Molluscs 02
Transcript of Molluscs 02
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Mollusca
mollusks
bltdjur
molluscs
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What is a mollusc?
Fundamental organization (hypothetical archimollusc):
- shell secreted by a layer of tissue called the mantle- mouth and anus at opposite end (but in gastropods both anterior)- mantle cavity bears gills (but pulmonate gastropods have no gills)- above mantle cavity is the visceral mass
with gut, nervous, circulatory and muscular system
- shell is of calcium carbonite (calcite or aragonite) (but may be secondaryly lost)- shell typicallyexternal (but in some groups it became internal)- grow by accretion (calcium carbonate is added to the edge of the shell by the mantle)- generally marine (but also few freshwater terrestrial groups)
Mollusca
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Mollusca
systematics main groups
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Mollusca systematics Bivalvia
Bivalvia
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Mollusca systematics Bivalvia
Bivalvia (=Pelecypoda, Lamellibranchia)
pair of valves (right and left valve) bilobed mantle valves articulate along a dorsal hinge line no head typically bilaterally symmetric
(plane of symmetry passing between the valves, = commissural plane) prominent ventral foot known since the Early Cambrian, but diversify not prior to Ordovician but still not a very common faunal element during the Paleozoic
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Mollusca systematics - Bivalvia
Main features of the shell
muscle scars ligament dentition lunule pallial line beak
homomyar internal cardinalia (escutcheon) sinupalliate orthogyrate
heteromyar external lateralia integripalliate prosogyrate !
monomyar amphidetic opisthogyrate
prosodetic
opisthodetic !
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Gills: Protobranchs (deposit feeders, most primitive)
Filibranchs (suspension feeders)
Eulammelibranchs (suspension feeders)
Septibranchs (carnivores, most derived)
Basic for systematics are the gill type and the hinge dentition
Mollusca systematics Bivalvia gill types
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Mollusca systematics Bivalvia dentition
Dentition: Various types and subtypestaxodont dysodont isodont schizodont desmodont pachydont heterodont
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Types of dentition
Taxodont many small similar teeth & sockets all along hinge plate (e.g., Glycimeris andArca)
Schizodont two or three thick teeth with prominent grooves (e.g., Trigonia)
Dysodont small simple teeth near the edge of the valve (eg Mytilus)
Heterodont few teeth varying in size and shape, distinquished as cardinal teeth, beneath the umbo, and
lateral teeth which lie obliquely along the hinge plate (e.g., most recent bivalves)
Isodont teeth very large and located on either side of a central ligament pit (e.g., Spondylus)
Desmodont teeth very reduced or absent (e.g., Mya) with a large internal process (the chondrophore)
carrying the ligament
Mollusca systematics Bivalvia dentition
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taxodont
Taxodont many
small similar teeth &sockets all along hinge
plate (e.g., Glycimeris
andArca)
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dysodont
no teeth just crenulation
Dysodont
small simple
teeth near the
edge of the
valve (eg
Mytilus)
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heterodont
cardinalia and lateralia
Heterodont few teeth varying in size and shape, distinquished as cardinal teeth,
beneath the umbo, and lateral teeth which lie obliquely along the hinge plate (e.g.,
most recent bivalves)
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isodont
two teeth correspond to two grooves
Isodont teethvery large and
located on either
side of a central
ligament pit (e.g.,
Spondylus)
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schizodont
teeth have crenulations (teeth with teeth)
Schizodont two or three thick teeth with prominent grooves (e.g., Trigonia)
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desmodont
internal process (the chondrophore) carries the ligament
Desmodont teeth very reduced or absent (e.g., Mya) with a large internal
process (the chondrophore) carrying the ligament
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pachydont
Pachydont large, heavy and massive teeth (e.g., rudists)
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Mollusca systematics Bivalvia orientation
Orientation of a bivalve shellwhat is posterior anterior right left ?
ligament typically posterior posterior adductor muscle scar stronger developed pallial sinus posterior / shell gaps posterior posterior part of shell typically better developed
umbo (beak) typically points anterior (prosogyre) byssal notch anterior
Oysters: left valve bigger/cemented
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Mollusca systematics Bivalvia orientation
right
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Mollusca systematics Bivalvia orientation
right
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Mollusca systematics Bivalvia orientation
right
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Mollusca systematics Bivalvia orientation
right
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Mollusca systematics Bivalvia orientation
left
Ecology
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Ecology
marine and fresh water
typically benthic, infaunal or epifaunal include burrowing, browsing, cemented, free lying, swimming, boring forms
filter feeders, deposit feeders, carnivores
Mollusca systematics Bivalvia ecology
Infaunal bivalves
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Infaunal bivalves
Both detrivorous and filtering strategies
Most Palaeozoic groups are infaunal detrivores
Probably the oldest of all bivalve life-modes
Burry thorugh sediment with the muscular foot
Extensions of the mantle (siphons) allow water
transportShell modified to specific substrate requirements
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Sessile Epibenthic bivalves
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Sessile Epibenthic bivalves
Attaches to hard subsrates and becomes immobile
Many groups have evolved this lifemode independently
Allows effective filterfeeding
Mytilus (common blue mussel) and many others attach
by chitinous threads (byssus) secreted by the footOysters attach by cementing one valve (left) to the
substrate and adapt to the shape of the substrate
Motile epibenthic bivalves
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Motile epibenthic bivalves
Lie exposed on the seabed
Mostly filterfeeders
Acute sensory system including photophores (eyes) and
sensory tentacles along the mantle edge
Escape strategy: Rapid closure of the valves createsjetstream and the mussel can thus swim short distances
Some J urassic bivalves may have been permanent
swimmers
Soft sediment recliners and mudstickers
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Some byssally and cementing forms have
evolved secondary soft sediment life-modes
Larvae attaches to small objects and develops
shapes that allows the bivalve to survive on the
sediment surface
Pinnate bivalvesGryphaea (devils toenail)
Reef-forming bivalves
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g
Modern Tridacna clam
Rudists (J urassic-Cretaceous) reef builders
Differential valves
Cone-shaped right valve
Left valve acts as a lid
Probably had symbiotic algae like modernTridacna
Evolved from oysters?
Reef-forming bivalves
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g
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Rock boring bivalves
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g
Several groups of bivalves can produce
livingchambers by boring through rock and wood
Lithophaga
calcareous substrates (corals, limestone
etc.)
Valves without gape
Exclusively chemical excavation
Pholadids
All types of substrates
Wood, corals, granite, lead cables, plastic,
amber etc.
Valves with wide anterior gape
Excavation by movement (abrasion)Shell ornament of teeth and rockfragments
wedged between them act as sandpaper
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Mollusca systematics Cephalopoda
Cephalopoda
Cephalopods
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Mollusca systematics Cephalopoda
p p
most highly evolved molluscs (especially eyes and brain) a high level of cephalization (concentration of sensory and neural centers in the head) group includes the modern Nautilus, argonauts, squids, octopuses, cuttlefishes
as well as the fossil ammonites and belemnites 2 main groups: Palcephalopoda (nautilids and endoceratids)
Neocephalopoda (orthoceratids, ammonites, belemnites)
typically bilaterally symmetrical shell, if developed, subdivided in chambers by septae chambers are connected by a tube (siphuncle) hyponome and tentacles are homologue to foot of bivalves and gastropods
mouth with powerful horny beaklike jaws and a radula radula less developed than in gastropods since Late Cambrian
Neocephalopods
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p p
Spirula
Loligo (Squid)
Sepia (Cuttlefish)
Octupus
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Palcephalopoda (Nautilus + fossils)
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Shell terminology
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Mollusca systematics Cephalopoda shell morphology
protoconch
septal neck
peristome
shell wall
growth line
camera /
chamber
septum
aperture
phra
gmoc
one
living
chamb
er
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Mollusca systematics Cephalopoda morphology shell
The suture = junction between septa and shell wall
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Mollusca systematics Cephalopoda morphology shell
saddles: point in apertural directionlobes: point backward
- most important for taxonomy and phylogeny ofAmmonitoidea
- particular types characterize distinct families and orders
prosuture primary suture
Shape of shell
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Mollusca systematics Cephalopoda
The cephalopod jaw
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Modern Cephalopods have a horny beak, either two simple plates or more
complex structuresThere is also a radula with rel. simple, undifferentiated teeth
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Mollusca systematics Cephalopoda morphology shell
Classification
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Old: Nautiloidea Ammonoidea Coleoidea
Palcephalopoda (~Nautiloidea) Neocephalopoda (Orthoceratoidea, Ammonoidea, Coleoidea)
Palcephalopodashell well developed and large, originally slightly curvedsiphuncle was situated between the center and the ventral surface.siphuncle generally large with internal deposits (important tax. feature)
Neocephalopodasiphuncle thin and emptyphragmocone originally straight with the siphuncle situated at or near the center
later the position of the siphuncle shifted to the ventral surface (Bactritida),the shell became coiled (Ammonoidea)the shell became internal, reduced or absent (Coleoidea)
Mollusca systematics Cephalopoda
Palcephalopoda (= Nautiloidea, + several Paleozoic groups, excl. orthoceratids)
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Mollusca systematics Cephalopoda
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Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)
Ammonoidea Ammonitida
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Mollusca systematics Cephalopoda
Ammonoidea - Ammonitida
Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)Ammonoidea heteromorphic ammonites
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Mollusca systematics Cephalopoda
Ammonoidea heteromorphic ammonites
SpirocerataceaeMiddle J urassic
Ancyloceratinalatest J urassic to end Cretaceous
ChoristocerataceaeLate J urassic
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Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)
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Coleoidea
Coleoids have little skeletal materialConsequently are rare as fossils
Fossisl date back to the CarboniferousProbably derived from orthoconeNeocephaolopds in the Devonian
Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)Coleoidea Belemnitida
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Mollusca systematics Cephalopoda
Belemnites were squid-like with internal shell (Phragmocone)The posterior of the phragmocone had mineralised deposits
(rostrum or guard)The rostrum is a massive, calcareous structure and hencefossilise extremely well (contrary the phragmocone)Probably worked as counterbalance (compare darts)Belemnites were common in the J urassic and CretaceousNo modern cephalopods produce a rostrum
Evolution
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Mollusca systematics Cephalopoda evolution
Plectronoceras
evolutionary explosionhigh diversity
increase in size
Ecology
entirely marine
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Mollusca systematics Cephalopoda ecology
entirely marine active predators (all are carnivorous)
active swimmersswimming is by rapidly expelling water from the mantle cavity
the water is forced out through the hyponome (jet propulsion)
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Cephalopod eyes
Camera eye fully comparable to ours
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Homo sapiens Octopus
Camera eye fully comparable to ours Famous case of convergent evolution
Forms from skin in the embryo, ours from extension of the brain Nautilus has very primitive, pin-hole camera type eye
Biostratigraphy
especiallyAmmonoidea
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Mollusca systematics Cephalopoda biostratigraphy
especiallyAmmonoidea
and in the Mesozoic
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Mollusca systematics Polyplacophora
Polyplacophora
Polyplacophora (chitons)
primitive molluscs with eight, articulating (overlapping) aragonitic plates
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Mollusca systematics Polyplacophora
(except one Palaeozoic lineage had seven)
generally oval in outline with a flattened body creeping foot, a primitive feature in molluscs radula, mineralized with magnetite head is poorly developed the girdle (perinotum), a band of muscular tissue, runs along the dorsal periphery
embedded in the girdle are small calcareous spines, scales or spicules known since the Late Cambrian (isolated plates)
Multiplacophorans
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Polysacos
Stem grouppolyplacophorans?Different numbers ofscleritesBest know is Polysacos
from the Carboniferous17 plates
i l i k d d i th i t tid l
Polyplacophoran Ecology
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marine, commonly occurring on rocks and seaweed in the intertidal zone
few species have also been found at depths down to 5000 meters
photoreceptor cells in the mantle and girdle.
the animal is thus able to detect light, which it responds negatively to
active at night, when they creep over rocks scraping algae
and other microscopic organisms off the surface with their radula
Mollusca systematics Polyplacophora
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Mollusca systematics Gastropoda
Gastropoda
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Torsion
twisting of the body [it is entirely different from the spiraling of the shell
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Mollusca systematics Gastropoda torsion
fossil evidence suggests that early, non-twisted molluscs already had coiled shells
some modern gastropods have uncoiled shells, or even no shell at all]
all gastropods undergo torsion during some stage of their development- displacement of many interior organs- digestive tract became U-shaped (anus and nephridia moved anterior)- nervous system acquires a twisted appearance (streptoneury)
Torsion
Advantages:ll d th ill b tt t t fl
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Mollusca systematics Gastropoda torsion
allowed the gills better access to water flow
allowing the animal to withdraw more deeply into the shellthe head was able to retract first (foot last, still able to swim)
Disadvantages:
anus and nephridia anterior the animal would be dumping its waste on its head
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The radula important taxonomic feature in modern gastropods no fossil radula confirmed, although there are descriptions
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Mollusca systematics Gastropoda radula
composed of chitinous material and arranged as a long, coiled band
consists of central, lateral, and marginal teeth
Shell terminology
coiling:
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Mollusca systematics Gastropoda
- dextral- sinistral
Gastropod opercula
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Traditional classification
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Mollusca systematics Gastropoda systematics
Prosobranchia (shelled gastropods in which torsion is complete)
classification based on gill and radula types -- unfortunately!
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Mollusca systematics Gastropoda classification
Archaeogastropoda: holostome aperture = no siphonal canal (since Cambrian)
Mesogastropoda: aperture typically with siphonal canal (since Ordovician)
Neogastropoda: aperture siphonostome, often very long siphonal canal (since Cretaceous)
Incertae Sedis (primitive forms - Archaeogastropoda in part)Order "Tropidodiscida" ("Bellerophontina" in part)
Modern classification
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Order Tropidodiscida ( Bellerophontina in part)
Order Bellerophontida ("Bellerophontina" in part)
Subclass Eogastropoda (primitive forms - Prosobranchia / Archaeogastropoda in part)Order "Platycerida" Order Patellogastropoda (Docoglossa)Order Cocculinida (polyphyletic?)Order Vetigastropoda
Subclass Orthogastropoda (all other gastropods)
Infraclass Neritimorpha (Archaeogastropoda in part)Infraclass ApogastropodaSuperorder Heterobranchia
Order OpisthobranchiaOrder Pulmonata
Superorder Caenogastropoda(Prosobranchia in part)Order ArchitaenoglossaOrder NeotaenioglossaOrder Neogastropoda
Mollusca systematics Gastropoda systematics
Patellogastropoda Neritimorpha
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Cellana radians
Haliotis (Haliotis) midae
Vetigastropoda
Turbo (Dinassovica) imperialis
Amblychilepas scutella
Caenogastropoda
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Oliva (Oliva) sericea textilina
Conus (Asprella) alabaster
Pusionella vulpina Morum (Oniscidia) exquisitum
Murex (Murex) aduncospinosus Malea ringens
Turritella ungulina
Heterobranchia
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Pulmonata
Philine angasi
Ophistobranchia
Glaucilla marginata
Ecology
most are aquatic, marine, brackish and fresh water
several groups lives on land (most are Pulmonates)
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Mollusca systematics Gastropoda ecology
marine forms typically live in shallow waters
highest diversity in tropical waters
but also known from arctic waters and hydrothermal vents in the deep sea
one of the most adaptable forms with respect to:
salinity preassure (water and air) temperature (water and air) humidity
most are herbivores, but also carnivore (Muricidae, Naticidae, Conidae) and omnivore
marine forms typically benthic, but also free swimming and floating forms
freshwater and terrestrial forms at least since Carboniferous
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Palaeozoic gastropods
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Bellerophontids
Planispiral coiling
Selenizone and deep sinus
Selenizone often raised
Extinct
Name derived from ancient Greek
hero Bellerophon in recognition of the
similarity to a greek helmet
Bucanella nana
Sinuites
Bellerophon
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Mollusca systematics Gastropoda
Modern gastropods
Cone shells
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Hunt with poisonous harpoons
Poison sometimes extremely
potent (deadly to humans)
Prey is ingested whole or
scraped with radula
Patellids
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Cap-shaped shell
Sticking to rocks and other hard things
Foot modified to function as a sucker
Why?
ProtectionConserve moisture
Feeding by scraping algae
Secondarily untorted
Obs! Convergent evolution
Predation by Gastropods
Several groups of gastropods feed by drilling holes
in mollusc shells
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Naticid
Muricid
Muricids are epibenthic with often highly ornateshells. Drill holes with straight sides
Naticids are infaunal with very smooth, rounded
shells. Drill countersunk holes by combining acid with
radular activity
Mesozoic marine revolution
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Predator-prey arms race
J urassic to present
Evolution of new predators (e.g. tools)
- Crab and lobster claws
Today: More shells are damaged than in
Palaeozoic
Led to new mollusc adaptions
- glossy shells
- varices on aperture
- narrow aperture
New inventions forced the opponent to
develop new counter methods
Affected all benthic marine animals