1 L2 Marine Invertebrates 1 Background • Life began in the sea bacteria, algae and other protists evolved first • Animals evolved in the sea >600mya • All animal phyla have marine representatives, some are secondarily marine • Plants evolved on land ~450mya • Some plants (seagrasses, mangroves) have reinvaded the sea
The Animal Kingdom • 30+ phyla • Heterotrophic – have to eat something to survive • Multicellular and have tissue layers o Deuterostome = second opening (invagination) to the embryo = mouth o Protostome = first opening to the embryo becomes the mouth • Nerve network Phylum Porifera • Simplest animals • Pore-bearers • Sponges • Asymmetrical • No gut, tissues or nervous system • Filter/suspension feeders
General body structure • Osculum: pore which filtered water is released out through • Spicules: calcareous or siliceous in matrix gives body form and strength • Spongocoel: central cavity • Choanocytes: line inside chamber, beating draws water in and they collect food particles, water exits via osculum o As size , water infiltrating system complexity (more channels) through folding – due to decreasing SA:V ratio = maintains filtration of water
Porifera Classes (based on skeletal structure) • Class Calcarea: small simple sponges with calcareous spicules • Class Hexactinellida: deep water, ‘glass’, 6 rayed silica network rigid skeleton • Class Demospongiae: common, can be very large, siliceous spicules • Class Homoscleromorpha: small group, encrusting sponges, spicules smaller and all similar shape, some species lost spicules.
2 Phylum Cnidaria • Two tissue layers (diploblastic) + intermediate mesoglea • Two body forms (medusa and polyp) same in structure • Cnidae: stinging cells concentrated on tentacles (predators) • Blind ended gut • No organs = diffusion for respiration, circulation and excretion • Nerve net • No ‘direction’ – radially symmetrical • Complex life histories (e.g. alternation of generations)
Cnidaria classes Class Hydrozoa • Very small (microscopic) with large diversity • Polyp (=zooid) mouth pointing upwards surrounded by solid tentacles • Colonial with specialisations for feeding, reproduction and defence • Hydrozoan medusa (rare) free swimming, released by asexual budding, reproduce sexually, releasing gametes which settle and grow into new polyp colonies = asexually reproducing polyp/sexually reproducing medusa (alternation of generations)
Class Scyphozoa • Medusae stage dominant (reduced polyp stage) • ‘jellyfish’ thick layer of jelly-like mesoglea between two body layers • Alternation of generations common (except when medusa dominant) o Medusa (adult) and polyp (juvenile) phase - layers ‘bud off’ to form asexual young medusa (in a colony), which then reproduce sexually • Circular shape with tentacles distributed around
Scyphozoan Life History
Class Cubozoa (e.g. Box jellyfish) • Medusae stage dominant (reduced polyp stage) • ‘jellyfish’ thick layer of jelly-like mesoglea between two body layers • Square bells and 4 groups tentacles
Class Anthozoa • POLYPS (large and not differentiated) • Hollow tentacles, pharynx and longitudinal filaments (mesenteries) • Two major subclasses: o Octocorallia (e.g. soft corals and sea pens) ▪ Eight tentacles and mesenteries ▪ Pinnate tentacles o Hexacorallia (e.g. stony corals and anemones) ▪ Six tentacles and mesenteries ▪ Smooth tentacles Phylum Ctenophora • Two body layers (diploblastic) • Intermediate mesoglea – but with cells = mesenchyme (feels like jelly) • Ctenes (bands of cilia) form comb rows used for movement • Lasso cells (instead of cnidae) = sticky = used for predation voracious predators! 3 • Simple life history • Can bioluminescence • E.g. comb jellies Phylum Platyhelminthes • Flat worms allows diffusion of oxygen • Not segmented and bilaterally symmetrical • Triploblastic (three layers) = complexity Few organs • Solid body (no coelom) • Blind ended gut = one opening (mid ventral = underneath) • Cephalised: “brain!” = information to central processing network o Primitive brain = cerebral ganglia o Longitudinal nerve cord o Directionality + increased awareness • Primitive sensory organs • Investment into reproductive system = complex • Excretory system • Predators Phylum Nemertea • Ribbon worms • Not segmented ant triploblastic • Cephalised • Full gut – mouth to anus • Excretory system • Circulatory system • Proboscis – used for predation and scavenging