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Herbivory: algivores (seaweed-eaters)

hot buttons for algivory part of BCCR hot button for fishes part of algivores section in BCCR hot button for sea-urchin part of algivores in BCCR hot button for crustaceans/chitons/snails part of algivores in BCCR

This section of algivores deals with crustaceans, chitons, & snails; information on other seaweed-eating reef animals can be accessed via the icons.

 

 
 

Herbivory: algivores: crustaceans/chitons/snails

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of sargassum-weed taken from a video

"What's this floating by? Oh! It's sargassum weed, introduced from Europe and a pest. It's not eaten by fishes, but often has lots of tasty small crustaceans, worms, and molluscs in it." - Little Cayman 2003

NOTE Sargassum sp.

 
 

The brown alga Sargassum is not a favoured food-item for herbivorous fishes, perhaps because of its content of secondary metabolites, but often there are resident invertebrates in the weed (mainly crustaceans) that may be preyed upon. The crustaceans themselves are mainly plant-eaters, although caprellids also prey on small zooplankters floating by in the water current. In one study, some 700 epifaunal organisms are collected from a single 100g mass of live Sargassum weed on a mid-summer's day in a shallow, boulder-stewn reef-flat area. These include amphipods, isopods, polychaetes, and gastropods, most of them herbivorous. Some feed directly on the Sargassum, others use it as a perch on which to catch large phytoplankton cells or algal bits floating by, whilestill others feed on the small nutritious plants (epiphytes) growing on the Sargassum. In return for providing a protective habitat for these invertebrates, the host Sargassum benefits from the removal of the light-blocking epiphytes. In turn, small fishes, such as bluehead wrasses, may feed upon these Sargassum-inhabiting invertebrates. Martin-Smith 1992 Proc 7th Intern Coral Reef Symp 2: 881.

NOTE these are chemicals in a plant or animal that have no apparent role in the normal metabolism of the organism, but that may be involved in defense

 
photograph of brown alga Sargassum including common crustaceans that live within it
 
  In the rocky intertidal zone of the reef and amongst shallow coral rubble, numerous chitons and snails feed on diatom scum and algal sporelings.
 
drawing of reef with photographs of chitons Acanthopleura and snails Nerita
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a conch taken from a video

"Let's sneak up on this conch...see if we can catch it feeding. Conches use a radula... sort of rasping tongue located at the end of a proboscis, to feed on algae. We can move around to the front, ever so slowly, but it's having none of us! Very timid!. But here's a little tentacle and an eyeball peeking out at us!" - Turks & Caicos 2003

NOTE milk conch Strombus costatus

 
 

photograph of the head end of a conch Strombus sp. showing eyes and proboscis
Conchs and other herbivorous snails feed on macroalgae and algal scum by rasping with a feeding apparatus known as a radula. The radula is situated at the end of a proboscis which, depending upon species, can be extended some distance from the body. In conchs, for example, the one featured here, the proboscis is comparatively short.

 

 

 

 

Conch Strombus sp. with black proboscis
extended 1.5X. The eyes are ringed
with yellow and seem very "eye-like",
but are not image-forming

  photo/drawing collage of the proboscis and radula of a snailThe radula is a highly muscular apparatus bearing teeth, or cusps, and the action of feeding is like a person licking an ice-cream cone. The radula is extended out of the mouth by special muscles and, as it is extended, its cusps splay outwards. When the radula is withdrawn, the splayed-out cusps rasp upwards on the food, breaking off bits. These bits of food matter are borne upwards and moved into the esophagus, and then into the gut for digestion.
 

photograph of sea hare Aplysia dactylomela crawling on a cageSea hares are a type of herbivorous snail that inhabit shallow, back-reef areas but, although relatively large in size, are not seen much by SCUBA divers because of the shallowness of where they live and their generally nocturnal habits. At dusk they emerge from under-rock hiding places to copulate and feed on algae until dawn, when they return to shelter.

 

 

Several sea hares Aplysia dactylomela
browse on algae growing on a cage in the
shallow subtidal region of the reef 0.33X

 

photograph of sea hare Aplysia dactylomela eating seaweedSea hares have jaws as well as a radula. The jaws grip the algae while the radula tears off bits for consumption. Visible in the photograph is the broad, muscular foot on which the animal crawls and, at the back of the body, the exhalent funnel. The funnel has 3 main functions: 1) it allows exit of the respiratory water flow after it passes over the gill; 2) feces and kidney excretions are carried out in the exhalent water flow; and 3) it allows exit, along with the opening between the fleshy mantle folds behind the head, of ink used in defense.

NOTE more on defenses in sea hares can be found at DEFENSES: INVERTEBRATES: TOXINS

 

 

Sea hareAplysia dactylomela being fed some red algae.
The jaws are the white structures. Part of the radular
apparatus is just visible within the jaw aperture

 


photograph showing details of sensory structures on the head of a sea hare AplysiaThe head of a sea hare Aplysia dactylomela bears 3 paired sensory structures, the eyes, the oral tentacles, and the rhinophores. The last 2 provide information on the location and type of seaweeds being sought for food. Sea hares feed almost continuously through the nighttime when they are active outside their protective hide-aways.

NOTE lit. "nose bearing", referring to perception of, in this case, water-borne chemical odours

 

 
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