Gonatids are mostly muscular squids of moderate size but are unusual in having the armature on the arms in four, rather than two, series (occasionally more than four series near the arm tips). The two medial series are usually hooks. Often the tentacular club has one very large centrally located hook. Species of the genus Gonatopsisare unusual in that they lack tentacles which are lost in the early juvenile stage.
An oegopsid ...
with tetraserial armature on arms.
usually with hooks on arms and often on tentacular clubs.
Arms with quadraserial armature except at arm tips in some species where sucker series increases.
Arms I-III with hooks in two medial series except in Berryteuthis anonychus; the latter with hooks only in females and only at bases of arms I-III.
Figure. Oral view of two arms of Gonatus steenstrupi with large hooks in the medial two series (white arrows) and a series of small suckers on each arm margin (black arrows). Transmitted-light photograph by M. Vecchione aboard the R/V G. O. SARS, MarEco cruise to the central North Atlantic.
Tentacular clubs with numerous irregular series of suckers; additional hooks in some genera.
Tentacular clubs with unique locking-apparatus in Gonatus and Eogonatus consisting of a series of elongated ridges with medial suckers and knobs.
Figure. Oral view of the proximal region of the club of G. steenstrupi. Arrow points to one ridge of the locking-apparatus. Transmitted-light photograph by M. Vecchione aboard the R/V G. O.SARS, MarEco cruise to the central North Atlantic.
Buccal-crown connectives attach to ventral margins of arms IV.
Funnel locking-apparatus with straight groove.
Photophores absent except in G. pyros (ocular photophores).
Gladius with primary conus.
The following table compares characteristics of subadults for the genera of Gonatidae.
Genus / Character
Hooks present on clubs
Single series of suckers and knobs along entire dorsal margin of manus.
Short series of suckers, knobs and elongate ridges at base of manus.
Short series of suckers, knobs and elongate ridges at base of manus.
Gonatids are pelagic squids that occur from the surface to over 1000 m depth. Some species undergo extensive diurnal (diel) vertical migrations, ascending at night and descending during the day. A few species are associated with the ocean floor over the continental slope. Gonatids occur in high latitudes of both hemispheres. One species lives in antarctic waters, two in the North Atlantic and 16 in the high North Pacific where they are among the most abundant oceanic squids.
Historically the paralarvae of gonatid squids have been virtually impossible to identify. Recent studies, however, found that the dorsal-head chromatophores can allow specific identification of the smallest paralarvae of at least six species in the North Pacific (Jorgensen, 2006). Jorgensen (2006) recognizes two basic patterns, Type 1 and Type 2, with species-specific variations in each.
Figure. Dorsal view of the two classes of head chromatophores presently recognized in gonatids. Type I has three tear-shaped chromatophores on each side that come to a point over each eye. Type II has three transverse rows of chromatophores with one chromatophore in the anterior row, two in the middle row and three in the posterior row. Drawings from Jorgensen (2007).
There have been several recent attempts to unravel the phylogeny of the Gonatidae. Nesis (1997) relied on similarity of morphology and, apparently, results of protein electrophoresis by Katugin (1993 and 1995) to derive the following relationships:
Figure. Phylogeny of the Gonatidae. Chart modified from Nesis (1997). Subgenera are in red. Nesis included only species that were well established as valid species. Nesis considered Eogonatus to be a subgenus of Gonatus.
Clearly if this phylogeny is correct then Gonatopsis is polyphyletic. More recently Katugin (2004) has reassessed the phylogeny of the gonatidae using electrophoretic analysis of allozymes. His results are seen below.
Figure. Phylogenetic tree of the Gonatidae. Chart modified from Katugin (2004). Numbers represent genetic distance based on protein electrophoresis. Branch lengths are not drawn proportional to genetic distance.
In the paper Katugin proposes a new classification of the Gonatidae with two subfamilies (Berryteuthinae and Gonatinae) based on the radula. Within the Berryteuthinae he includes Berryteuthis (B. magister), Boreoteuthis (for Gonatopsis borealis; he elevates the subgenus Boreoteuthis to generic level) and a new genus Okutania for Berryteuthis anonychus. In the Gonatinae he includes Gonatus (including Eogonatus) and the remaining species of Gonatopsis. Katugin's phylogeny based on allozymes is similar to that of Nesis (1997) however the allozyme data shows the Gonatinae nested within the Berryteuthinae.
Recent molecular data analyzed using parsimony from 12S rRNA, 16S rRNA, and COI genes (Lindgren et al. 2005) offer a somewhat different phylogeny of the family but without good bootstrap support for most nodes. This data suggests that the Gonatus is paraphyletic. Gonatopsis borealis groups more closely with Berryteuthis than other species of Gonatopsis as suggested by both Nesis (1997) and Katugin (2004).
Figure. Phylogenetic tree of the Gonatidae. Chart simplified from Lindgren et al. (2005) by collapsing nodes with bootstrap support of 50 or less, and eliminating uncertain identifications. Numbers represent bootstrap support for relationships derived from combined molecular analyses. Numberered G. middendorffi represent different morphological types.
Because of the uncertainty in gonatid phylogeny, we retain here the standard classification for the family.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:166 Specimens with Sequences:125 Specimens with Barcodes:125 Species:19 Species With Barcodes:18 Public Records:66 Public Species:17 Public BINs:17
The Gonatidae, also known as armhook squid, are a family of moderately sized squid. The family contains about 19 species in three genera, widely distributed and plentiful in cold boreal waters of the Pacific Ocean. At least one species is known from Antarctic waters, and two from the North Atlantic.
Morphologically, armhook squid are fairly uniform: all species are characterised by the suckers of their arms, which are arranged in four rows (series) rather than the typical count of two. In most species, the arm's two mesial rows of suckers have been modified into hooks and the tentacular clubs—which are covered with many irregular rows of tiny suckers—may possess an enlarged central hook, with or without several smaller hooks. In the magister armhook squid (Berryteuthis magister), only the females possess hooks. Species of the genus Gonatus differ from the rest of the family (and from most squid) by their lack of tentacles as adults.
Gonatids typically have muscular, cylindrical bodies with very soft, reddish to purplish-brown skin. The arms are thick and capable; the fins vary in shape and size, from sagittate and about 50% of the mantle length, to reniform and about 30% of the mantle length. Of moderate size, these squid range in size from 11 to 40 cm—most species are 25 cm or less. Females are somewhat larger than males.
These squid are pelagic, associated with the continental shelf and may roam as deep as 4,500 m or more, depending on the species. Their habits are poorly studied, but the squid are thought to undertake diel migration; by day, the squid remain in the blackness of the depths in midwater. By night, they ascend to the upper layers of the water column to feed by starlight. One species, however, Gonatopsis octopedatus, has curiously recurved arms, suggesting a benthic existence.
Little is known about the reproductive cycle of armhook squid. Most squid species whose reproduction has been obeserved have been seen to deposit eggs on the sea floor, then leave the eggs to hatch on their own. Five female Gonatus onyx squids have been observed in Monterey Canyon dragging a membrane sack containing 2,000 to 3,000 developing eggs. It is uncertain if this behavior extends to other members of the Gonatidae family or if it is particular to this species.