On sandy bottom, fish and cuttlefish
Rocky bottoms, soft sediments and seagrasses beds occur in shallow water along the Antiparos coast.
Providing a stable and hard substrate, massive rocks often allow a large variety of benthic organisms to settle, forming diversified benthic communities where food and hiding places are particularly various. In contrast, soft sediments look at first sight ‘desertic’, however by observing them more closely, life is well present and diversified.
Soft sediments (gravel, sands, mud) are loose substrates made of particles of various sizes and nature (mineral or organic). Mineral particles mainly originate from soil and rock erosion (they reach seas and oceans being transported by rivers, winds or melting glaciers). Biogenic particles fall down the water column accumulating on the seafloor, they result from living forms activities (e.g., faeces, gametes, decay of dead animals and plants, skeleton/shells, …).
In shallow waters soft sediments are subject to continuous reworking owing to waves, tides and currents mainly, but also to animal activities such as foraging and displacement. Consequently, the settlement of benthic organisms is at first sight ‘problematic’ on this unstable substrate! They have to grow fast, or to bury, or to develop root-like stems as do seagrasses (the only large plants living on soft sediments).
Animals colonizing soft sediment are buried (infauna) or live at its surface (epifauna). A major part of them feeds on detrital particles by being deposit- or suspension feeders. Scavengers also occur on the sediment, feeding on decaying organisms (animals or plants).
The infauna and epifauna can be preyed by fishes, crustaceans or mollusks passing by, and such predation may leave traces on the seafloor. When snorkelling over sediments areas, one can easily observe fishes behaving in a special way while adopting a particular strategy to reach their preys. This has intrigued me … I cannot resist presenting the fishes I regularly meet on soft sediment with a brief description of the way they behave.
- A fish that digs sediments leaving typical cup-like depressions all over the prospected surface:
The striped seabream, Lithognathus mormyrus (Linnaeus, 1758) – Sparidae
Silver-Grey (or yellowish) colour, back and flanks bearing up to 15 aligned black or brown vertical stripes, belly silver-grey; size of observed individuals 10 – 25 cm (it may however reach up to 50 cm). Elongated body, with a pedunculate forked caudal fin; one dorsal fin (it extends up to the caudal peduncle). The head is strong with a typical curved dorsal profile. The snout is long and large, slightly flattened ventrally, the mouth is protractile with strong jaws and teeth. Large eyes, rather dorsal and far from the mouth.
This carnivore fish digs sediments with its strong snout to capture worms, crustaceans, mollusks, sea urchins (infauna). It leaves typical cup – like depressions (see pictures below).
Sometimes it is accompanied by other fishes trying to benefit from a part of the meal!
Geographic distribution. Mediterranean Sea, East Atlantic coast (Channel, North Sea, Biscay Gulf up to South Africa), Indo-Pacific Ocean, Red Sea
2. Fishes that stay immobile waiting for preys passing by:
The greater weever, Trachinus draco Linnaeus, 1758 – Trachinidae
Text and pictures coming soon