Bathothauma lyromma

Tentacles

• Tentacular club with suckers only.
• Tentacular stalk with two series of suckers and pads proximally, four series near club forming elongate carpal group.

Arms

• Arm suckers with 8-28 rounded teeth around entire circumference of sucker ring (teeth can be more angular on smaller suckers).

Funnel

• Funnel valve absent.
• Funnel organ with inverted V-shaped dorsal pad

Mantle

• Tubercles absent (or virtually absent) from funnel-mantle fusion.

Fins

• Fins paddle-shaped, subterminal.
• Fins widely separated (insert on lateral, expanded ends of transverse extensions of posterior end of gladius).*

Photophores

• One on each eye.**

Figure. Juvenile B. lyromma left eye showing ventral photophore : (a) ventral, (b) lateral, (c) anterior perspectives.  Illustration from Evans (2018), specimen 75 mm ML.

*Within Cranchiidae, unique to this genus.
**Unusual feature within Cranchiidae; shared only with Helicocranchia.

Geographical distribution

The genus is found circumglobally in tropical and subtropical waters except for an apparent absence from subtropical waters of the South Atlantic. It also extends into temperate waters in the northeastern Atlantic and South Pacific (Voss, 1992).

Vertical distribution

The vertical distribution of B. lyromma off Hawaii exhibits a clear ontogenetic descent. No adults were taken in this study, but they probably occur at depths greater than 1000 m.

Figure. Vertical distribution chart of B. lyromma, Hawaiian waters. Captures were made with both open and opening/closing trawls. Bars- fishing depth-range of opening/closing trawl. Circle- Modal fishing depth for either trawl. Blue-filled circles- Night captures. Yellow-filled circles- Day capture. Chart modified from Young (1978).

Males exhibit secondary sexual modifications on arms I (first seen developing around 50 mm ML). Arm sucker series change from 2 series of suckers to four series of much smaller suckers on distal half of Arms I. 

Figure. Male arm modification (a), Arm I of B. lyromma showing sucker arrangement, (b) enlargement of modified sucker (NIWA 90017, ML 110 mm ML). Image from Evans, 2018.

The paralarva is very distinctive with extremely long eye-stalks, a long brachial pillar, widely separated fins and an unusual chromatophore pattern.

Figure. Left - Dorsal view of a large paralarva (ca. 50 mm ML) B. lyromma, Hawaiian waters. Photograph by R. Young. Right - Ventral and dorsal views of a small paralarva (ca. 5 mm ML) of B. lyromma, Hawaiian waters. The drawings emphasize the eye-rostrum by showing the eyes in different orientations: The left eye is seen in a side view and the right eye in either a ventral view (left drawing) or a dorsal view (right drawing). Drawings by R. Young. The scale bar is 1 mm.

Although not a peer-reviewed source of information, this REDDIT link shows one of the few public video's of Bathothauma lyromma in the wild.