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Term Project: Willa Neilsen and Emily Palmer

Note: We added to an existing page that included; the first paragraph, before Taxonomy, and the species description box. All sub-titled sections were added by us.

The southern lanternshark (Etmopterus granulosus) is a shark of the family Etmopteridae found in the southeast Pacific between latitudes 29°S and 59°S, at depths of between 220 and 1,460 m. This species has been found off Northland, off the Chatham Islands, on the Campbell Plateau, all in New Zealand waters. Its length is up to 60 cm. Reproduction is ovoviviparous, with 10 to 13 pups in a litter, length at birth about 18 cm. They exhibit bioluminescence.

Parasites of the southern lanternshark, studied off Chile, include Monogeneans, Digeneans, Cestodes, Nematodes, and Copepodes.

In June 2018 the New Zealand Department of Conservation classified E. granulosus as "Not Threatened" with the qualifier "Secure Overseas" under the New Zealand Threat Classification System.

Taxonomy
The southern lanternshark (Etmopterus granulosus) and the giant lanternshark (Etmopterus baxteri) are now considered conspecifics thanks to DNA sequencing. They had previously been considered separate species based on the assumption that E. granulosus was confined to the regions surrounding South America while E. baxteri was thought to occupy a range around New Zealand and Australia.

This is a relatively large species of lanternshark, which is part of the Etmopterus spinax clade of the family Etmopteridae. Etmopteridae is one of only three families containing species of bioluminescent sharks, Etmopteridae, Dalatiidae, and Somniosidae.

Distribution
Although patchy, the distribution of the Southern Lanternshark is widespread throughout the Southern Hemisphere. They are thought to have a circum-Antarctic distribution. Populations have been reported from the Southwest (Argentina, Falkland Islands) and Southeast Atlantic Ocean (South Africa, Angola), Western (southern Madagascar Ridge, Amsterdam Island) and Eastern (southeast Australia) Indian Ocean, and Southwest (Tasman Sea, New Zealand) and Southeast (Chile) Pacific Ocean.

Actual population sizes and trends for this species are unknown in most of its range, with most information coming from New Zealand and Australia.

Reproduction
The Southern Lanternshark has been found to reach maximum lengths of 88cm, however size at maturity varies according to region. Generally, males mature at 46-68 cm total length, and females mature at 62-69 cm total length. Age at maturity has been estimated as 20 and 30 years for males and females, respectively.

Aspects of the life history of E. granulosus could imply that mate availability is a limiting factor to this species reproduction. Monogamy is common in populations, with a female skewed sex ratio and segregation by size class and sex being contributing factors by limiting access to mates. Another factor that may contribute to monogamy among populations of the Southern Lanternshark may be asynchronous reproduction. It may be difficult to come across a viable mate if the timing and location of mating in this species is unpredictable.

This unpredictable reproductive cycle may be related to the consistent environment of the deep sea, as they do not see the same seasonal changes that species inhabiting surface waters experience. This irregularity in reproductive patterns may be beneficial due to the fact that larger females of this species do not have the best fecundity. The absence of a specific mating and reproductive season allows individuals to do so year-round. It also means that regular fishing of this species will not result in a reduction in population sizes.

E. granulosus, although not verified, is generally classified as having lecithotrophic viviparous, as well as asynchronous, reproduction, and a confirmed reproductive cycle has not been established.

Feeding
The diet of the southern lanternshark has not been studied in detail. In New Zealand and Australia, it has been reported that their diet consists mostly of fish, including orange roughy and squid. More specifically, the diet of the southern lanternshark is made up mostly of teleosts. Cephalopod prey is also important and crustaceans are a minor component.

The importance of teleost prey in the diet seems to increase with the total length of the individual, while crustaceans are more important in the diet of smaller individuals.

Light Generation and Control:
** An image of E. granulosus bioluminescent pattern and photophore structure can be found in Figure 2, panel E of Mallefet et al. (2021). Note: the image could not be uploaded because of copy-write laws **

Etmopterids, such as E. granulosus, have specialized cells called photophores which produce a blue coloured bioluminescent light, rather than harbouring bioluminescent symbionts. However, the mechanisms of bioluminescence among elasmobranchs, including Etmopterids, is still poorly understood although recent studies (as of 2022) have begun to identify some compounds which influence the bioluminescence of Etmopterids. In a controlled study, tissue samples from three Etmopteridae species produced a bioluminescent reaction when exposed to the luciferin coelenterazine, which suggests that the coelenterazine luciferin-luciferase system is a mechanism of bioluminescence among Etmopterids. However, some species (E. spinax) tissues have not reacted to coelenterazine and it is still unclear if the Etmopteridae species that did react produce coelenterazine metabolically or obtain it from their diet.

The bioluminescence of Etmopterids also appears to be at least partially hormone controlled. Tissue samples from E. granulosus displayed an increase in the intensity of the bioluminescent light they produced when exposed to the hormone melatonin at a concentration of 10-6 mol/L. The light intensity produced by E. granulosus photophores decreased when exposed to a 10-6 mol/L treatment of alpha-melanocyte-stimulating hormone.

Photophore Structure and Distribution:
Etmopterids tend too have large and complex photophores although the exact structure may differ among species. E. granulosus’ photophores appear typical of Etmopterids; having three photocytes positioned underneath an iris-like structure, which is cupped by a pigmented sheath, all underneath a single lens cell. The photophores are part of the epidermis and are distributed amongst placoid scales. The highest concentration of photophores were on the ventral side, as well as below the pectoral fins and along the bottom side of the tail, however their distribution is heterogeneous and creates a species specific ‘pattern’ of bioluminescence.

Functional Uses:
The highest concentration of photophores are on the ventral side of E. granulosus which indicated that their bioluminescence functions primarily in camouflage through counter-illumination. This is a common use of bioluminescence in the twilight zone as there are no structures to hide from predators, counter-illumination ‘masks’ the shark's shadow so it is difficult for larger predators below to see them.

The species within the Etmopteridae family have undergone extensive sympatric speciation and the observations of species specific bioluminescent patterns, have led to the hypothesis that these patterns, often referred to as flank markings, are used for conspecific recognition and have contributed to the reproductive isolation of species.