Practical 1 - Molluscan Feeding Strategies

Question 1

Suspension feeding

Answer 1

The collection of particles (ranging from bacteria to small zooplankton) from the water column.

Question 2

Ciliary-mucous suspension feeding

Answer 2

Mucus is used to entrap suspended particles, but the arrangement of cilia enables particles that are either too large or too small to be rejected - selective particle feeding. This method has been adopted in a wide variety of benthic invertebrates, e.g. the polychaete families Serpulidae and Sabellidae (these use a crown of ciliated tentacles), bivalve molluscs (gills and labial palps), bryozoans (a ciliated lophophore).

Question 3

Mucous-bag or mucous-sheet suspension feeding

Answer 3

The polychaete worm Chaetopterus produces a thimble–shaped bag of mucus which is stretched across its ‘U’–shaped tube between a pair of highly specialised parapodia. When this sheet is saturated with particles it is ingested and a new bag secreted. Ascidians (tunicates) use a mucous–sheet secreted over the inner face of the perforated pharyngeal basket; ciliated tracts move the sheet towards the mouth. Here feeding is essentially non–selective.

Question 4

Setose suspension feeding

Answer 4

This method is found only in arthropods, e.g. barnacles, copepods, some amphipods and anomurans. Currents created by movement of thoracic limbs are drawn through filters formed by appendages richly furnished with setae.

Question 5

Tentacle-tube feet suspension feeding

Answer 5

Crinoids and some brittle stars used their tube feet to secrete mucus in which food particles are ensared; these particles are then carried by other tube feet to the food groove and then to the mouth. Some polychaetes (e.g. spionids) use tentacles in a similar fashion. Other tentacle–feeders include anemones and corals, although it is difficult to define where suspension feeding on small zooplankton ends and carnivory begins.

Question 6

Sponges (suspension feeding)

Answer 6

Use flagellated cells (choanocytes) to phagotose small food particles. Some sponges take advantage of velocity gradients above the bottom; with laminar flow the velocity decreases as the bottom is approached. The stronger water flow over the chimney–like excurrent openings (oscula) creates a pressure differential such that water is drawn into the incurrent openings (ostia) closer to the substratum.

Question 7

Deposit feeding

Answer 7

Feeding on small particles in soft sediments on or in the bottom and digesting the attached microbial organisms.

Question 8

Swallowers (deposit feeding)

Answer 8

These ingest many sediment particles at a time with little selectivity with respect to size or quality, e.g. lugworm Arenicola and the mud snail Hydrobia. Because sediments contain only c. 2% organic matter, these organisms must process large quantities of sediment.

Question 9

Tentacles-feeders (deposit feeding)

Answer 9

These use tentacular structures to gather detrital particles and transport them to the mouth, e.g. sea cucumbers, nuculoid bivalves (use palp probosces), polychaetes such as Amphitrite.

Question 10

Surface siphon feeders (deposit feeding)

Answer 10

This method is restricted to the tellinacean bivalves, e.g. Tellina, Scrobicularia, Macoma; siphons are separate and of unequal size and the large inhalent siphon operates like a hoover picking up detrital particles from the sediment surface.

Question 11

Sectose deposit feeders

Answer 11

i.e. Corophium uses setose appendages to trap sedimentary particles. Some deposit feeders will ingest their own faecal pellets (coprophagy) stripping them of microbial flora, e.g. Hydrobia, the prawn Palaemonetes.

Question 12

Herbivorous browsers

Answer 12

These graze algae from hard surfaces or directly consume plant material. Some herbivore browsers (e.g. urchins) are also capable of consuming small animals as well as plants.
Chitons and many gastropods (e.g. Littorina, Gibbula, Patella) use a ribbon of chitinous teeth (= radula) to scrape food from hard surfaces. Some gastropods (e.g. Aplysia) use a crude jaw system to seize or cut pieces of seaweed.
Sea urchins posses a complex series of teeth (Aristotles’ lantern) for rasping or tearing macroalgae. Many crustaceans consume algae – isopods such as Ligia cut up seaweeds using their mandibles; gammarids such as Orchestia feed on algae and algal fragments using their maxillipeds.
Polychaetes like Nereis tear plant material with their jaws.
Wood–boring invertebrates have the ability to digest cellulose or else depend on microbiota in the gut.

Question 13

Carnivores and scavengers

Answer 13

Carnivores seize and capture animal prey. many are scavengers when live prey is unavailable. Hunters are of three broad types: (a) pursuit hunters - these are highly active animals like cephalopods, (b) searchers - these seek out prey which are less active than themselves, (c) ambushers - are essentially sedentary and wait for prey to come to them. There are numerous predatory marine invertebrates - asteroid starfish (e.g. Asterias) use suckered tube feet to pull open bivalves; others (e.g. Astropecten) ingest their prey whole using suckerless tube feet to cram prey into the mouth. Coelenterates use tentacles loaded with specialised nematocysts; nermeteans use an eversible proboscis sometimes armed with piercing stylets. Many gastropods (e.g. Nucella) use their radula to drill holes through molluscs and barnacles. Nudibranchs graze sponges, and hydroid coelenterates – some even consume nematocysts, store them undischarged in dorsal sacs (cerata) and use them for defense. Many polychaete worms (e.g. Glycera, Nepthys, Nereis) are voracious carnivores. Amongst the arthropods, crabs are important predators, whilst pycnogonids feed on bryozoans, sponges and hydroids.