Magnoteuthis microlucens (Young, Lindgren & Vecchione 2008)

Introduction

Magnoteuthis microlucens, previously listed here as Mastigoteuthis sp. A, is the most common species of the genus around the main Hawaiian Islands. It has numerous tiny photophores that lie beneath the outer layer of integumental chromatophores. The photophores are so small that they cannot be recognized as photophores without the aid of a microscope.

Brief diagnosis:

A Magnoteuthis with ...

•  minute integumental photophores.
•  a tropical Pacific habitat.

Characteristics

1. Arms
   1. Arms II moderate in length, 46% of ML, 69% of arms II length.
   2. Large arm suckers generally with closely-set, rounded teeth on the distal margin, occasionallly fused; sometimes smaller, more proximal teeth with narrower, more distinctly separated teeth.
      

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      Figure. Oral view of an arm sucker (sucker 26, arm III) of Mg. microlucens, paratype. Photograph by R. Young.

      Scanning electron micrographs of the arm and tentacle suckers can be seen here.

2. Head
   1. Beaks. Description of the beaks can be found here in 2D.
   2. Beaks: Descriptions can be found here in 3D: Lower beak; upper beak.
      
3. Funnel
   1. Funnel component of the funnel/mantle locking-apparatus flask-shaped with narrow stem.
   2. Mantle component without a nostril.
      

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      Figure. Funnel-mantle locking-apparatus of Mg. microlucens, holotype, Equatorial Pacific. Left - Frontal view of funnel component. Middle - Frontal view of mantle component. Right - Side view of mantle component. Blue stain = methylene blue. Photographs by R. Young.

4. Photophores
   1. Minute ("lens" diameter less than 0.1 mm) integumental photophores present but unrecognizable to naked eye due to small size and structure.
   2. Integumental photophores present on aboral surfaces of all arms, on dorsal and ventral surfaces of Head, mantle and fins, on ventral surface of funnel.
   3. Integumental photophores lie beneath the surface layer of chromatophores.
   4. Eyelid photophores absent.
      

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      Figure. Ventral view of the head of Mg. microlucens, holotype, Equatorial Pacific. Outer chromatophores were lost during capture leaving the scattered photophores visible. Enlargement at right allows photophores to be recognized by the presence of small, oval lenses. Photographs by R. Young.

      

      Figure. Photophores of Mg. microlucens. Left - Anteriolateral view of the head near the eye opening looking at the lenses of the photophores. Right - High magnification of two photophores (one indicated by the arrow) and a few, lighter-red chromatophores that are more superficially located from a damaged region of the side of the head, 160 mm ML, Hawaiian waters.

Comments

More details of the description can be seen here.

We identify structures here as photophores on the basis of a dark, pigmented "cup" and a spherical, whitish "lens" and their distribution as a separate layer beneath most chromatophores. No bioluminescence has been observed or attempted to be observed. Mg. microlucens is unique among members of Magnoteuthis in having photophores. If the surface chromatophore layer is intact, the deeper lying photophores can be difficult to recognize even under a microscope. Mastigoteuthis sp. A also differs from Mg. magna in the more elongate shape of the posterior bulb of the funnel locking-apparatus, dentition of the arm suckers and details of the beaks.

Molecular Characteristics

Mg. microlucens is most closely related to Mg. magna based on morphological evidence (see the Magnoteuthis page). Molecular data indicate substantial divergence across the three  genes examined (see Table below) and confirms that Mg. microlucens is a separate species from Mg. magna. Hebert et al (2003) hypothesized a 2% rate of divergence in the COI locus as a general guideline for identifying distinct species, here the rate of divergence between Mg. microlucens and the morphologically similar Mg. magna was almost 10%.  The more morphologically distinct Mastigopsis hjorti had a COI sequence divergence rate 12% with Mg. microlucens, only slightly greater than that of Mg. magna which further emphasizes the surprising result of 10% difference seen with Mg. magna (Young, Lindgren and Vecchione, 2008) (see also the Discussion of Phylogenetic Relationships on the Mastigoteuthidae page).

More details on the squid that provided the molecular data can be found here.

Life History

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Figure. Ventral and dorsal views of Mg. microlucens paralarvae, Hawaiian waters. Left - 4.6 mm ML. Right - 7.2 mm ML. Scale bar = 1 mm. Drawings from Young (1991) labeled as Mastigoteuthis inermis.

Paralarvae.

At 5 mm ML  Mg. microlucens paralarvae are separated from similar-sized paralarvae of Echinoteuthis. famelica, the only other common mastigoteuthid in Hawaiian waters, by the broader body shape, larger eyes, more posterior digestive gland and tentacular suckers and chromatophores restricted to the distal end of the tentacle. At 7 mm ML the presence of short fins (1/5 of ML vs 1/3 of ML) distinct anterior and posterior fin lobes (vs somewhat lanceolate fins) and tentacle suckers restricted to the club (vs along full tentacle) also distinguishes Mg. microlucens. At larger sizes, the slender shape, elongate fins and, by at least  17 mm ML, skin tubercules easily distinguish Echinoteuthis famelica. The differences between the two species diminish at sizes below 4.5 mm ML. The position of the digestive gland is useful to at least 3.5 mm ML. The smallest Mg. microlucens paralarva captured had a mantle length of 2.5 mm ML.

Adults.

We have seen one mature male, 215 mm ML but no mature females.

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Figure. Mature male of Mg. microlucens, 215 mm ML, Hawaiian waters, showing large penis containing white spermatophores and two whitish sacs of the distal reproductive tract emerging from the genital sac. Photograph by R. Young.

Distribution

Geographical Distribution 

Type locality: Equatorial Pacific south of Hawaii.

We have captured Mg. microlucens in the region of the Hawaiian Archipelago to about ca. 26°N and south to the equator. This is the most common mastigoteuthid encountered in the region of the high (main) Hawaiian Islands.

Vertical Distribution 

The vertical distribution of Mg. microlucens in Hawaiian waters was examined by Young (1978). During the day most captures came from depths of 675-870 m; nighttime captures came from 255 to 725 m with most squid taken between 250 and 450 m.

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Figure. Vertical distribution chart of Mg. microlucens. Captures were made with both open and opening/closing trawls. Bar - fishing depth-range of opening/closing trawl. Circle - Modal fishing depth for either trawl. Blue-filled circle - Night capture. Yellow-filled circle - Day capture. Note the breaks in the x-axis. Chart modified from Young (1978, labeled as Mastigoteuthis inermis).

References

Hebert, P.D., S. Ratnasingham, and J.R. de Waard. 2003. Barcoding animal life: cytochrome c oxidase subunit I divergences among closely related species. Proc. R. Soc. Lond. B., 270: S96-S99

Young, R. E. 1978. Vertical distribution and photosensitive vesicles of pelagic cephalopods from Hawaiian waters. Fish. Bull., 76: 583-615.

Young, R. E. (1991). Chiroteuthid and related paralarvae from Hawaiian waters. Bull. Mar. Sci., 49: 162-185.

Young, R. E., A. Lindgren and M. Vecchione. 2008. Mastigoteuthis microlucens, a new species of the squid family Mastigoteuthidae (Mollusca: Cephalopoda). Proc. Biol. Soc. Wash. 121(2): 276-282.