how many species of sea gulls exist?

Answer 1

55 species according to wikipedia which sounds about right.

Genus Larus

Dolphin Gull, Larus scoresbii
Pacific Gull, Larus pacificus
Belcher's Gull, Larus belcheri
Olrog's Gull, Larus atlanticus
Black-tailed Gull, Larus crassirostris
Grey Gull, Larus modestus
Heermann's Gull, Larus heermanni
White-eyed Gull, Larus leucophthalmus
Sooty Gull, Larus hemprichii
Common Gull or Mew Gull, Larus canus
Audouin's Gull, Larus audouinii
Ring-billed Gull, Larus delawarensis
California Gull, Larus californicus
Great Black-backed Gull, Larus marinus
Kelp Gull, Larus dominicanus
Glaucous-winged Gull, Larus glaucescens
Western Gull, Larus occidentalis
Yellow-footed Gull, Larus livens
Glaucous Gull, Larus hyperboreus
Iceland Gull, Larus glaucoides
Thayer's Gull, Larus thayeri
Herring Gull, Larus argentatus
Heuglin's Gull, Larus heuglini
American Herring Gull, Larus smithsonianus
Yellow-legged Gull, Larus michahellis
Caspian Gull, Larus cachinnans
East Siberian Herring Gull, Larus vegae
Armenian Gull, Larus armenicus
Slaty-backed Gull, Larus schistisagus
Lesser Black-backed Gull, Larus fuscus
Great Black-headed Gull, Larus ichthyaetus
Brown-headed Gull, Larus brunnicephalus
Grey-headed Gull, Larus cirrocephalus
Hartlaub's Gull, Larus hartlaubii
Silver Gull, Larus novaehollandiae
Red-billed Gull, Larus scopulinus
Black-billed Gull, Larus bulleri
Brown-hooded Gull, Larus maculipennis
Black-headed Gull, Larus ridibundus
Slender-billed Gull, Larus genei
Bonaparte's Gull, Larus philadelphia
Saunders' Gull, Larus saundersi
Andean Gull, Larus serranus
Mediterranean Gull, Larus melanocephalus
Relict Gull, Larus relictus
Lava Gull, Larus fuliginosus
Laughing Gull, Larus atricilla
Franklin's Gull, Larus pipixcan
Little Gull, Larus minutus
Genus Rissa

Kittiwake or Black-legged Kittiwake, Rissa tridactyla
Red-legged Kittiwake, Rissa brevirostris
Genus Pagophila

Ivory Gull, Pagophila eburnea
Genus Rhodostethia

Ross's Gull, Rhodostethia rosea
Genus Xema

Sabine's Gull, Xema sabini
Genus Creagrus

Swallow-tailed Gull, Creagrus furcatus

Answer 2

at least 43, that we know of....
SCIENTIFIC CLASSIFICATION

COMMON NAME: gull
KINGDOM: Animalia
PHYLUM: Chordata
CLASS: Aves
ORDER: Charadriiformes
FAMILY: Laridae
GENUS SPECIES: Most species in the genus Larus; roughly 43 species total

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FAST FACTS

DESCRIPTION: In general, gulls are robust, long winged birds that have bills that are stout and hooked and fully webbed feet except for the hind toes. Unlike terns, which are found in similar habitats, gulls have broader wings and squared-off or rounded tails. Many gull species exhibit different colorations due to seasonal changes or maturity level and it is often very difficult to identify a given gull's species. Bill and leg coloration are excellent distinguishing characteristics to use identifying gulls.
MALE Males are generally slightly larger than females and have larger bills.

SIZE: Generally gulls range in length from 28-81 cm (11-32 in.)

WEIGHT: No data

DIET: Gulls are mainly scavengers, but they also prey on fishes, insects, mollusks, crustaceans, worms, mice, young birds, bird eggs, seaweed and berries.

INCUBATION: For kelp gulls, the eggs begin to hatch after about 27 days, and the chicks leave the nest soon after. Mortality rates can be high, as egg and chick cannibalism by other gulls results in up to 50% mortality rates in some colonies.
CLUTCH SIZE: Gulls usually nest in colonies, laying 2-3 eggs in shallow nests composed of stones, seaweed, mosses, and feathers.

know of...

Answer 3

From my birders guide, there are over 120 species of sea gulls and shore birds.