Download Lecture 09, molluscs 2 - Gastropoda - Cal State LA

Phylum Mollusca
Class Aplacophora - Small + wormlike, primitive (no shell)
Class Monoplacophora - Deep sea, cap-like shell; segmented?
Class Polyplacophora - Chitons
Class Gastropoda - Snails + slugs
Class Bivalvia - Clams, mussels, oysters
Class Cephalopoda - Octopus, squid, cuttlefish, nautilus
Class Scaphopoda - Tusk shells
Modern gastropod phylogeny
Orthogastropoda
Opisthobranchs (sea slugs)
Pulmonates (land snails + slugs)
Caenogastropoda
- No nacre in shells; operculum made of chiton
- Head with eyes at base of cephalic tentacles
- Radula with only 1-3 teeth per row
- Mantle forms incurrent siphon on left
- Loss of right-side organs, including right atrium of heart
- Separate sexes, males w/ penis
- Concentrated nervous system
Operculum is used for
protection and to prevent
dessication
Caenogastropods seen in lab
not sure if I have these for lab,
but you will see them
everywhere on the field trip
Vermetids are ecosystem
engineers much like the tube
worm Phragmatopoma –
they make reefs of heavy
calcareous tubes
Vermetid gastropod,
They use a mucus web to
filter feed like the Innkeeper
the tube-dweller
Serpulorbis squamigerous
Caenogastropods seen in lab
Cowries are a hyper-diverse
(= many species) group in
the tropics, but we have only
one local species
Why is the shell so polished
and smooth?
How does the animal respond
when you touch its shell?
Chestnut cowrie:
Cypraea spadicea
Mantle or Shell?
1) Snail is touched by arm of Pisaster
2) Immediately covers shell w/ mantle
3) Runs away
Mantle can present
deterrent chemicals
to allow escape from
would-be predators
Predation by Cone Snails
Cone snails use modified, harpoonlike radula to inject paralyzing
neurotoxins into fish, worms or
molluscs to capture prey
Then engulf prey with pharynx
Some have venom lethal to man
Only “drug from the sea” so far is a
back pain drug - a cone snail toxin!
Many caenogastropods lay benthic (attached to bottom)
egg masses that larvae develop in, and either..
a) hatch as veligers, or..
b) complete development + metamorphose inside,
emerging as crawl-away juveniles (direct development)
egg masses may
contain many
unfertilized
nurse eggs that
are food for
growing juvenile
snails, a form of
maternal nutrition
made possible by
development in a
“container”
Sea slugs!!
Heterobranchia
“opisthobranchia” (sea slugs)
Cephalaspidea
- Navanax + its prey Bulla
Anaspidea
- sea hares
Nudibranchia
- colorful predatory slugs
Sacoglossa
- specialist herbivores
Marine heterobranchs often have an enlarged “head shield”
divided into two lobes, extending out in front
Body often flows around and over the thin shell, or shell is lost
Cephalaspidea – “bubble shells”
- species have external or internal, reduced shell, and a wide
frontal head shield used to plow through sand
Chelidonura
Hydatina
Navanax inermis is an aggressive
predator -- of other sea slugs!
http://www.life.uiuc.edu/slugcity/movie/navanax_eat_hem.mpg
Order Anaspidea - Sea Hares
- includes world’s largest gastropod, Aplysia vaccaria
- parapodia folded over mantle cavity
- herbivorous: eat particular macroalgae, or cyanobacteria
- defensive chemistry + inking
release stored
toxins from algal
diet (in this case,
red algae)
Anaspideans - Sea Hares
Nobel Prize in medicine in 2002 awarded for work on learning
& memory in sea hares, for studies of how nerves habituate
to repeated stimuli (poking the siphon over and over)
also famous for group sex
Nudibranchia
branchial plume, a circle of gills, used
instead of ctenidia; surrounds anus on
dorsal surface, can be withdrawn
mantle -- margin contains secretory
glands that release defensive
chemicals from sponge or coral diet
rhinophores = antennae used to chemically detect prey
Nudibranchia
group Doridacea (Dorid nudibranchs - predators)
Chromodoris
Phyllidia
Halgerda
Glossodoris
Nudibranchia
group Doridacea – genus Hypselodoris
Hypselodoris iacula
Hypselodoris
kanga
Chromodoris annae
Hypselodoris fucata
Hypselodoris nigrostriata
Crypsis vs. Aposematism
Crypsis:
avoid being
seen by
predators
Aposematic coloring:
make yourself obvious
- common in chemically defended
taxa
- Batesian mimicry
Aposematic coloring often leads to Batesian mimicry,
where a non-toxic organism evolves to “impersonate” a
toxic animal to obtain protection from predators
both nudibranchs are highly toxic
nudibranch
Chromodoris
magnifica
Nudibranch Phyllidiella pustulosa (left)
& flatworm Pseudoceros imitatus
flatworm Pseudoceros sp.
Nudibranchia
Aeolidacea - Aeolid nudibranchs
- cerata on backs store cnidae from prey, for slug’s defense
- one radular tooth per row, with jaws of chiton to tear off hunks
of flesh from prey
Hermissenda
crassicornis
Godiva
quadricolor
Recycling Cnidae
Immature cnidae are sorted in digestive diverticula, passed to
cnidosac where they mature; function in defense of slug
Melibe leonina
- catches tiny crustaceans
under oral hood
- resembles a moving
Venus fly trap
The fabulous Bornella anguila
- feeds on hydroids
- swims like an eel
Nudibranch Reproduction
Mating is reciprocal; penis
on right side of head
Aeolids: coiled egg ribbons
Dorids: colorful, spiral egg ribbons
Sacoglossa – the ultimate vegetarians
- specialist herbivores that feed on siphonaceous algae
( the kind that are one big cell…)
- radula is just 1 tooth, used to pierce algae and suck out juice
- each species feeds on a different species of host algae..
- allows many species to co-exist without competing
- may promote speciation by switching onto new algae
Some species exhibit kleptoplasty:
chloroplasts from their food algae
are kept in cells lining digestive
diverticula which wind thru body
Diverticula surround dorsal vessels:
clear tubes may be reverse gills to
remove O2 from photosynthesis, to
avoid DNA damage
Elysia chlorotica: slugs stop
eating + live off photosynthesis
of their hijacked chloroplasts
for up to 9 months!
Problem: Chloroplasts burn out
their light-harvesting proteins
Elysia chlorotica in a month
A retrovirus has picked up >50 genes from the algae, and
moved them onto the slug’s chromosomes where they
supply the missing chloroplast proteins
Slug is thus a genetic fusion of algae and animal
... but at the end of each breeding season, after slugs lay
their eggs, the virus emerges from all slugs’ DNA, kills
every adult slug, then re-infects the settling larvae
Elysia chlorotica: slugs stop
eating + live off photosynthesis
of their hijacked chloroplasts
for up to 9 months!
Problem: Chloroplasts burn out
their light-harvesting proteins
Elysia chlorotica in a month
A retrovirus has picked up >50 genes from the algae, and
moved them onto the slug’s chromosomes where they
supply the missing chloroplast proteins
Slug is thus a genetic fusion of algae and animal
Video presentation by Skip Pierce available at:
www.youtube.com/watch?v=TGZdiXYpY30
Sacoglossan Reproduction
Sacoglossans are hermaphrodites, by many mate by
hypodermic insemination (also seen in some flatworms)
- sperm are injected anywhere on the recipient’s body;
swim through tissues to locate eggs or storage organs
- may not involve consent
- results in complex mating
behaviors and strategies:
when should you be the boy?
when should you be the girl?
Hypodermic Insemination
Large slugs aggressively inseminate smaller
specimens, even of the wrong species
Mating results in tissue damage that lowers egg
production (cost of mating)
Here, 3 slugs inseminate each other by injecting sperm
through a weak spot in body wall
Species you may see in lab
Alderia willowi – the only animal that can switch between lots
of tiny eggs (planktotrophy) and a few big eggs (lecithotrophy)
My lab named this species, which we discovered living in
southern California up to San Francisco
Recent papers have claimed that either:
a) pulmonates (land snails + slugs) are paraphyletic, and
sea slugs evolved from within the pulmonates
b) sea slugs are paraphyletic, and pulmonates evolved from
a sea slug ancestor
No phylogeny has ever resolved the placement of the group
I study, Sacoglossa – no idea where it goes on mollusc tree
To try to answer this, we analysed 102 gene sequences…
Siphonaria: only
snail with both
gill & lung
sea
slugs
1) opisthobranchs are indeed paraphyletic
2) closest relative of pulmonates is… my sacoglossans!
3) basal pulmonate Siphonaria has both a sacoglossan-like gill
and a primitive version of the pulmonate lung
New Mollusc Phylogeny
Two studies in Nature
analysed hundreds of
gene sequences to resolve
the phylogeny of molluscs
Suggests cephalopods and
snails independently evolved
large brains (“smartness”)
Conchifera: ancestor
had a one-piece shell
bivalves evolved a
2nd shell valve
Kocot et al. 2011
(308 genes)