<<

Lecture 2 Anemones, , , hydras and relatives by John R. Finnerty

Animal Phylogeny

Porifera Cnidaria Deuterostomia Chordata Arthropoda Annelida Hemichordata Echinodermata Nematoda Platyhelminthes Silicispongiae Calcispongia PROTOSTOMIA Phylum Cnidaria

Cnidaria (Greek: “stinging thread”) distinguished by the possession of cnidae 10,000 described —sea anemones, corals, jellyfishes and hydras diploblast = 2 germ layers ( & ) blind gut (single opening) net & muscle cells radial symmetry??? sexual & asexual

Cnidocytes & Cnidae

Diagnostic of cnidarians. However, both ctenophores (Haeckelia [=Euchlora]) and aeolid nudibranchs may re-deploy cnidae that they have obtained from their cnidarian prey. (Barnes, , 1987) & Cnidae

The is a sensory-effector containing a cnida. Each cnida is a rounded proteinaceous capsule, with an opening on the apical surface that is often covered by a hinged operculum. At the surface, where the cnida opens, there are generally found a number of modified cilia, called cnidocil, which assist in the perception of tactile stimulation and chemical stimulation. In sac, there is a long hollow thread. Upon mechanical contact and receipt of appropriate chemical stimuli, this thread is explosively everted from the sac. The cnidae may serve to deliver , like a hypodermic needle. Many “nematocysts” function like this. The cnidae may also serve to anchor the to a or to adhere to a prey item. This adherent role can be performed by various subtypes of nematocysts but also by spirocysts and ptychocysts.

Cnidae

(Barnes, , 1987)

(Pechenik, of the Invertebates, 2000) Cnidae Discharge cytoplasm of cnidocyte

cnida cnida cnida

H2O stimulation eversion of tubule macromolecule calcium release (e.g., ) increase in osmotic pressure Ca2+ rushes in by osmosis

Diploblasty

Cnidaria diverged from prior to the of . So, Cnidaria lack mesoderm. Cnidaria are diploblasts, having only two germ layers, the primary germ layers: ectoderm and endoderm. Outer ectodermal (ectoderm) Inner gastrodermal epithelium (endoderm) Central layer of of varying thickness. Mesoglea is a gelatinous, largely acellular substance. It may have a few living cells within it— often mobile amoeboid cells. However, the cells are not organized into a like true mesoderm. The mesoglea can act as a hydrostatic , providing support to the rest of the cnidarian body which is really just two thin layers of epithelium

mouth

gastrodermis (endoderm) (ectoderm) enteron mesoglea

disc

“For those contemplating reincarnation, a drawback to as a cnidarian would seem to be the absence of an anus. All undigested material passes through the same opening through which the food enters: the mouth. This is not particularly appetizing from the point of view, but the shortcomings of life without an anus are not merely aesthetic. The sequential disassembly of particulate food material that occurs in an open-ended tubular gut is not possible in the cnidarian digestive system and, indeed, the animal must expel the undigested remains of one meal before it can ingest more food.” — Pechenik, 2001 In the through gut, different functions are localized to different sections of a linear tube.

For example, consider your own digestive tract.

Small Large Mouth Bowel Bowel Mechanical Protein Protein Water processing hydrolysis hydrolysis resorption Initial Pepsin Trypsin (pH < 6) (pH 7-9) digestion Carbohydrate digestion

One-way gut

In the one way gut, it is widely thought that you cannot have specialized regions of . In other words, all extracellular digestion would have to occur in the same physio-chemical environment. In with one-way guts, there tends to be a greater emphasis on , where undigested food are phagocytosed into the cells lining the gut. However, in both Cnidaria and Ctenophora, there is evidence for distinct gut regions with distinct extracellular environments and Medusa enteron

mouth mouth mouth gastrodermis (endoderm) pharynx epidermis (ectoderm) enteron enteron mesoglea

basal disc (Oliver & Coates, in The , 1992)

Medusa = pelagic drifter are mixing the ?

A theoretical model for the relative contributions of Darwinian mixing and turbulent wake mixing is created and validated by measurements of swimming jellyfish using a newly developed scuba-based laser velocimetry device. Extrapolation of these results to other animals is straightforward given knowledge of the animal shape and orientation during vertical migration. On the basis of calculations of a broad range of species, we conclude that biogenic mixing via Darwin’s mechanism can be a significant contributor to mixing and transport.

Polyp = sessile benthic (but capable of some movement) Cnidarian

Cnidarian Nerve Net Spread of Excitation in Cnidarian Nerve Net

Cellular Composition Cnidarian Muscle Histology

Scale bar = 1.0 µm

(Blanquet and Riordan, 1981)

Cnidarian Diversity & Evolution

ANTHOZOA — polyp body form only; simpler life histories; bilateral and biradial symmetry : sea anemones, corals, sea pens, etc. — most have both polyp and medusa; radial and tetraradial symmetry Class Cubozoa: box jellyfishes Class : true jellyfishes Class : hydras, hydroids, hydromedusae Hydrozoan Life-History & Bodyplan Diversity

Polyp Medusa _Anthomedusae yes yes yes yes no _Leptomedusae yes yes yes yes no _Limnomedusae reduced yes yes no no _Trachymedusa no yes yes no no e _Hydra yes no no no no _Siphonophora yes yes yes yes no _Buddenbrockia no yes no yes yes _Myxobolus no yes no yes yes

Cnidarian Phylogeny

Trachyline Other Anthozoa Cubozoa Scyphozoa hydrozoa hydras hydrozoa

- -

+ MEDUSOZOA

(Bridge et al. 1997) (Anthozoa)

Broadcast Planula spawning

Polyps

Sexual Reproduction (Medusozoa)

Broadcast Medusae spawning

Planula larva

Polyps Embryogenesis &

Zygote Planula Polyp

Asexual Reproduction in Cnidaria

Transverse fission

Budding

Nematostella Strobilation

Metridium Pedal laceration “Cnidarians are radially symmetrical animals.”

-Audesirk et al., 2001 -Barnes et al., 2001 -Brusca & Brusca, 1990 -Campbell et al., 2002 -Enger & Ross, 2003 -Lewis et al., 2004 -Mader, 2004

Hydra is radially symmetrical

ectoderm mesoglea

endoderm

colenteron (gut) CBA BILATERIA

RADIAL SYMMETRY BILATERAL SYMMETRY primary body axis (oral-aboral) primary body axis (A-P) & secondary body axis (D-V) oral dorsal

. post.

ventral

aboral

Nematostella — the starlet

oral

Head

Column

Foot

aboral column foot

mouth pharynx gut

gut cavity endoderm mesoglea pharynx mesentery ectoderm

siphonoglyph *

directive axis retractor after Stephenson, 1926 muscle

SYMMETRY IN CNIDARIA

Porpita (Hydrozoa) Aurelia (Scyphozoa) Radial Tetraradial

Nematostella (Anthozoa) (Anthozoa) Biradial Bilateral Why did bilateral symmetry originate?

What was its original selective advantage?

The Standard Explanation: Directed Locomotion An alternate scenario…. In the Cnidaria, locomotion is not correlated with symmetry.

Modern Cnidaria are either sessile, or they locomote in a manner that is random with respect to their secondary axis.

The bilaterally symmetrical corals and anemones are essentially sessile.

The ancestral Cnidarian was a sessile polypoid animal. (Bridge et al., 1992, 1995, Collins 2003, and others)

Therefore, bilateral symmetry did not evolve under selection for directed locomotion in the Cnidaria. In the Cnidaria, symmetry IS correlated with internal ciliary circulation…...

Why is a sessile bilateral?

siphonoglyph pharynx mesentery

ciliary filaments on asulcal septum

coelenteron Alcyonaria polyp Modified from Kaestner, 1984 An alternate scenario…. The correlation of symmetry with internal circulation holds for bilaterally symmetrical forms, bi-radially symmetrical forms, and tetradially symmetrical forms.

Among polyps, true radiality characterizes the smallest hydrozoan polyps.

Size interacts with symmetry and the location of ciliary tracts to affect the efficiency of internal circulation.

Size dependency of internal polyp

Anthozoa

biradial or bilateral

Scyphozoa

tetraradial

Hydrozoa radial biradial Can this selective explanation be extrapolated back to the Cnidarian- Bilaterian Ancestor?

If we assume:

Homology of bilateral symmetry in Bilateria and Cnidaria (Finnerty et al., 2004)

That the Cnidarian-Bilaterian ancestor was a sessile benthic animal (e.g., Collins, 2004).

An alternate scenario….

Ancestral Cnidarian Ancestral Bilaterian Benthic Benthic gut Sessile gut gut Crawling Bilateral symmetry Bilateral symmetry (manifest primarily (manifest internally & internally) externally)

! Pronounced external manifestations of bilateral symmetry ! Centralized ! Directed locomotion.

Cnidarian-Bilaterian Ancestor Bilateral symmetry (developmentally plastic?) Hox , dpp

Finnerty, BioEssays 2005 Predictions and Implications…

The location of ciliary tracts was under the control of “dorsal- ventral patterning genes” in the Cnidarian-Bilaterian ancestor. Perhaps this aspect of developmental regulation is conserved among modern Cnidaria and Bilateria.

Variation in the arrangement of ciliary tracts within Cnidaria may be attributable to variation in the expression of dpp and other genes that pattern the “directive” axis.