Strombidae


Strombidae, commonly known as the true conchs, is a taxonomic family of medium-sized to very large sea snails in the superfamily Stromboidea, and the Epifamily Neostromboidae. The term true conchs, being a common name, does not have an exact meaning. It may generally refer to any of the Strombidae but sometimes is used more specifically to include only Strombus and Lambis. The family currently includes 31 extant, and 10 extinct genera.

Distribution

Strombid gastropods live mainly in tropical and subtropical waters. These animals are widespread in the Indo-West Pacific, where most species and genera occur. Nearly 40 of the living species that used to belong to the genus Strombus can be found in the Indo-Pacific region. They also occur in the eastern Pacific and Western Atlantic, and a single species can be found on the African Atlantic coast. Six species of strombids are found in the wider Caribbean region, including the queen conch Aliger gigas, the goliath conch Titanostrombus goliath, the hawk-wing conch Lobatus raninus, the rooster tail conch Aliger gallus, the milk conch Macrostrombus costatus, the West Indian fighting conch Strombus pugilis, and the Florida fighting conch Strombus alatus. Until recently, all of these species were placed in the genus Strombus, but now many species are being moved into new genera.

Morphology and life habits

Strombids have long eye stalks. The shell of a strombid has a long and narrow aperture and a siphonal canal. The shell margin has an indentation near the anterior end which accommodates one of the eye stalks. This indentation is called a strombid or stromboid notch. The stromboid notch may be more or less conspicuous, depending on the species. The shells of most species in this family grow a flared lip upon reaching sexual maturity. They lay eggs in long, gelatinous strands. The genera Strombus and Lambis have many similarities between them, both anatomical and reproductive, though their shells show some conspicuous differences.
Strombids were widely accepted as carnivores by several authors in the 19th century, an erroneous concept that persisted for several decades into the first half of the 20th century. This ideology was probably born in the writings of Lamarck, who classified strombids alongside other supposedly carnivorous snails, and was copied in this by subsequent authors. However, the many claims of those authors were never supported by the observation of animals feeding in their natural habitat. Nowadays, strombids are known to be specialized herbivores and occasional detritivores. They are usually associated with shallow-water reefs and seagrass meadows.
Unlike most snails, which glide slowly across the substrate on their feet, strombid gastropods have a characteristic means of locomotion, using their pointed, sickle-shaped, horny operculum to propel themselves forward in a so-called leaping motion.
Burrowing behavior, in which an individual sinks itself entirely or partially into the substrate, is also frequent among strombid gastropods. The burrowing process itself, which involves distinct sequential movements and sometimes complex behaviors, is very characteristic of each species. Usually, large strombid gastropods, such as the queen conch Eustrombus gigas and the spider conch Lambis lambis, do not bury themselves, except during their juvenile stages. However, smaller species such as Strombus canarium and Strombus epidromis may bury themselves even after adulthood.

Taxonomy

For a long time, all conchs and their allies were classified in only two genera, namely Strombus and Lambis. This classification can still be found in many textbooks and on websites on the internet. Based on morphological and molecular phylogenies in addition to an extensively documented fossil record, both genera have been subdivided into several new genera by different authors.

Genera

The family Strombidae comprises 31 extant genera and 10 extinct genera.
;Extant genera
;Extinct genera
File:Striatostrombus blanci.jpeg|thumb|right|The fossilized holotype of †Striatostrombus blanci held at MNHN, Paris
;Genera brought into synonymy
  • Afristrombus Bandel, 2007 is a synonym of Persististrombus Kronenberg & Lee, 2007
  • Aliger Thiele, 1929 is a synonym of Lobatus Swainson, 1837
  • Decostrombus Bandel, 2007 is a synonym of Conomurex Bayle in P. Fischer, 1884
  • Eustrombus Wenz, 1940 is a synonym of Lobatus Swainson, 1837
  • Fusistrombus Bandel, 2007 is a synonym of Canarium Schumacher, 1817
  • Gallinula Mörch, 1852 is a synonym of Labiostrombus Oostingh, 1925
  • Hawaiistrombus Bandel, 2007 is a synonym of Canarium Schumacher, 1817
  • Heptadactylus Mörch, 1852 is a synonym of Lambis Röding, 1798
  • Latissistrombus Bandel, 2007 is a synonym of Sinustrombus Bandel, 2007
  • Millipes Mörch, 1852 is a synonym of Lambis Röding, 1798
  • Ministrombus Bandel, 2007 is a synonym of Dolomena Wenz, 1940
  • Monodactylus Mörch, 1852 is a synonym of Euprotomus Gill, 1870
  • Margistrombus Bandel, 2007 is a synonym of Neodilatilabrum Dekkers, 2008
  • Pterocera Lamarck, 1799 is a synonym of Lambis Röding, 1798
  • Pyramis Röding, 1798 is a synonym of Strombus Linnaeus, 1758
  • Solidistrombus Dekkers, 2008 is a synonym of Sinustrombus Bandel, 2007
  • Strombella Schlüter, 1838 is a synonym of Strombus Linnaeus, 1758
  • Strombidea Swainson, 1840 is a synonym of Canarium Schumacher, 1817

    Phylogeny



The evolutionary relationships within the family Strombidae have been studied multiple times using various scientific approaches. One important contribution came in 2005, when Brazilian malacologist Luiz Ricardo L. Simone published a detailed monograph analyzing the morphology and anatomy of several related families: Aporrhaidae, Strombidae, Xenophoridae, and Struthiolariidae. In this work, Simone identified the Strombidae as a monophyletic group, meaning all its members descend from a common ancestor and share unique traits not found in other groups. He based this conclusion on 13 shared derived traits and recognized at least eight distinct genera within the family. Simone considered the genus Terebellum to be the most primitive member of the Strombidae. It stood apart from other strombids by its own set of 13 unique features, such as having a rounded foot. Although Tibia was not included in his formal analysis, Simone believed it was closely related to Terebellum due to morphological similarities.
Most of the genera analyzed, except for Lambis, had traditionally been grouped under the broad genus Strombus. However, Simone found that only three species, Strombus gracilior, Strombus alatus, and Strombus pugilis fit the stricter definition of the genus, based on at least five defining characteristics.
The other species, which had often been treated as subgenera, were elevated to full genus status. For instance, Eustrombus included species like Eustrombus gigas and Eustrombus goliath. Likewise, the genus Aliger included species such as Aliger costatus and Aliger gallus, the former of which is now treated under Macrostrombus.
The remaining taxa were previously considered as subgenera, and were elevated to genus level by Simone in the end of his analysis. The genus Eustrombus, in this case, included Eustrombus gigas and Eustrombus goliath ; similarly, the genus Aliger included Aliger costatus and Aliger gallus.


A different approach, this time based on sequences of nuclear histone H3 and mitochondrial cytochrome-c oxidase I genes, was proposed by Latiolais and colleagues in a 2006 paper. The analysis included 32 strombid species that used to, or still belong in the genera Strombus and Lambis. Despite issues with individual gene cladograms, the combined analyses were statistically congruent and reasonably represented the phylogeny of Strombus and Lambis. The findings indicated that Strombus s.l. as defined by Abbott in 1961 was paraphyletic, supporting an earlier suggestion that Strombus s.l. was polyphyletic. Relationships within Lambis were consistent with Stone’s systematics, although Abbott’s subgeneric classifications did not align with the new phylogeny. Based on strong statistical support, the study also proposed that Tricornis might be the sister group to Lambis. Further, the study found mixed support for uniting many strombid subgenera based on a glazed lip character, with some taxa lacking outer lip glazing but still clustering with those that possess it, suggesting that the character might be homoplasious among strombids. While the study was not able to fully test the monophyly of all subgenera defined by Abbott, it indicated that some, such as Tricornis and Lentigo, might not be monophyletic.
In 2019, Maxwell and colleagues proposed a new crown clade known as Neostromboidea to differentiate Strombidae, Rostellariidae, and Seraphsidae from their sister families Struthiolariidae and Aporrhaidae. This revision was based on distinct morphological similarities, including the position of the eye, foot shape, radular configuration, and shell structures. Members of the proposed clade Neostromboidea are characterized by having eyes situated at the tips of peduncles, with a cephalic tentacle located near its distal end. Their foot is laterally compressed, and their shells exhibit a stromboid notch that allows the pedunculated eyes to protrude. In contrast, Struthiolariidae and Aporrhaidae possess a broader, flattened foot and have eyes located at the base of their tentacles, not on peduncles. These families also lack the anterior notch found on Neostromboidea shells. According to the authors, these structural differences reflect the distinct life habits and evolutionary histories of these groups.