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Pterotrachea coronata

Pterotrachea coronata is a species of large floating sea snail, technically known as a pelagic gastropod mollusc. Of the four species in the family Pterotracheidae, P. coronata is the largest (1).

Though similar in appearance, this pelagic snail is not at all closely related to the pelagic opistobranch s such as the sea angels and sea butterflies. Being in the clade Littorinimorpha, and it is much more closely related to families such as tritons (Ranellidae) and tun shells (Tonnidae).

Description:
One of the most distinguishable body parts of P.coronata is the mouth, which is long and proboscis-like, giving this creature the nickname of “sea-elephant”. Above the “trunk” are two dark, cylindrical eyes, which vary in shape and size between different species of Pterotrachidae. The body is almost entirely colorless and transparent, but the visceral nucleus (a liver-like organ) is a clearly visible white sliver towards the tail. Next to the visceral nucleus, feathery gills are present, along with genitalia, which is on the right side of the body. There are true sexes in this species, meaning males and females have been observed, but their reproductive habits are not well documented. A dorsal fin propels the creature forward. The maximum recorded body length is 260 mm.

P. coronata, like all other gastropods, have a balance sensory receptor called a statocyst, made of a sac-like structure lined with innervated sensory hairs (setae), and a mineralized mass (statolith) which rolls around according to gravity and the creature’s movements, subsequently stimulating the hairs and allowing a sense of orientation. However, P. coronata is unique in that the statocysts have been found to directly influence fin movement. The sensory hairs are concentrated on the upper part of the sac, and form in a ring-like structure around a “giant central cell”, which is equipped with around 500 cilia, compared to the smaller cells having around 30-80 cilia. Studies have speculated that this structure gives increased control over interneural activity to the fin. (2) The fin is a hollow organ, with inner and outer muscle layers on the left and right surfaces. To propel the body forward, the fin employs a lateral stroke, combined with an undulating motion along the body axis. These complex fin movements are modulated by visual, sensorimotor, and statoreceptive stimulation, and controlled by nerve centers in the pedalganglia. (2)

P. coronata is morphologically distinguishable from the other species of Pterotrachidae, due to its large body size, along with distinctive eye and visceral nucleus shapes. The smaller species are much harder to distinguish, and therefore species-specific information about Pterotrachidae has been scarce and inaccurate, on occasion (4)

While genetic information on P. coronata is lacking, Pterotraches hippocampus Philippi, a close relative, was found to have a chromosomal number of 2n=31 and 32 for males and females, respectively. (5)

Habitat:
This pelagic species has been recorded at 0m to 250m depth, and is ubiquitously distributed throughout most warm oceanic regions, yet their lifestyle is not well documented. They have been seen eating jellyfish, specifically of the genus Forskalia, so it is likely that they live in a similar habitat to their prey.

References:

 * 1) Aravindakshan P.N. (1973) “Distribution and Ecology of Pterotrachea coronata (Forskål)”. The Biology of the Indian Ocean. Ecological Studies, vol 3. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-65468-8_33
 * 2) Wolff, HG. (1991) “Perception of Gravity and Fin Movement in the Marine Gastropods,  Pterotrachea-coronata and Carinaria-Mediterranea (Heteropoda)”. Zoologische Jahrbücher- Abteilung für allgemeine Zoologie und Physiologie der Tiere (95) 453-463.
 * 3) Jennings RM, Bucklin A, Ossenbrügger H, et al. (2010) “Species diversity of planktonic gastropods (Pteropoda and Heteropoda) from six ocean regions based on DNA barcode analysis”. Deep Sea Research Part II: Topical Studies in Oceanography (57) 2199-2210
 * 4) Seapy RR (2000) “Species Discrimination Among Pelagic Heteropods: Resolution of the Pterotrachea hippocampus-P. minuta Problem” Journal of Molluscan Studies (66) 99–118, https://doi.org/10.1093/mollus/66.1.99
 * 5) Vitturi R, Catalano E, Colombera D et al. (1993) “Multiple sex-chromosome system and   other karyological characterizations of Pterotrachea hippocampus (Mollusca: Mesogastropoda)” Marine Biology (115) 581–585.