Reef Organisms: Invertebrates Flashcards
How much of the ocean floor is covered by coral reefs?
<2% of the ocean floor are coral reefs (relatively small area)
What proportion of marine organisms depend on coral reefs?
~25% of marine organisms depend on them (big significance despite small area)
Note: this may not be species living on the reef specifically
List three key reef invertebrate phyla
- Sponges
- Echinoderms
- Molluscs
What are the three types of sponge growth morphology?
- Erect (upward growth)
- Encrusting (along reef)
- Boring (drill into calcareous material and can push coral off skeleton with toxic chemicals)
What are the main structural components of a sponge?
- Small pores along the surface allow water to enter the sponge through channels
- Holes called oscula (osculum) allow the water to exit the sponge
Why are sponges important for reef systems? (4 reasons)
- Import and retain nutrients in coral reefs (“sponge loop”)
In this way, sponges are the largest source of dissolved inorganic nitrogen (DIN) in nutrient-poor reefs - They are a habitat for commensalistic organisms
- Important contributors to the bioerosion of calcareous structures (mechanical and biochemical forces chip away, forming sediment) -> allows seagrasses, which like soft sediment, to establish nearby
- If sponges inhabit the skeleton, they can weaken live corals (again forms soft sediment)
How do sponges increase bioavailability of nutrients?
Sponges take up nutrients (nitrogen, phosphate etc.) from phytoplankton/microbial plankton (bacteria) in the water column via filter feeding.
They then excrete ammonia, nitrate and phosphate as dissolved inorganic nutrients (make it bioavailable).
In this way, nutrients are funneled from water passing the reef straight to the reef.
What is an example of Asteroidea (echinoderms) that has impacts on the reef system? What are the impacts?
Crown of Thorns (COTS – Acanthaster planci):
- Corallivorous – consume coral from skeleton (shows how reefs can be destroyed by high numbers of echinoderms -> outbreaks are major drivers of reef decline)
- Nutrient enrichment is suspected to promote outbreaks as larvae are more successful with more phytoplankton
In summary: mostly negative
What is an example of Echinoidea (echinoderms) that has impacts on the reef system? What are the impacts?
One example is the sea urchin Diadema sp.
- They are key herbivores
- Disease induced reduction in these populations has been seen to lead to a phase shift to macroalgal dominance in Caribbean reefs
- However, if too many they start to erode the coral skeleton by nibbling away at the top layers of calcareous material to eat microscopic algae
In summary: mostly positive, but can be negative if too many
How do sea cucumbers (Holothuria within Echinoderms) impact reef systems?
- Stop sediments from becoming anoxic or not bioavailable by bioturbation of sediments and recycling of organic matter and nutrients
- Commercially important species
- Overfished in many places and subject to management
In summary: positive impacts
What are four specific examples of key mollusc species?
- The Tridacna sp. = a big colourful clam (bivalve) which contributes to the calcium carbonate structure
- The lithophaga sp. = date mussel/boring bivalve which drills into calcareous coral skeleton for protection
- The Charonia tritonis = huge snail (gastropod) that can feed on the Crown of Thorns (regulate the population)
- Coralliophila sp. = sessile snail (gastropod) that sit inside poritis corals, eating the coral skeleton as it grows towards them
Why does the coral look pink around the Coralliophila sp.?
the coral grows faster around the parastic Coralliophila sp. to over grow the snails (this is why it is pink) - this is exploited by the snails, which consumes the skeleton as it grows towards them -> this creates an equilibrium, which keeps the snail in the same place as it gets food, but it does not leave (as it is successfully getting food).
How do molluscs interact with the reef system?
- Filter feeders (take up nutrients from water)
- Potential coral competitors -> also have symbiotic zooxanthellae and acquire nutrients using similar strategies
- Some infest live coral -> weaken skeleton (this is thought to be promoted by nutrient-rich water )
- Some attach to coral (such as sessile gastropods), damaging coral with growth
- Some are corallivorous (predators) and therefore damage reef
- Some consume COTs (prevent COTs outbreaks)
What is the main biological principle driving the outcome of direct and indirect interactions in a coral system? What does it mean?
Goldilocks Principle
i.e., an optimal number of things exists that are good for the system - there is no good and bad!
