Chapter 13 Cnidarians

Cnidaria Characteristics

Cnidocytes present with stinging cells nematocysts
Entirely aquatic
Radial symmetry/biradial symmetry around a longitudinal axis
Oral and aboral end, no head
Two types of individuals: polyps and medusae
Diploblastic
Mesoglea (jelly like cell matrix) sits between layers, can form the ectoderm in some
Incomplete gut (gastrovascular cavity); often branched/divided with septa
Extracell digestion in gastrovascular cavity, intracell in gastrodermal cells
Tentacles around mouth
Muscular contractions via epitheliomuscular cells
Sense organs: statocysts (organs of balance) and ocelli (photosensitive organs)
Nerve net with symmetrical and asymmetrical synapses; diffuse
conduction; two nerve rings in hydrozoan medusae
Asexual reproduction by budding in polyps
Sexual reproduction by gametes in all medusae and some polyps;
monoecious or dioecious; holoblastic indeterminate cleavage; planula larval form
No excretory or respiratory system
No coelomic cavity

coral bleaching

Ocean temp increase with global warming, increasing CO2 in atmosphere means water takes more CO2 than needed, CO2 increases acidity and reduces calcium carbonate availability, so coral can’t rebuild and can even dissolve. Also the zooxanthellae leave in worsening conditions, leaving coral hungry as well

siphonoglyph

ciliated groove (around pharynx) that generates water currents

pedal disc

broad base by which attachment to a surface occurs

septa

partitions that divide the gastrovascular cavity into 8 in Anthozoa

acontia threads

contain nematocysts and gland cells and are protruded through mouth or pores in the body wall to aid in prey capture or defense

zooxanthellae

photosynthetic algae that live inside the tissues and provide nutrients to the cells, in return get protection and shelter in form of the cnidarian

Anthozoa

All marine
Exclusively polyps; no medusa stage
Solitary or colonial
Gastrovascular cavity is subdivided by septa
Three subclasses: Hexacorallia (reef building corals, sea anenomes), Octocorallia, and Ceriantipatharia

reef building coral anatomy

gastrovascular cavity divided by septa in multiples of six
hollow tentacles around mouth, but with no siphonoglyphs
no pedal disc, but the base epidermis makes a skeletal cup they can retract into when not feeding
exoskeleton is made of calcium carbonate

Anthozoan life cycle

the polyp is the gamete-producing form and the cycle is embryo>larva>polyp, NO MEDUSA STAGE

lifecycle of a hydrozoan colony

free swimming larva (planula)>polyp>asexually reproduce into colonies>some become specialized and release free-swimming medusa (look like jellyfish),reproduce sexually> make new planula

cnidocyte

specialized cell containing a fluid-filled membranous capsule with organelles (cnidae)

found throughout epidermis and i nsome parts of the gastrodermis

cnida

inside cnidocytes, one is nematocysts and the other are for entangling prey OR adhesive, act for attachment/movement

nematocyst

stinging organelle to stun prey and protection

colonial

single organism made of many individuals (zooids), created by asexual budding

dimorphism

organism has 2 distinct forms

polymorphism

single genotype produces different body forms
(special cells for this are gastrozooids, gonozoids)

gastrovascular cavity

organ that performs both digestion and circulation

nerve net

diffuse nervous system: nerve cells are spread throughout an organism, no large concentration (brain)
synapse to sense cells OR effector organs (nematocysts, epitheliomuscular cells)
has 2 parts, one at the base of the epidermis and the other as the base of the gastrodermis

mesoglea

extracellular matrix “jelly” in between the 2 layers, Mostly water and acts like an elastic skeleton

hydrostatic skeleton

fluid-filled cavity that supports the body and allows for movement

epitheliomuscular cells

cells with contractile fibers at base
responsible for locomotion (contract and expand to move)
polyps: sheets of longitudinal and circular fibers
medusae: coronal muscle around the margin of the bell and most also have radial muscles

rhopalium

sensory organ with organs for sensing light (ocelli), chemoreceptors, and movement or direction with respect to gravity (statoliths)

nerve ring

nerve net but for medusas, coordinate swimming and tentacles, also have rhopalia

asexual reproduction

most common in polyps, but can happen in medusae
fragmentation and budding

sexual reproduction

most are dioecious, some monoecious
depending on the group, sexual reproduction can occur in both polyps and medusa

Hydrozoa

can be colonial (hydroids, Portuguese man-of-war) or solitary (freshwater medusa, freshwater hydra)
complex life cycle with the medusa stage, medusa create gametes for new planula larvae and they make colonies where they asexuall reproduce for a while)

hydrozoan colony structure

interconnected polyps, shared gastrovascular cavity, connected by tube-like stolons, forming a branching colony where individual polyps are specialized for different functions like feeding, reproduction, and defense

polyp

tube shaped, sessile, up facing mouth, no manubrium or sense organs, reproduce both ways

medusa

bell shaped, free floating, down facing mouth, yes manubrium and sense organs (statocyst and photoreceptors), reproduce sexually

manubrium

tube that supports the mouth and connects it to the main body cavity

overall cnidarian anatomy

Central gastrovascular cavity
Body wall consisting of an outer epidermis, inner gastrodermis, with mesoglea in between

Digestion and feeding

nematocysts paralyze prey, tentacles drag them to their ‘opening’, the gastrovascular cavity releases digestive enzymes (extracell digest), waste is pumped out through the ‘opening’ and the rest of digestion is finished intracellularly in hydrozoans