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The octopus of the genus Tremoctopus from the Arabian Sea has not been investigated in the past. This study reports on the rare pelagic octopus Tremoctopus gracilis from the south-eastern Arabian Sea. Twenty females with dorsal mantle lengths (DML) ranging between 62 and 160 mm and weighing between 31 and 719 g were examined. These specimens were collected from commercial fishing grounds, and amongst them was a single mature female with 12,618 oocytes in the ovary (size varied from 0.61 to 1.67 mm (mean = 1.13 mm). The detailed morphometric and morphological characteristics were recorded, and the specimens were identified up to the genus level. Further molecular species identification was carried out through mitochondrial markers (COI & 16S rRNA). Phylogenetic analysis, based on partial sequences of the above markers, generated a single clade that included sequences from the Indian and Pacific Oceans, including those misidentified as Tremoctopus violaceus from the Pacific. Evolutionary divergence estimation revealed a high inter-specific genetic divergence from T. violaceus sequences in the Mediterranean Sea and Atlantic Ocean, thus confirming the Tremoctopus species in our study as T. gracilis and authenticated that GenBank sequences of T. violaceus from the Pacific Ocean are indeed T. gracilis.
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Vol.:(0123456789)
Marine Biology (2024) 171:47
https://doi.org/10.1007/s00227-023-04376-0
ORIGINAL PAPER
Resolving taxonomic ambiguity anddistribution ofTremoctopus
gracilis (Cephalopoda: Tremoctopodidae) based onmolecular
evidence
KurichitharaK.Sajikumar1· GeethaSasikumar1· DivyaViswambharan1· SummayaRahuman1· N.S.Jeena1·
NedumpallyVineesh1,2· C.P.Binesh1· K.P.SaidKoya1· V.Mahesh1· PrathibhaRohit1· P.K.Asokan1·
SmruthuMohan1· KolliyilS.Mohamed1
Received: 21 January 2023 / Accepted: 6 December 2023 / Published online: 6 January 2024
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2024
Abstract
The octopus of the genus Tremoctopus from the Arabian Sea has not been investigated in the past. This study reports on the
rare pelagic octopus Tremoctopus gracilis from the south-eastern Arabian Sea. Twenty females with dorsal mantle lengths
(DML) ranging between 62 and 160mm and weighing between 31 and 719g were examined. These specimens were col-
lected from commercial fishing grounds, and amongst them was a single mature female with 12,618 oocytes in the ovary
(size varied from 0.61 to 1.67mm (mean = 1.13mm). The detailed morphometric and morphological characteristics were
recorded, and the specimens were identified up to the genus level. Further molecular species identification was carried out
through mitochondrial markers (COI & 16S rRNA). Phylogenetic analysis, based on partial sequences of the above mark-
ers, generated a single clade that included sequences from the Indian and Pacific Oceans, including those misidentified as
Tremoctopus violaceus from the Pacific. Evolutionary divergence estimation revealed a high inter-specific genetic divergence
from T. violaceus sequences in the Mediterranean Sea and Atlantic Ocean, thus confirming the Tremoctopus species in our
study as T. gracilis and authenticated that GenBank sequences of T. violaceus from the Pacific Ocean are indeed T. gracilis.
Keywords Arabian Sea· Blanket octopus· Identification· Taxonomic ambiguity· Molecular
Introduction
The octopus Order Octopoda comprises nearly 315 valid
species (MolluscaBase 2023) that are widely distributed in
the demersal and pelagic ecosystems of the world oceans
(Allcock etal. 2014), whilst merely 45 species of the families
Octopodidae, Tremoctopodidae and Argonautidae abound in
the Indian Seas (Silas etal. 1985; Sreeja etal. 2015; Saji-
kumar etal. 2016, 2020; Sundaram etal. 2019; Vargheese
etal. 2022). Tremoctopodidae is one of the four families
of incirrate octopus under the superfamily Argonautoidea
which exhibit considerable sexual dimorphism in body size,
with females being two orders of magnitude larger than
males in length (Norman etal. 2002). During mating, male
argonautoids detach and transfer the hectocotylized arm to
the female’s mantle cavity, which serves as a sperm storage
organ (Naef 1923; Norman etal. 2002).
The monogeneric family Tremoctopodidae, also known
as blanket octopuses, is characterised by the typical trailing
elongated webs from the dorsal arms, and is represented by
four recognised species, Tremoctopus violaceus delle Chiaje,
1830, T. gracilis (Souleyet 1852), T. gelatus Thomas 1977
and Tremoctopus robsonianus Kirk, 1884 (Mangold etal.
1996). Tremoctopus violaceus and T. gracilis are known to
inhabit the surface to mid-depths of the subtropical and trop-
ical oceans, whilst T. gelatus is deep-living and presumably
mesopelagic, with circumtropical and temperate distribution,
and T. robsonianus has been reported from New Zealand
waters (Mangold etal. 2018).
Tremoctopus gracilis (Souleyet 1852) was initially
described from the Pacific Ocean (106°W; 08°N, Type
Responsible Editor: R. Rosa.
* Geetha Sasikumar
geetha.sasikumar@icar.gov.in
1 ICAR-Central Marine Fisheries Research Institute, Post Box
No. 1603, Kochi682018, India
2 Department ofZoology, University ofTartu, Tartu, Estonia
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