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. When snorkelling over sediments areas, one can often 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 their behaviour.

1. Typical cup-like depressions left by 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 all over its prospected area (see pictures below).

Other species of fishes may swim close to the stripe seabream trying to benefit from dislodged little preys!

Geographic distribution. Mediterranean Sea, East Atlantic coast (Channel, North Sea, Biscay Gulf up to South Africa), Indo-Pacific Ocean, Red Sea.

2. Ambushing fishes… the weevers

Weevers live on soft bottom; they are partly or almost completely buried (their eyes then only emerging from the sediment). They display a particular technique of hunting by ambush, feeding on small fishes (gobies, sand eels, anchovies) and crustaceans (shrimps, mysids) passing by. They use their powerful tail to pounce on a swimming prey.

Important to notice: these fishes can be particularly dangerous for swimmers when walking in shallow waters.  Indeed, weevers synthetize a highly toxic venom. The venom glands are often associated to spines of the dorsal fin and of the opercula. Very painful injuries are induced when someone incidentally walks on the fish or manipulates it. The venom affects the nervous and circulatory systems. As the toxic substance in the venom is thermolabile between 40° and 50°C, a prolonged bath of the wounded limb in hot (40-42°C) water or the application of hot compresses can be a first-aid attempt. It is however necessary to see a doctor for an adequate treatment.

More infos in: https://en.wikipedia.org/wiki/Greater_weever, and, https://doris.ffessm.fr/Especes/Trachinus-draco-Grande-vive-637

Four species of weevers occur in the Mediterranean Sea. They can easily be distinguished by the size and position of their eyes, and by the shape of their pupil (among other features). See for instance the comparative pictures in: https://doris.ffessm.fr/Especes/Trachinus-radiatus-Vive-leopard-2625

I met two species in shallow water (between 0.5 and 3 m depth), the greater weever and the starry weever (also named streaked weever).

The greater weever, Trachinus draco Linnaeus, 1758 – Trachinidae

Body laterally compressed and elongated (length: 20-30 cm), with the flanks having brown and blue oblique stripes (the brown stripes may be ‘fragmented’ into aligned brown patches). The mouth is oblique, opening upwards, giving a fierce/angry look. Eyes are large, closed to each other’s; they have a round pupil and are bordered by a gold-coloured line that displays a typical upper notch. 

The yellow arrow indicate the presence of a wide-eyed flounder (Bothus podas) that closely stay next to the weever, presumably waiting for remains of its meal …

Venom apparatus. There are two dorsal fins, the first one is short (triangular), dark-coloured, with 5-7 spines each connected to a venom gland. The second dorsal fin only bears soft rays. Each opercula also bears a venom spine. 

Reproduction from June to August (with regional variations). Eggs and larvae are pelagic (planktonic), the eggs hatch after 3 months. There is no parental care and seemingly no territoriality. 

Geographic distribution. Mediterranean Sea, Black Sea, North Sea and Northeast Atlantic from southern Norway to Morocco.

The starry weever (or streaked weever), Trachinus radiatus, Cuvier, 1829 – Trachinidae

Like the greater weever, the starry weever has an elongated body, a mouth oriented upwards, the same attitude and feeding method. It differs from the greater weever, by its eyes, its flanks color and its more massive outline. The size is ca. 15-20 cm for the fish I met; it can however reach up to 50 cm.

The eyes are close to each other’s, quite dorsally located; their pupil is narrow and elongated. Behind its eyes, radiating bony ridges (not always clearly visible) give this weever fish its name. The head of the starry weever exhibits rapid colour changes: it can darken dramatically against a violet to reddish background (see pictures above).

The body background colour is whitish to brownish, but flanks and head are mottled with brown or black, with large brownish patches, some forming ring surrounding a clearer center. The belly is uniformly whitish.

Venom apparatus. There is a large venom spine on each operculum, and 2 or 3 smaller ones on the dorsal edge of the eye orbit. There are 6 venom spines on the first dorsal fin (a small dark fin, pointed by the yellow arrow on the picture below) and no venom spines on the second dorsal fin. The anal fin bears two venom spines.

Reproduction. Occurs in spring or summer (with regional variations). Eggs and larvae are planktonic. The starry weever migrates to shallow waters to spawn, a behaviour that increases the risk of envenomation for swimmers.

Geographic distribution. Mediterranean Sea, East Atlantic Ocean from Portugal to South Africa.

3. Benefiting from others … The wide-eyed flounder, Bothus podas (Delaroche, 1809) – F. Bothidae

The wide-eyed flounder is a flat fish that usually follows other fishes, trying to benefit from their foraging activities. Its food consists of benthic small fishes, worms, crustaceans, mollusks and sea urchins, i.e., preys notably dislodged by fishes like the striped red mullet (see pictures below), the weever fishes and the striped seabream.

A few words about flatfishes… Flatfishes are for sure bizarre animals! They are laterally compressed and asymmetrical, with one side (left or right) facing the seafloor and the other one, the water column. Both eyes are thus on the upper flank. This asymmetric morphology is acquired during the metamorphosis of the juvenile when one eye migrates next to the other one across the top of the head.  Eyes are either on the right or on the left body side; accordingly, the flatfishes are said right-eyed (dextral) or left-eyed (sinistral). This is often a species feature, but not always.

Flatfishes swim owing to the undulations of their dorsal and anal fins that line the body margins (pectoral fins are less conspicuous).

The wide-eyed flounder is a sinistral flatfish (both eyes on the left side; the right side facing thus the sea bottom). The eyes are rounded, large and prominent with one eye more frontal, next to the mouth. 

Male and female differ by their size (males reaching 45 cm and females 20 cm in length), by the larger inter-orbital space in male (higher than the eye diameter) and by the body profile, very straight in males, more rounded one in females (see the three pictures below)

Camouflage. The body side facing the seafloor is whitish or brownish while the side facing the water column mimics the background colour (‘camouflage’). Changes of colour are due to pigmented cells, the chromatophores, which size and distribution vary according to stimuli exerted by the immediate surrounding. Colour changes are not only a way to avoid predation,  but also to communicate on the mood state:  e.g., angry/aggressive fishes can become dark-coloured while fishes showing submission are greyish.

A challenging situation in the above picture!

Reproduction. Sexes are separated. Reproduction occurs from May to August, during early morning hours. Males are territorial, they display a courtship and may mate with several females. Sperm and eggs are released on the sediment where fecundation occurs. Larvae and juveniles are pelagic. Larvae are symmetrical i.e., resembling ‘typical fish’. The asymmetric morphology is acquired during the metamorphosis of the juvenile, when one of the eyes migrates next to the other one. 

Geographic distribution.  Mediterranean Sea, South-East Atlantic, from Angola up to Portugal, Madeira, Azores, Canary Islands.