Abralia trigonura Berry 1913

Introduction

A. trigonura belongs to the Heterabralia subgenus. It is characterized by having two or three hooks on the manus, the left arm hectocotylized, and complex eye photophores. This species resembles A. andamanica but  the latter is separable by its robust tail and dark body color.

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Abralia trigonura, female, 41 mm ML, ventral view. Drawing from Burgess (1992).

Characteristics

1. Tentacle clubs
   1. Two or three hooks on ventral side.
   2. Two rows of large suckers on dorsal side of manus.
   

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   Figure. Oral view of a tentacular club of A. trigonura, Hawaiian waters. Drawing from  Burgess (1991, p. 115, Fig. 1J).

2. Hectocotylus
   1. Left ventral arm of male hectocotylized.
   2. Hectocotylus with two different sized off-set flaps.
   

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   Figure. Oral view of the hectocotylus of A. trigonura, Hawaiian waters. Drawing from  Burgess (1991, p. 115, Fig. 1I).

3. Suckers
   
   1. Suckers dentate all aroung.
   

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   Figure. Sucker rings of A. trigonura showing dentition. X - Inner sucker ring shelf showing indentation. Drawings from Burgess (1991, p. 115, Fig. 1D, E).
   

4. Head
   
   1. Eye Photophores: Five, complex organs: two large terminal opaque organs and three intermediate silvery organs.
   2. Beaks: Descriptions can be found here: Lower beak; upper beak.
      
      


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Figure. Ocular photophores of A. trigonura. Left - Ventral view. Drawing from Tsuchiya (2000). Right - Postero-ventral view. Photograph by R. Young of a fresh squid with overlying skin removed.

5. Integumental Photophores
   1. Ventral mantle and head with scattered arrangement of integumental organs.
      
      
6. Mantle apex ("tail")
   1. Broad and extends well beyond conus of gladius.
      
      
7. Reproductive structures
   
   

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   Figure. Dorsal view of the receptacle for spermatangia in female A. trigonura. Anterior is left. Left side - Note the nuchal cartilage and collar on either side. Middle - Deep pit that holds the spermatangia. Right - Mantle and gladius folded posteriorly showinng the receptacle located where the mantle and visceral sac fuse. Drawing from Burgess (1991, p. 115, Fig. 1K)

Comments

A. trigonura is similar to A. andamanica, A. siedleckyi, A. heminuchalis, and A. veranyi in its complex ocular photophores and left ventral arm hectocotylization.

Life history

Age and reproduction

Female longevity is up to 6 months, and sexual maturity is reached at 3.5 months. Male longevity is the same as female, and sexual maturity is reached at 2.5 months. The smallest mature female is 31mm DML, and 80% of females are mature at 35mm DML. Males sexually mature between 23-27mm DML. This species seems to be a multiple spawner, and females spawn eggs every few days. Female batch fecundity was roughly 290-430 eggs (Young and Mangold, 1994).

Eggs

Spawned eggs are slightly ovoidal, 0.9 mm x 0.79 mm, usually with a slightly greenish tint, clear, and with a sticky colorless jelly coating (Young and Harman, 1985).

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Figure. Left - "Egg" (actually early embryo) of A. trigonura taken from a plankton tow off Hawaii. The egg is surrounded by a gelationus oviducal secretion with debris attached. Right - Newly hatched paralarva of A. trigonura. Photographs by R. Young.

 Paralarvae

The distinctive chromatophore patterns, especially that of the ventral mantle, can be followed from hatching to the largest paralarva illustrated (5.0 mm ML) where each ventral mantle chromatophore is accompanied by a photophore. That is, the initial chromatophore pattern is duplicated with photophores as the paralarva grows.

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Figure. Ventral and dorsal views of A. trigonura paralarvae. 1.0 mm ML - At hatching. 1.3 mm ML - Seven days after hatching, yolk exhausted. Others - Trawl captured. Drawings from Young and Harman (1985).

Distribution

Vertical Distribution

In the Hawaiian waters, this species was considered to be an important component of the mesopelagic boundary community (Reid et al., 1991), and vertically migrates from upper mesopelagic depths during the day to the upper 50 m at night (Young, 1978). It generally stays during the daytime >5m above the ocean floor (Young, 1995). Young stages occur primarily outside the mesopelagic boundary zone. Adults inhabit the outer mesopelagic boundary zone during daytime, and migrate to the inner mesopelagic boundary zone during night (Young, 1995).

Geographical Distribution

This species is broadly distributed in tropical West to Central North Pacific (Hidaka and Kubodera, 2000).

References

Hidaka, K. and T. Kubodera. 2000. Squids of the genus Abralia (Cephalopoda: Enoploteuthidae) from the western tropical Pacific with a description of Abralia omiae, a new species. Bull. Mar. Sci. 66: 417-443.

Tsuchiya, K. 2000. Illustrated book of the Enoploteuthidae. In: Okutani T., ed. True face of Watasenia scintillans. Tokai University Press, Tokyo, p 196–269. (in Japanese)

Young, R.E. 1995. Aspects of the natural history of pelagic cephalopods of the Hawaiian mesopelagic-boundary region. Pacific Science, 49:143-155.

Young, R.E. and Harman, R.F. 1985. Early life history stages of enoploteuthin squids (Cephalopoda: Teuthoidea: Enoploteuthidae) from Hawaiian waters. Vie Milieu, 35:181-201.

Young, R.E. and Mangold, K.M. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Marine Biology, 119: 413-421.