One way to classify seagrass animals i to divide them into those that move around (mobile) and those which do not (sessile)
There are many different species occurring along the Australian coast. Some of these animals, such as snails, amphipods, isopods, gastropods. limpets and sea urchins are grazeing herbivores. These animals do not feed on the seagrass itself, but rather on the plants growing attached to the seagrass leaves, known as epiphytes..
Sea urchins are grazers and use their hard jaw plates located in their mouths to scrape away at seaweed fronds or encrusting epiphytes on seagrass leaves.
Predators include some snails, sea stars, crabs and some fish,
Vertebrates found in seagrass meadows include include fish (such asmullet), rays (such as cow tail rays and eagle rays), sharks (including juvenile Port Jackson sharks), birds (such as cormorates), reptiles (including sea snakes and green turtles) and mammals. In warmer areas dugongs and turtles graze on seagrass meadows. In temperate regions, garifsh, leatherjackets and black swans graze on seagrass as does the Red Swimmer crab (Nectocarcinus integrifrons). They can eat and digest the tough seagrass leaves.
Sessile invertebrates are attached permanently to a surface, such as a seagrass leaf, and cannot move from this position. Sessile animals include mussels, oysters, barnacles, bryozoa, sponges, Spirorbid Polychaetes (tube dwelling segmented worms), and sea squirts. Animals such as mussels and oysters are abundant on or near seagrass beds.
Many filter feeders are associated with seagrass, such as ascidians and sponges. They feed on the plankton in the waters around seagrass. Detritivores associated with
seagrass include shrimp, prawns and crabs. There are a group of invertebrate animals called deposit feeders that pass large quantities of sediment through their gut, extracting detrital and microalgal particles rich in micro-organisms in the process. Deposit feeders associated with seagrass include peanut worms and polychaetes.
Many carnivorous invertebrates are found in or amongst seagrass beds. Some examples
include many species of molluscs (e.g. pygmy squid, dumpling squid, blue-ringed
octopus and welks) and crustaceans.