Hooked squid

Abstract: Cephalopod molluscs possess a variety of reproductive organs and display diverse sexual behaviours, justifying their nickname as ‘oceanic intellectuals’ (Boyle & Rodhouse, 2005). However, while courtship and mating of shallow-water cuttlefish, octopuses and squid have been filmed and well documented by divers and remotely operated vehicles (Sauer et al., 1997; Hanlon, 1998), what happens during mating of deepwater cephalopods remains largely a mystery (Nesis, 1995), apart from one observation of possible mating behaviour in Brachioteuthis beanii (Roper & Vecchione, 1996). When squid mate, a male transfers its sperm to a female enclosed in complex structures called spermatophores. These are accumulated in the spermatophoric sac, a storage organ inside the mantle cavity, before ejaculation through the penis. Squid that spawn in shelf waters and epipelagic waters of the open ocean usually have short penes hidden completely inside the mantle. Males pick the ejaculated spermatophores from inside their mantle with a specially modified arm called the hectocotylus, to transfer them to the female. Females spawning in shallow water have special places for spermatophore attachment on the body, both externally (skin ring around the mouth, and back of the head) and internally (oviducal gland openings near gills) (Nesis, 1995). As female squid lack a vagina, the use of a highly articulated arm (hectocotylus) for transfer and placement of spermatophores is more precise than by means of the comparatively poorly articulated penis. The majority of squid spawning in deep water lack hectocotyli (except Ancistrocheiridae and Cranchiinae; Nesis, 1995), and they have longer penes than their shallow-water counterparts. The only exception is the giant squid Architeuthis dux, which has the tips of both ventral arms modified and also a long penis (Nesis, 1995; Gonzalez et al., 2002). However, it is unknown whether the modifications of the ventral arms in A. dux are functional hectocotyli involved in the transfer of spermatophores (Hoving et al., 2004). It has been suggested that squid with long penes mate in a head-to-head position and transfer spermatophores directly by using the penis, as in Onykia ingens (Jackson & Jackson, 2004). The location of spermatophore attachment in squid with long penes is more variable than in squid with short penes, and can be virtually everywhere outside the body and inside the mantle (Nesis, 1995). However, it has remained unclear how such males could reach distant parts and internal areas of the female body, because mating of deep-water squid has not been described. Observations of captured and dead mature males have shown that their penes are either almost entirely concealed within the mantle with only the apical end protruding beyond the mantle margin (Jackson & Jackson, 2004), or that they significantly extend from the mantle (Jackson & Mladenov, 1994). It has been speculated that the males may somehow hydraulically ‘inject’ their spermatophores ‘under pressure’ to the arms and internal coverings of the females, as in giant squid A. dux (Norman & Lu, 1997). Here we report on a rare sighting of penis enlargement in a deep-water squid, which sheds some light on the copulating mode of these animals. A mature moribund male of the greater hooked squid Onykia ingens (Smith, 1881) (38.5 cm mantle length, 1180 g body mass) was caught on the Patagonian slope south of the Falkland Islands (July 2006, 538200S, 598310W, 1050 m depth). When the mantle of the squid was opened for maturity assessment during processing of the catch onboard, the penis of the squid, which previously had extended only slightly beyond the mantle margin, suddenly started to erect. It became rigid and quickly elongated to 67 cm total length, almost the same length as the whole body of the animal (mantle, head and arms; Fig. 1). Immediately after elongation, several spermatophores were ejaculated from the penis tip. The capacity to elongate, or to erect, the penis in this way has not previously been recorded in squid. This observation suggests how mating may occur in deep-water squid that do not have the modified arm (hectocotylus) to perform spermatophore transfer. During copulation, the male probably holds the female with its arms. The females of deep-water squid usually lack special organs for spermatophore attachment, but by means of such enormous penis elongation males could reach everywhere on the female body, including internal organs within the mantle cavity. In mature female O. ingens, spermatangia (attached spermatophores) have been found beneath the skin both inside and outside the mantle and funnel (Jackson & Jackson, 2004). In other deep-water squid the placement of the spermatophores is more restricted, e.g. near the mantle cuts in Chaunoteuthis mollis (Arkhipkin & Nigmatullin, 1997) or on the specially modified gelatinous and rugose tissue on the mantle surface of Octopoteuthis sicula (Hoving, Lipinski &Videler, et al., 2008), but within these areas precise targeting is probably not necessary. After attachment, spermatophores dissolve the outer tissues and penetrate the muscle where they remain until spawning (Hoving & Laptikhovsky, 2007). Insertion of spermatophores under the skin or through cuts made by male hooks is likely painful or unpleasant for the female (Nesis, 1995). One may assume that males need to act quickly to avoid rejection, and direct transfer of spermatophores by the penis should be faster than indirectly by means of a hectocotylus. We suggest that evolutionary development of means of spermatophore transfer in squid has followed two distinct pathways. The first is characteristic mainly of shelf and epi/mesopelagic squid, and involved the development of specialized seminal receptacles or spermatophore attachment sites (i.e. near oviducal gland openings) in females, and elaboration of modified arms (hectocotyli) in males for the targeted placement of spermatophores in these sites/receptacles. The second pathway is restricted mainly to deep-water squid, in which spermatophores can be placed on any part of the female body by the long (in the case of O. ingens erectile) penis, where they subsequently penetrate under the skin to the muscle. This copulatory