Transcript Larval Transport

Migration, Dispersal, and Larval Ecology:
• Many abundant commercially exploited fish spp.
conform to the spawning and migration cycle - where
the juveniles drift from a spawning area to a nursery
area and are recruited to an adult stock area that feeds
a feeding ground; adults then migrate to the spawning
area
• The gradual movement of fishes from the nursery
grounds to different adult feeding grounds (not rich
enough) may reflect competition between juveniles
and adults for limited food resources
• Spawning
area - region where eggs are laid and
inseminated. Eggs may be laid in midwater
(pelagic) or on the bottom (benthic)
• In salmonids, spawning grounds are located on
the bottom of freshwater streams and young
smelts then migrate down stream to the sea
• In Atlantic salmon - migration occurs towards
the sea after 2-3 years; in Pacific salmon, it
happens in a few weeks
• In
the Atlantic herring, Clupea harengus, eggs
are attached to plant stems and gravel by the
female while the males swim and deposit sperm
• In contrast, the cod, Gadus morhua, spawns in
the water column - male and female fishes swim
in contact as eggs and sperm are released
• Anadromous
- (salmon, shad, sea lamprey)
spend most of their time in the sea and
return to freshwater to breed
• Catadromous (eels) spend their adult lives
mainly in freshwater and migrate to the sea
to reproduce
• Oceanodromous - (herring, cod, plaice) live
and migrate in the ocean
Invertebrate Larval Development
Planktotrophic Development:
• In many invertebrate spp., the time between release
of larvae in the plankton and final metamorphosis is
genetically programmed and mediated through a series
of developmental stages
Risks of mortality and benefits of dispersal
• Metamorphosis may not occur if no suitable
substratum exists for adult life – can lead to death
• M. edulis - 4-5 weeks (veligers); Balanus sp.. 3-4
weeks (nauplii) -
Lecithotrophic Development:
• Larva has a short planktonic stage and uses a large
egg yolk as a source of energy during development in
like plankton
• Ectoprocts - may brood eggs in socially modified
chambers called ovicells – Bugula spp.
• Even though the dispersal phase is short,
lecithotrophy must have evolved for the purpose of
dispersal because even the existence for a short period
like an hour will permit a dispersing larva to travel
hundreds of meters and escape crowded conditions
Direct Release From Egg Cases:
• Planktonic phase completely absent development in an egg case is often
supplemented by some form of parental
protection
• Young released into immediate environment
Gene Flow:
• Allozyme differences of the mud snail
Ilyanassa obsoleta - a species with longdispersing planktotrophic larvae - was compared
with the intertidal snail Littorina saxatilis which
has direct release
• Littorina has more local geographical
differences than does the long disperser I.
obsoleta
Within Parent Development (Viviparous):
• Complete development occurs within the
parental animal up to the point that the embryo
is capable of escaping to the bottom as a fully
developed juvenile - few young produced
• Littorina saxatilis - eggs arrive in the brood
chamber already fertilized; brooding occurs in
an altered part of the oviduct “uterus” containing
young in various stages - young snails then
emerge as fully shelled juveniles
Distribution
• Latitudinal Trends - Murray (1895-1898);
Thorsen (1950) - about 70% of contemporary
species of marine-bottom invertebrates have
planktonic development
• However, in the Arctic 90% of marine species
develop without phase and with large yolky eggs
• Arctic planktotrophic larvae confront 2
problems: a short phytoplankton season and cold
temperatures
• Antarctic
species - many are brooding species
• 30-45% of sea stars
• Although rich planktonic production occurs in
Antarctic seas, production takes place at the very
surface of the open ocean while bottom organisms
inhabit partly shallow-bottom shelves where food
reserves can be low
•A second trend suggested by Thorsen (1950) is a
variation in reproduction with depth of water.
Claimed that dominance of non-planktonic
development in Arctic and Antarctic oceans held for
deep sea as well (low food resources)
• Scheltema
(1971)- discovery of the large-scale
occurrence of long-lived planktonic larval stages in the
tropics promoted large-scale survey with larvae of
benthic stages occurring in the Gulf Stream, N.
Atlantic drift, and equatorial currents
• Scheltema termed such ocean-going larvae of shelf
invertebrates teleplanic larvae
• estimates range of 42-320 days of possible life in
the plankton for planktotrophic gastropod larvae
• one result of long-distance dispersal is the
possibility of large-scale geographic ranges of
tropical marine invertebrates
Settlement of Larvae:
• 3 major stages larvae must pass before
successful settlement:
• successful development
• retention nearshore
• substratum selection
• Reasons for not succeeding - food shortage,
wastage of larvae, predation, crowding
Metamorphosis:
1) physical characteristics of the substratum such as
the presence of pits and grooves or the presence of
sand grains of suitable diameter
2) presence of adults of the same species (gregarious
settling)
3) contact with a substance produced by a species that
predictably co-occurs with adults of the larva in
question
4) contact with some generalized biological
substratum feature - bacterial films
Physical Substratum:
• grain size - polychaete Ophelia bicornis - wellrounded sand grains
- Ilyanassa - fine-grained
• gregarious settling
Theoretical Considerations:
• Vance (1977) attempted to predict optimal
reproduction and dispersal strategies
2 questions:
1) Why do we observe only exclusively lecithotrophic
or exclusively planktotrophic development for most
species? Few species have an extended period of
dependence on yolk reserves with subsequent
dependence on feeding planktonic larva
2) Under what conditions should a larva feed in the
plankton or the benthos, develop living off a yolky
egg, or simply develop within an egg case or parent
and hatch as a free-living juvenile?
Strathman (1974) Model:
a) egg content energy
b) length of pre-feeding period
c) length of feeding period
d) total larval developmental period
e) mortality rates
Model Predicts- 1) Only extremes of the possible
ranges of egg size and method of nutrition are stable in
an evolutionary sense
2) Over range of environmental parameters, 2
developmental types are both evolutionary stable
states
3) Planktotrophic development is more efficient than
lecithotrophic development when more planktonic
food is available than the reverse situation
4) Benthic pre-feeding development results in greater
efficiency when lecithotrophic developmental time is
long and/or plankton predation is more intense than
benthic predation
Reproductive Strategies
Allocation of Resources:
• Growth rate of Balanus balanoides decreases
during the reproductive or spawning period egg production
• Semelparous - once (Gadgil & Bossert,
1970)
• Iteroparous - repeatedly
• Current reproductive activity and future
reproductive success are inversely correlated
(Williams, 1966)
• Must
marine animals have a life cycle that can
be divided between larval and adult stages.
• In such life cycles the risk of being a larva may
be compared with adult mortality - selection of
reproductive changes
• Under high adult mortality the species with the
highest rmax will win. So, when there is
significant danger to adults, we expect early
reproduction and one-time spawning
(semelparity)
• Iteroparity is favored if a random component
to juvenile mortality is introduced. Under these
circumstances, reproducing more than once
increases the probability that a year-class of
adults will successfully recruit another yearclass of juveniles
Asexual vs. Sexual:
• Advantages of asexual - proliferation of
genotype that successfully colonizes a given
microhabitat
• Secondly, the proliferation of this genotype is
not hampered by the suite of adaptations
necessary for gamete union
• All are identical - poorly suited for sudden
changes
Binary fission and fragmentation – fission (i.e.,
diatoms – typical doubling time 0.6-6 per day),
frag. – multicellular (i.e., Enteromorpha –
fragmentation leads to new indiv. (buds)
Parthenogenesis – unfertilized eggs develop into
normal indiv. – (Rotifers) – sperm required but
does not contribute to genes (rare)
Vegetative Reproduction – division of an animal
into many indiv. which may or may not be
connected (encrusting sponges and ectoprocts)
Anthopleura elegantissima:
• Contiguous aggregations composed of individuals
from a single clone
• planula larva settles, metamorphoses, and divides
asexually producing hundreds of clones
• Anthopleura also has separate sexes and normal
sexual reproduction. Thus, the need for genetic
variation is maintained
• Colonies may have structural function - elkhorn coral
- Acropora palmata - minimize effects of currents and
waves
Hermaphroditism:
• Single individual contains gametes for both
sexes
• Can reproduce with any other individual in the
population; self-fertilization is RARE
• Barnacles – Balanus spp.
• Sequential hermaphrodites - can change from
male to female (protandry) or female to
male (protogony)
• Protandry is the general rule for many marine
invertebrates
• Crassostrea
virginica - smaller young make
transformation into female after a few years Self-Fertilization is Possible
• Also suspension-feeding gastropod Crepidula
fornicata - larger older females below
• Some sequential hermaphroditism in fishes coral reefs (i.e., wrasses)
• Labroides dimidiatus (wrasse) - small school,
about 15 individuals - largest member is male,
all others female
• If male dies - largest member of the group of
females will change into male
• Male to female change is dependent on
predation
• This form of sex change is common among
reef fishes
Separate Sexes:
a) Both eggs and sperm may be shed in the
water
b) Sperm may be shed in the water and fertilize
eggs held by the female
c) Sperm may be mechanically transformed
from male to female
• Epidemic
spawning - a few individuals induce
mass spawning by others
• May be correlated with lunar phases - within a
few hours, all have spawned
• Reef sponges - spawn at same time – large
clouds in the water
• Adult nereid, syllid, eunicid polychaetes change morphology into individuals filled with
gametes (epitokes) that swim to the surface and
spawn - nuptial dance
Sexual Reproduction in Benthic Algae
• May be very complex with alternations of
diploid and haploid phases
• gametophyte - gametes in gametangia
• motile – brown algae
• non-motile – red algae