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| Aenigmachanna gollum Britz, Anoop, Dahanukar & Raghavan 2019 Aenigmachannidae Britz, Dahanukar, Anoop, Philip, Clark, Raghavan & Rüber, 2020 Gollum Snakehead Fishes |
Abstract
Pronounced organism-wide morphological stasis in evolution has resulted in taxa with unusually high numbers of primitive characters. These ‘living fossils’ hold a prominent role for our understanding of the diversification of the group in question. Here we provide the first detailed osteological analysis of Aenigmachanna gollum based on high-resolution nano-CT scans and one cleared and stained specimen of this recently described snakehead fish from subterranean waters of Kerala in South India. In addition to a number of derived and unique features, Aenigmachanna has several characters that exhibit putatively primitive conditions not encountered in the family Channidae. Our morphological analysis provides evidence for the phylogenetic position of Aenigmachanna as the sister group to Channidae. Molecular analyses further emphasize the uniqueness of Aenigmachanna and indicate that it is a separate lineage of snakeheads, estimated to have split from its sister group at least 34 or 109 million years ago depending on the fossil calibration employed. This may indicate that Aenigmachanna is a Gondwanan lineage, which has survived break-up of the supercontinent, with India separating from Africa at around 120 mya. The surprising morphological disparity of Aenigmachanna from members of the Channidae lead us to erect a new family of snakehead fishes, Aenigmachannidae, sister group to Channidae, to accommodate these unique snakehead fishes.
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| Aenigmachanna gollum. (a) 60.6 mm individual alive (KUFOS 2019.8.226), green arrow marks position of vent; (b) CT scan image of skeleton of 90.2 mm holotype (BNHS FWF 966), green arrow marks position of vent and red arrow the first caudal vertebra; (c) CT scan image of head of holotype, yellow arrows mark postorbital process and posterior tip of greatly elongated maxilla; (d) CT scan image of caudal skeleton of holotype, note absence of Day’s bone and presence of distally bifurcated haemal spine (yellow arrow) on second preural centrum; (e) CT scan image of iodine stained 124.5 mm specimen (KUFOS 2019.8.225) in lateral view, swim bladder is shown in blue above the digestive system (green), note swim bladder ending at level of 8th postanal vertebra. |
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Head anatomy of a channid (Parachanna, left side) and Aenigmachanna (right side) in comparison. Neurocranium of Parachanna africana, (MTD-F39824, c&s, nasals not removed) 150 mm (a,c,e) and Aenigmachanna gollum, (KUFOS 2019.8.224) 81.8 mm in dorsal (a,b), lateral (c,d) and ventral (e,f) view. Cleared and stained jaws and hyopalatine arch in lateral view of (a) Aenigmachanna gollum, (KUFOS 2019.8.226) 60.6 mm, (b) Parachanna africana, (MTD-F39824) 150 mm and (c) Channa punctata, (MTD-F39825) 118 mm. |
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| Lateral views of skeleton of the generalized percomorph Lates (a), the anabantoid Ctenopoma (b), the aenigmachannid Aenigmachanna (c) and the channid Parachanna (d). Green arrow marks position of vent, red arrow position of first caudal vertebra (anterior most vertebra with haemal spine). Note more or less equal separation of vertebral column in Lates (a) and Ctenopoma (b) into abdominal and caudal vertebrae (a,b,d adapted from33) with position of vent and first caudal vertebra close to each other and caudal and postanal region of vertebral column of similar length. Note partial abdominalisation of postanal vertebral column in Aenigmachanna and almost complete abdominalisation in Parachanna. |
Taxonomy
Aenigmachannidae new family
(Gollum snakehead fishes)
Type genus: Aenigmachanna Britz, Anoop, Dahanukar & Raghavan 2019
Diagnosis: A family of the acanthomorph clade Labyrinthici (Anabantiformes), as evidenced by the shared derived possession of a parasphenoid tooth patch (Fig. 2d,f). Aenigmachannidae are distinguished from all other Labyrinthici by the following autapomorphies: (a) a very long maxilla reaching caudally beyond the anterior margin of the preopercle (Figs. 1b,c, 3a), (b) presence of a prominent postorbital process on the maxilla (Figs. 1c, 3a), (c) the frontal suturing with the parasphenoid forming a complete interorbital septum (Fig. 2f), (d) the unique count of 29–32 abdominal and 29–31 caudal vertebrae (Fig. 1b), (e) a series of five median predorsal bones (supraneurals or rayless pterygiophores) in front of the dorsal fin (f) 83–85 scales in a lateral series, and (g) a high number of 40–44 anal-fin rays (Fig. 1a,b). It differs further from all Anabantoidei and Channidae by the swim bladder being short (Fig. 1e), not reaching the parhypural and by the absence of a suprabranchial cavity and organ (Fig. 2h,j). Aenigmachannidae share with Channidae long nasal tubes (Fig. 1a), cycloid scales, the absence of fin spines in dorsal- and anal-fins (Fig. 1a), an increase in the number of vertebrae, a single posterior swimbladder extension combined with abdominalisation of the anterior ten postanal vertebrae (Fig. 1), and five branchiostegal rays, but differ from them by the autosphenotic being excluded from the skull roof and the frontal broadly sutured only to the pterotic (Fig. 2a–f), prootic and basioccipital forming equal parts of the bulla for the sacculith (Fig. 2b,c), the presence of numerous caudal vertebrae and therefore the lack of an abdominalisation of the posterior vertebral column (Fig. 1b), and by absence of the uncinate process of the metapterygoid (Fig. 3a), absence of Day’s bone (Fig. 1d), and of a body lateral-line canal.
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| Phylogenetic relationships of Aenigmachannidae (marked in red) based on 46 morphological characters. Shown is the strict consensus tree of two most parsimonious trees (tree length = 64 steps, consistency index (CI) = 0.72, retention index (RI) = 0.89). Numbers at nodes correspond to bootstrap values (1000 replicates). Plesiomorphic character states of Aenigmachanna in relation to Channidae discussed in the text are shown on the tree with corresponding numbers. |
Ralf Britz, Neelesh Dahanukar, V. K. Anoop, Siby Philip, Brett Clark, Rajeev Raghavan and Lukas Rüber. 2020. Aenigmachannidae, A New Family of Snakehead Fishes (Teleostei: Channoidei) from subterranean waters of South India. Scientific Reports. 10, 16081. DOI: 10.1038/s41598-020-73129-6
Neue Familie für den Gollum-Schlangenkopffisch
