Levinton 2001
Reproduction, dispersal and migration are the
fundamental processes that allow living
populations to grow and exploit new
environments.
Definitions:
Reproduction: replication of individuals.
Dispersal: spread of progeny to locations that differ from
the parent.
Migration: a directed movement between specific areas
Nonsexual Reproduction
• Non-sexual reproduction:
Descendants are genetically identical
- clones
Colonial species produce a set of
individuals that are genetically
identical, known as a module; each
module may have arisen from a
sexually formed zygote
Organisms that benefit from group living commonly
reproduce asexually because group living benefits are
greatest when you are related.
Bryozoans (top left), tunicates
(top right), anemones (bottom
left), corals, and sponges are
all examples of organisms that
reproduce asexually.
Cost of Sex
Sex is a species property whereby different
individuals have the chance to exchange or
combine DNA to produce offspring.
• FEMALE: invests more energy in production
of eggs, and offspring’s DNA is only half
hers.
• Sex involves expenditure of energy and time
to find mates, combat among males
Benefits of Sex?
• genetic diversity - sex increases combinations of
genes - resistance against disease
• Alternative to sex: clones, must wait for
mutations to occur
• Sex - recombination produces variable gene
combinations, meiosis enhances crossing over of
chromosomes: new gene combinations and
variants
Types of Sexuality
• Separate sexes –gonochoristic
• Hermaphroditism -individual can have
male or female function
Hermaphroditism
• Simultaneous
Many nudibranchs (above) are
simultaneous hermaphrodites
Sequential
Protandrous - first male,
then female
Protogynous - first
female, then male
Simultaneous Hermaphrodites
What are the advantages?
• can mate with anyone you run
in to.
What are the costs?
•High cost in energy over time
to produce both eggs and sperm
What if any individual releases
sperm during mating, but then
does not release eggs??
Sequential Hermaphroditism:
Protandry and the size advantage model
• Eggs are costly in terms of
resources, so more offspring
produced when individual
functions as female when large
• Male function does not produce
great increases in offspring when it
gets larger.
Therefore, there is a threshold size
when female function begets more
offspring. Smaller individuals do
better as males.
Prawn (above) are
frequently protandrous.
Number of offspring produced
Male at advantage
Female at advantage
Female
Male
Body size
The size advantage model for Protrandry
Example: Selection in a Fishery
that illustrates the size
advantage model
• Shrimp Pandalus jordani, protandrous
• Danish, Swedish catch:
1930-1956: stable, increased slowly
1956- 1960: catch tripled to (2000 to 6300 ton/y)
Pandalus jordani fishery
Changes in Body Size
Period
1949-1950
1954-1957
1961-1962
% over 80 mm Somatic
growth
change
44%
0
25%
0
14%
0
Changes in Size of Change from Male to Female
Period
Before 1954
1954
1955-1962
% females < 75 mm
long
0
7% (65-74 mm)
14 % (55-74 mm)
Protogyny
• Male function must result
in more offspring when
male is older and larger
• Important when
aggression is important in
mating success, e.g., some
fishes where males fight
to maintain group of
female mates.
• Example: Blue-head
wrasse and other coral
reef fish species.
Terminal phase of a
Bluehead Wrasse (above).
•14 families of fish, 11 of
which are common in coral
reefs (porgies, damsels,
wrasses, parrotfish)
Trends observed in many fish families that are sequential
hermaphrodites:
 Fish that form harems or defend spawning sites are usually
protogynous.
 Fish that live in schools and are not closely associated with
the bottom, where mating is random are usually protandrous.
 Within protogynous species, population density plays a role in
the percent of the population that changes sex. At low densities,
large males can defend a harem or spawning site, and there are
usually no small males present (all males are the result of sex
change). At high densities, the population is nearly 50% primary
males (born male), basically gonochoristic.
3 important factors: (a) relative number of gametes produced,
(b) mating behavior of the species, and (c) density of the
population.
Sex - factors in fertilization
• Copulation: ( + choice of mate, increases
success of fertilization)
• Sperm applied to body of other individual or
egg clutch.
• Free spawning: shed gametes into water
column.
Fertilization success is affected by:
1. Volume of gametes
2. Sperm transfer
3. Distance between males and females
4. Water turbulence
5. Timing
Free Spawning
If there is anything truly unique about the
marine environment, it is the frequent
occurrence of free spawning.
What are the costs and benefits to
spawning??
Costs to Spawning in Success of Fertilization
1. Fertilization success is low if distance is great
and/or turbulence is strong.
- turbulence dilutes sperm and eggs, dispersing
them in all directions which makes encounters less
likely.
2. Timing must be simultaneous, tuned to a lunar cycle
or cued by other individuals
3. Gametes can encounter gametes of other species
and high densities of gametes.
4. Predation of eggs and sperm.
Planktonic sperm and eggs to prevent
fertilization by different species:
• Specialized binding/fertilization proteins
in sperm and receptors in eggs (bindin in
sea urchin sperm, lysin in abalone sperm)
• Sperm attractors in eggs
• Binding proteins are species-specific,
proteins with high rates of evolution