Evolution of the Tribe Electronini (Myctophifonnes, Myctophidae)
81
monophyletic subgenus Hierops which occur outside the Southern Ocean
region appear to have evolved along two separate lines. Each possesses
Convergence or South Temperate representatives. Protomyctophum
thompsoni (subArctic: North Pacific) and P. arcticum (Boreoarctic: North
Atlantic) are grouped with the Convergence / HolosubAntarctic species P.
subparallelum and P. parallelum. Protomyctophum beckeri (North
Central: Pacific) and P. crockeri (North Temperate: North Pacific) are
grouped with South Temperate P. chilensis. Greater resolution of these
relationships is required. It is possible that the history of these species
includes both a northern dispersion and subsequent allopatric speciation.
Such a scenario may have occurred more than once, as is indicated by the
later evolution and subsequent northern distribution of E. risso (Fig. 3).
The phylogeny of species of the subgenus Protomyctophum also
requires greater resolution. Their phylogeny, and those of Electrona (with
the exception of E. antarctica) and Metelectrona, appear to be associated
with an evolution from ancestral SubAntarctic and Convergence Pattern
species in association with possible vicariance events. The multiplicity of
HolosubAntarctic and Convergence Pattern pairs suggests that such events
may have also been operational on more than one occasion. The evolution
of the only Antarctic Pattern species, E. antarctica, appears late in the
phylogeny (Fig. 3). A proposed model for the evolution of the tribe (Fig.
4) suggests the following possible stages:
Stage 1: The opening of the Drake Passage to deep water, together with
the northern migration of Australia during the Early Miocene (22 Ma) led
to the development of the Circumpolar Antarctic Current and the STC,
and to the restriction of Tethys circulation [7,19]. In association with
continuing climatic cooling from the beginning of the Oligocene, this
resulted in the commencement of gyral decoupling and the subsequent
isolation of those Tethyan (Panthalassian) species living in high southern
latitudes. The derivatives of these Austral species (sensu White [8]) are
represented by lanternfish species which have a Broadly Antarctic Pattern
(sensu Hulley [14]). In the Electronini such species include K. anderssoni,
P. tenisoni and P. bolini. Recent unpublished but fragmentary data suggest
that P. choriodon may have to be reassigned to this pattern type.
Stage 2: During the Middle Miocene (14-12 Ma), continued gyral decoupling and the formation of the Antarctic ice sheets were evident [7).
Sinking of the Iceland Ridge below sea level created fundamental changes
in deep water circulation [19]. In the Southern Ocean, these changes led to
development of a system of hydrographic fronts and consequent
partitioning of the environment. Isolation of the populations gave rise to
81
monophyletic subgenus Hierops which occur outside the Southern Ocean
region appear to have evolved along two separate lines. Each possesses
Convergence or South Temperate representatives. Protomyctophum
thompsoni (subArctic: North Pacific) and P. arcticum (Boreoarctic: North
Atlantic) are grouped with the Convergence / HolosubAntarctic species P.
subparallelum and P. parallelum. Protomyctophum beckeri (North
Central: Pacific) and P. crockeri (North Temperate: North Pacific) are
grouped with South Temperate P. chilensis. Greater resolution of these
relationships is required. It is possible that the history of these species
includes both a northern dispersion and subsequent allopatric speciation.
Such a scenario may have occurred more than once, as is indicated by the
later evolution and subsequent northern distribution of E. risso (Fig. 3).
The phylogeny of species of the subgenus Protomyctophum also
requires greater resolution. Their phylogeny, and those of Electrona (with
the exception of E. antarctica) and Metelectrona, appear to be associated
with an evolution from ancestral SubAntarctic and Convergence Pattern
species in association with possible vicariance events. The multiplicity of
HolosubAntarctic and Convergence Pattern pairs suggests that such events
may have also been operational on more than one occasion. The evolution
of the only Antarctic Pattern species, E. antarctica, appears late in the
phylogeny (Fig. 3). A proposed model for the evolution of the tribe (Fig.
4) suggests the following possible stages:
Stage 1: The opening of the Drake Passage to deep water, together with
the northern migration of Australia during the Early Miocene (22 Ma) led
to the development of the Circumpolar Antarctic Current and the STC,
and to the restriction of Tethys circulation [7,19]. In association with
continuing climatic cooling from the beginning of the Oligocene, this
resulted in the commencement of gyral decoupling and the subsequent
isolation of those Tethyan (Panthalassian) species living in high southern
latitudes. The derivatives of these Austral species (sensu White [8]) are
represented by lanternfish species which have a Broadly Antarctic Pattern
(sensu Hulley [14]). In the Electronini such species include K. anderssoni,
P. tenisoni and P. bolini. Recent unpublished but fragmentary data suggest
that P. choriodon may have to be reassigned to this pattern type.
Stage 2: During the Middle Miocene (14-12 Ma), continued gyral decoupling and the formation of the Antarctic ice sheets were evident [7).
Sinking of the Iceland Ridge below sea level created fundamental changes
in deep water circulation [19]. In the Southern Ocean, these changes led to
development of a system of hydrographic fronts and consequent
partitioning of the environment. Isolation of the populations gave rise to