Sea butterfly

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Sea butterflies
Temporal range: Late Paleocene–recent
LimacinaHelicinaNOAA.jpg
Limacina helicina
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Heterobranchia
Clade: Euopisthobranchia
Order: Pteropoda
Suborder: Thecosomata
Blainville, 1824
Families

Limacinidae
Cavoliniidae
Cliidae
Creseidae
Cuvierinidae
Praecuvierinidae
Peraclididae
Cymbuliidae
Desmopteridae

Contents

The Thecosomata (collective/plural: thecosomes, [1] meaning "case/shell-body"), [2] or sea butterflies, are a taxonomic suborder of small, pelagic, free-swimming sea snails known as holoplanktonic opisthobranch gastropod mollusks, in the order Pteropoda (also included within the informal group Opisthobranchia). Most pteropods have some form of calcified shell, although it is often very light, even translucent. [3]

The sea butterflies include some of the world's most abundant gastropod species; [1] as their large numbers are an essential part of the ocean food chain, they are a significant contributor to the oceanic carbon cycle. [3] [4]

Morphology

Unlike other sea snails, or even land snails, sea butterflies float and swim freely through the ocean, traveling along with the currents. This has led to a number of evolutionary adaptations in their bodies, including complete or near-complete loss of the shell and the gill in several families. Their gastropodal foot has also taken the form of two wing-like lobes, or parapodia , which propel the animal through the sea by slow flapping movements.

Unidentified thecosome Sea butterfly.jpg
Unidentified thecosome

Most thecosomes have some form of calcified shell, although often very light. [3] They are rather difficult to see, since their shell, if present, is mostly transparent, fragile, and usually tiny (less than 1 cm in length). Although their shell may be so fine as to be transparent, it is nevertheless calcareous, and an important part of the oceanic calcium cycle. [4] Their shells are bilaterally symmetric and can vary widely in shape, ranging from coiled or needle-like to triangular or globular.

The shell is present in all life cycle stages of the Cavolinioidea (euthecosomata). In the Cymbulioidea (pseudothecosomata), adult Peraclididae also bear shells; the Cymbuliidae shed their larval shells and develop a cartilaginous pseudoconch in adulthood. Only the Desmopteridae lack any rigid covering when mature.

Behavior and distribution

Swimming kinematics

Molluscan pteropods develop their feet into a pair of wing-like parapodia in the growing phase. These 'wings' are highly flexible, as the orientation of the muscles is different, and they have a hydrostatic skeleton filled with a pressurized fluid. Thus, the high bending-angle supports the parapodia to diminish the drag forces generated by the classic "clap-and-fling" maneuver; additionally, it aids in carrying the extra weight of a shell and ascending the water column for the diel vertical migration. [5]

The power-stroke for L. helicina starts with a sharp rotation of it is body accompanied by an increase in swimming speed; it then rotates its shell in the opposite direction to initiate the recovery-stroke and swims upward, with a speed less than the power phase. There is a drop in overall speed between power and recovery strokes, which develop a sawtooth trajectory in the sagittal plane. The hyper-pitch of the round shell of L. helicina diminishes the rotational drag and the moment of inertia; the extreme shell rotation also assists in raising the wingtips at the end of each stroke to create a figure-eight pattern, common for flying insects. In contrast, flying insects and shell-less pteropods encounter higher resistance forces that limit the body rotation. [6]

Sea butterflies range from the tropics [7] to the poles. [8] They are "holoplanktonic"—they spend all their lives floating amongst plankton, rather than remaining planktonic during their larval stage. [lower-alpha 1] Thus, thecosomans are most common in the top 25 metres (82 ft) of the ocean—in terms of diversity, species richness, and abundance—and become scarcer with increased depth. [7]

Occasionally, thecosomans swarm in large numbers, and can sometimes be found washed ashore in flotsam, especially along the coastline of eastern Australia.

Diurnal vertical migration

Thecosomata beat their wing-like parapodia to "fly" through the water. [8] [9] When descending to deeper water, they hold their wings up.

They migrate vertically from day to night, so the community structure changes on a 24 hour cycle; during the day many organisms take refuge at water depths in excess of 100 m. [7]

Feeding

Little is documented of the dietary habits or behaviour of sea butterflies, yet they are known to have a peculiar way of feeding. [3] Being generally herbivorous, and mostly passive plankton-feeders, they live their lives according to the currents and find food by floating ventral-side up; some may more actively forage, at times. [3] They ensnare planktonic food by entangling it in a mucous web [8] measuring up to 5 cm wide—many times larger than their own bodies. If disturbed, they simply abandon the web and flap slowly away.

Each day, they embark on a regular diel vertical migration through the water column in their pursuit of planktonic prey. At night, they forage at the surface and return to deeper waters by the morning. [10]

Fossil record

Geologically-speaking, Thecosomata is a rather young group, being known from the Late Paleocene of the Cenozoic Era. [11]

Sea butterfly pseudoconch Sea butterfly pseudoconch (12286).jpg
Sea butterfly pseudoconch

The group is known within the fossil record from shells of those groups within the clade that mineralized. [12] [13] These carbonate shells are a major contributor to the oceanic carbon cycle, making up as much as 12% of global carbonate flux. [3] However, the low stability of their aragonitic shells means that few end up being preserved within sediments as viable fossils; rather, they are mostly deposited in shallow, tropical seas. [3]

Importance in the food chain

These creatures, which range from lentil- to orange-sized, are eaten by various marine species, including a wide variety of fish that are, in turn, consumed by penguins and polar bears. The sea butterflies form the sole food source of their relatives, the Gymnosomata. [8] They are also consumed by sea birds, whales, and commercially important fish. However, if sea butterflies are consumed in large quantities fish can get "black gut", which makes them unsellable. [1]

Taxonomy

Along with its sister group, the sea angels (Gymnosomata), the sea butterflies (Thecosomata) are included in the order Pteropoda. [3] The validity of the pteropod order is not universally accepted; it fell out of favour for a number of years, but recent molecular evidence suggests that the taxon should be revived. [14] Although most Thecosomata have some form of calcified shell, mature Gymnosomata have none. [3]

Ponder & Lindberg

Order Thecosomata de Blainville, 1824

Bouchet & Rocroi

In the new taxonomy of Bouchet & Rocroi (2005) Thecosomata is treated differently :

Clade Thecosomata : [15]

Bouchet & Rocroi (2005) move the family Limacinidae into the superfamily Cavolinioidea, making redundant the superfamily Limacinoidea erected for it in Ponder & Lindberg's taxonomy. The families Creseidae and Cuvierinidae are demoted to subfamilies of Cavoliniidae (Creseinae and Cuvierininae). The infraorder Pseudothecosomata becomes the superfamily Cymbulioidea. The family Peraclididae is included in the superfamily Cymbulioidea as the family Peraclidae, making the superfamily Peraclidoidea redundant.

See also

Footnotes

  1. Compare the sea butterflies unusual whole-life residence in the plankton with the more common behavior of most other marine gastropods, whose veliger larvae are part of the meroplankton, but who leave the plankton once they reach adult form.

Related Research Articles

<span class="mw-page-title-main">Sea angel</span> Clade of gastropods

Sea angels are a large group of small free-swimming sea slugs, not to be confused with Cnidarians, classified into six different families. They are pelagic opisthobranchs in the clade Gymnosomata within the larger mollusc clade Heterobranchia. Sea angels were previously referred to as a type of pteropod.

<span class="mw-page-title-main">Opisthobranchia</span> Informal group of gastropods

Opisthobranchs is a now informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping.

<span class="mw-page-title-main">Limacinidae</span> Family of gastropods

The Limacinidae are a family of small sea snails, pteropods, pelagic marine gastropod mollusks in the clade Thecosomata.

<i>Limacina</i> Genus of gastropods

Limacina is a genus of swimming predatory sea snails commonly known as sea butterflies in the family Limacinidae. This genus contains some of the world's most abundant gastropod species.

<span class="mw-page-title-main">Cavolinioidea</span> Superfamily of gastropods

The superfamily Cavolinioidea is the most speciose group of sea butterflies. They belong to the suborder Euthecosomata.

<span class="mw-page-title-main">Cymbuliidae</span> Family of gastropods

Cymbuliidae is a family of pelagic sea snails or "sea butterflies", marine gastropod mollusks in the superfamily Cymbulioidea.

Desmopteridae is a family of pelagic sea snails or "sea butterflies", marine gastropod mollusks in the superfamily Cymbulioidea.

The Notobranchaeidae, or "naked sea butterflies", are a taxonomic family of floating sea slugs, specifically under the subclass Opistobranchia, also called "sea angels".

<span class="mw-page-title-main">Orthogastropoda</span> Historic group of molluscs

Orthogastropoda was a major taxonomic grouping of snails and slugs, an extremely large subclass within the huge class Gastropoda according to the older taxonomy of the Gastropoda.

Paedoclione doliiformis is a species of sea angel, a small floating sea slug, a pelagic marine gastropod mollusk in the family Clionidae.

<span class="mw-page-title-main">Pteropoda</span> Order of molluscs

Pteropoda are specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The monophyly of Pteropoda is the subject of a lengthy debate; they have even been considered as paraphyletic with respect to cephalopods. Current consensus, guided by molecular studies, leans towards interpreting the group as monophyletic.

<span class="mw-page-title-main">Atlantidae</span> Family of gastropods

Atlantidae is a family of sea snails, holoplanktonic gastropod molluscs in the clade Littorinimorpha.

The taxonomy of the Gastropoda, as revised by Winston Ponder and David R. Lindberg in 1997, is an older taxonomy of the class Gastropoda, the class of molluscs consisting of all snails and slugs. The full name of the work in which this taxonomy was published is Towards a phylogeny of gastropod molluscs: an analysis using morphological characters.

<span class="mw-page-title-main">Pterotracheoidea</span> Superfamily of molluscs

The Pterotracheoidea is, according to the Taxonomy of the Gastropoda, a taxonomic superfamily of sea snails or sea slugs, marine gastropod molluscs in the clade Littorinimorpha. They are commonly called heteropods or sea elephants.

<span class="mw-page-title-main">Cymbulioidea</span> Superfamily of gastropods

Cymbulioidea is a taxonomic superfamily of pelagic "sea butterflies", one group of swimming sea snails. They are holoplanktonic opisthobranch gastropod molluscs in the clade Thecosomata.

<span class="mw-page-title-main">Cavoliniidae</span> Family of gastropods

The family Cavoliniidae is a taxonomic group of small floating sea snails, pelagic marine opisthobranch gastropod mollusks.

Clione limacina, known as the naked sea butterfly, sea angel, and common clione, is a sea angel found from the surface to greater than 500 m (1,600 ft) depth. It lives in the Arctic Ocean and cold regions of the North Atlantic Ocean. It was first described by Friderich Martens in 1676 and became the first gymnosomatous "pteropod" to be described.

<i>Protatlanta</i> Genus of gastropods

Atlanta is a genus of pelagic marine gastropod molluscs in the family Atlantidae.

<i>Limacina helicina</i> Species of gastropod

Limacina helicina is a species of small swimming planktonic sea snail in the family Limacinidae, which belong to the group commonly known as sea butterflies (Thecosomata).

Cuvierina atlantica is a species of sea mollusk, commonly known as a sea-angel pteropod.

References

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Sources