(Spix & Agassiz, 1829)
Synodus intermedius, the sand diver, is a species of fish in the lizardfish family, Synodontidae, a basal ray-finned fish in the class Actinopterygii. The etymology of Synodus is of Greek origin: "syn", symphysis or grown together, and "odous" which means teeth. Sand divers inhabit subtropical marine ecosystems, (37°N-17°N), including sandy bottom areas on continental shelves, coral reefs, estuaries, bays, and reef structures. They are demersal or benthic fish, which means they live on or close to the sea bed. Distribution ranges from the northern Gulf of Mexico south to the Guianas, and western Atlantic north to North Carolina and Bermuda. They are a common lizardfish in the West Indies. They grow to approximately 40 cm (16 in) total length, and weigh around 1 kg (2.2 lb).
The trunk of sand divers is a pallid color, ranging from gray to brownish gray with mottling on the head and trunk grading to a whitish underbelly. They have variable markings, including a black patch on the shoulder girdle, reddish brown vertical bars across the back, and thin yellow-gold striped lateral lines running from behind the gill flap or operculum to the base of the tail or caudal fin. They are able to alter their color, becoming paler or darker to blend with the background.
Taxonomy and evolution
Lizardfishes are aulopiforms in the order Synodontidae which is represented in the western Atlantic by 3 genera: Synodus, comprising 6 species, Saurida, comprising 4 species, and Trachinocephalus which includes a single species.
The fossil record indicates an evolutionary divergence of Aulopidae dating back to the Early Cretaceous or Late Jurassic, with most extant families appearing by the Late Cretaceous to the Eocene. Aulopoids are a small group of benthic fishes that inhabit coral reefs and continental shelves, including lizardfishes (Synodus, Saurida) and flagfin fishes (Aulopus). Integrated studies regarding deep-sea evolutionary adaptations, simultaneous hermaphroditism, and tubular eyes suggest that the stem species of lizardfishes arose in a marine environment with separate sexes, and round eyes that were laterally directed. The study also identified the suborder Alepisauroidei as the largest and oldest known vertebrae clade with the reproductive strategy of simultaneous hermaphroditism.
Sand divers have an elongated body and can grow to a length of up to 46 cm (18 in). The dorsal (upper) surface is dappled brownish-gray and there is a black patch on the shoulder girdle at the upper end of the gill slits. There are about eight pairs of reddish-brown bars on the top and sides of the body, broadest at the lateral line and narrowing towards the belly. There are also slender yellowish-gold longitudinal lines running along the body. The ventral (under) surface of the fish is pale. The tip of the lower jaw is rounded, without a fleshy knob, and the soft rays of the dorsal fin are relatively short, the tip of the front one lying on top of the origin of the hind one when depressed. These characteristics helps to distinguish this species from the offshore lizardfish (Synodus poeyi). The anal fin of the sand diver has ten to twelve soft rays which distinguishes it from the otherwise similar diamond lizardfish (Synodus synodus) which has eight to ten. Another fish with which it might be confused is the Atlantic lizardfish (Synodus saurus), but that species is uncommon in the Caribbean and has slender blue or turquoise longitudinal lines.
As is common among benthic and demersal fish, the cornea of the eye has an iridescent layer, the function of which may be to offer protection to the eye from very bright light. The mouth is wide and filled with numerous needlelike teeth. There are two rows of teeth in the upper jaw; the inner row teeth are longer than the outer row. There are three rows of teeth on the lower jaw; the outer row are covered by lips, the middle teeth increase in size. Both jaws have posterior teeth that slant slightly toward the back. There are also 3 to 4 rows of depressible teeth on the palate. The tongue has 5 rows of teeth, the largest positioned closest to the anterior tip, slanting toward the back.
Distribution and habitat
Sand divers are found in the subtropical western Atlantic Ocean between 37°N and 17°N, the Caribbean Sea and Gulf of Mexico. Their range extends from North Carolina and Bermuda, southwards to Santa Catarina in Brazil, and includes the West Indies, Caribbean Sea and northern, eastern and southern Gulf of Mexico. They have been reported from São Tomé and Príncipe in the eastern Atlantic, though that record is questionable, and are also known from the Philippines. Sand divers are usually found on or near the seabed in sandy areas among boulders or in sandy corridors in reefs, but they will also rest on the tops of reef structures, propping themselves up on their pectoral fins. They are found at depths down to 320 m (1,050 ft), however they seldom occur near shore and most records are in the depth range 38 to 110 m (125 to 361 ft).
Sand divers are piscivorous ambush predators once thought to occupy a mid-trophic position as a lie in wait ambush predator. Research has since discovered they occupy a high trophic position as active hunters that feed on other predatory fishes. In some systems they are considered apex predators. They can be found on top of the sand or on reef tops, and also bury themselves in the sand with only their head exposed. Sand divers feed on a variety of reef fishes such as bar jacks, blue chromis, wrasses, fairy basslets, grunts, and yellowhead jawfish.
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