Phylum Porifera

Question 1

What are the key features of Porifera?

Answer 1

• No true tissues or organs, although has specialised cells
• Flexible body form
• Body plan must allow for sufficient oxygen and food by maximising water flow > filter-feeders
• Ostia (narrow incurrent pores) and oscula (wider excurrent pores)
• Supporting material (mesohyl) contains motile cells and skeletal elements (spicules) in a gelatinous matrix

Question 2

What makes cell differentiation in Porifera unique?

Answer 2

Totipotent cells allow for limitless* potential. Cells can move between specialised (choanocytes) and unspecialised forms (amoebocytes).

Question 3

What is the role of choanocytes?

Answer 3

Choanocytes are specialised cells found in the gastrovascular cavity walls that enable the sponge to filter-feed. By beating their flagella, they pump water through the sponge’s body and capture food particles in their collar, where they are then engulfed within the cell and nutrients are transported into the sponge’s body by amoebocytes.

Question 4

What is the difference between choanoflagellates and choanocytes?

Answer 4

Choanoflagellates are single-celled protists that are considered to be one of the closest living relatives to animals. Choanocytes are specialised cells found in the multicellular sponge, used for generating water current and collecting food. Although similar in structure, choanoflagellates are free-living, single-celled organisms (can be colonial), whereas choanocytes are specialised cells within sponges.

Question 5

What are the three different body plans seen in sponges, and why might the most complex body plan be advantageous (*in certain environments)?

Answer 5

Asconoid –> Syconoid –> Leuconoid
- The asconoid body plan has a single cavity with no chambers - choanocytes line the spongocoel (cavity) = less choanocytes
- The syconoid body plan has small chambers lined with choanocytes branching off the main cavity = more choanocytes
- The leuconoid body plan has a complex system of small chambers lined with choanocytes = lots of choanocytes. This would be advantageous as when the water flows through, it is exposed to a greater surface area and therefore more choanocytes. This means that more food particles can be extracted before the water is pushed out through the oscula.

Question 6

What provides structural support in sponges?

Answer 6

• Spicules: can be calcareous (calcium) or siliceous (silica), some sponges lack them entirely
• Collagen fibrils: dispersed in mesohyl or in framework (spongin)

Question 7

How does the sponge’s form and function generate flow to enable feeding?

Answer 7

Ostia - small x-area = fast flow
Choanocyte chambers - large x-area = slow flow
Osculum - small x-area = fast flow
- This allows them to take in a lot of water and slow the water flow in the choanocyte chambers to capture as many food particles as possible, to then shoot the water quickly out the osculum.

Question 8

How does digestion occur in sponges?

Answer 8

The choanocytes capture food particles in their collar cells and engulf them, then pass the nutrients to the amoebocytes to transport nutrients to other cells throughout the sponge body.

Question 9

What does sexual reproduction look like in sponges?

Answer 9

Sponges are hermaphroditic, and the totipotent cells (amoebocytes and choanocytes) can differentiate into sperm and oocytes that are released into the water. Fertilisation usually occurs externally but can occur in the mesohyl. Fertilisation results in larvae.

Question 10

What does asexual reproduction look like in sponges?

Answer 10

• Gemmules: Usually seen in fresh-water sponges. Overwintering, dormant, “seed-like” capsules. Formed from aggregates of amoebocytes.
• Budding: pieces broken off or intentional fragmentation.
• Aggregation: sponges can reassemble themselves

Question 11

What are the key features of class Hexactinellida?

Answer 11

• “Having six-rays”
• Marine, mostly deep-sea
• Siliceous spicules, always 6-rayed
• Syconoid or leuconoid
• Little mesohyl

Question 12

What are the key features of class Calcera?

Answer 12

• “bony” sponges
• Marine, shallow water
• Calcareous spicules with simple structure (3-4 rays)
• Asconoid, syconoid or leuconoid

Question 13

What are the key features of class Demospongiae?

Answer 13

• “common sponges”
• Highly successful, marine or freshwater, can be carnivorous
• All leuconoid
• Supported by silicious spicules, spongin or both

Question 14

Why are sponges ecologically important?

Answer 14

• Some sponges bore into corals and shells - important bioeroders. Others cement reefs together.
• Indeterminate growth: shape can vary with the environment.
• ## Protected from predation due to spicules, and sometimes toxic compounds (although predation does still occur).