2. THE SPECIES OF KRILL
13
identification are associated with species in other genera, for
example Thysanoessa raschii and T . inermis, but it is not intended to
give a detailed account of their histories. Instead, anyone who is
particularly interested in this aspect of the taxonomy of euphausiids
should refer to the next chapter where the main synonymy of all species
is presented.
The characteristics that are common to all euphausiids are the small
size of the exopodite of the maxilla, the similarity in structure of all the
thoracic limbs with none modified into maxillipeds, the presence of a
single series of gills attached to the exopodites of the thoracic limbs,
and the absence of a statocyst, or balancing organ. Holt and Tattersall
(1905a) considered that they formed a family, the Euphausiidae, which
could be divided into three sub-families. These were the Bentheuphausinae, containing the genus Bentheuphausia, characterized by
reduced eyes and the absence of photophores ; the Euphausinae with the
genera Euphausia, Pseudeuphausia, Thysanopoda, Nyctiphanes, and
Meganyctiphanes, characterized by the absence of a grasping foot on
any of the thoracic limbs ; and the Nematoscelinae, which included the
genera Nematoscelis, Thysanoessa, Tessarabrachion, Stylocheiron, and
Nematobrachion, all with a grasping foot on one of the thoracic limbs.
This classification would not appear to be well founded because some
species of Thysanoessa show no evidence, a t any stage of their life, of
possessing a grasping foot. Colosi (1917b) regarded the group as an
Order, the Euphausiacea, and divided it into two families, the
Bentheuphausidae, with Bentheuphausia, and the Euphausidae, which
was further subdivided into four sub-families. These were as follows :
Euphausinae
Euphausia, Pseudeuphausia
Thysanopodinae Thysanopoda, Nyctiphanes, Meganyctiphanes
Nematoscelinae
Nematoscelis, Thysanoessa, Tessarabrachion
Stylocheirinae
Stylocheiron, Nematobrachion
Zimmer and Griiner (1956) kept the same two families, but called
them Bentheuphausiidae and Thysanopodidae. They did not, however,
propose any division of the latter into sub-families. Instead they listed
the ten genera in that order which they considered best to represent
their affinities one with another, namely :
Thysanopoda
with 14 species
Meganyctiphanes
Nyctiphanes
with 4 species
Euphausia
with 30 species
Pseudeuphausia
with 2 species
with 2 species (norvegica and calmani)
13
identification are associated with species in other genera, for
example Thysanoessa raschii and T . inermis, but it is not intended to
give a detailed account of their histories. Instead, anyone who is
particularly interested in this aspect of the taxonomy of euphausiids
should refer to the next chapter where the main synonymy of all species
is presented.
The characteristics that are common to all euphausiids are the small
size of the exopodite of the maxilla, the similarity in structure of all the
thoracic limbs with none modified into maxillipeds, the presence of a
single series of gills attached to the exopodites of the thoracic limbs,
and the absence of a statocyst, or balancing organ. Holt and Tattersall
(1905a) considered that they formed a family, the Euphausiidae, which
could be divided into three sub-families. These were the Bentheuphausinae, containing the genus Bentheuphausia, characterized by
reduced eyes and the absence of photophores ; the Euphausinae with the
genera Euphausia, Pseudeuphausia, Thysanopoda, Nyctiphanes, and
Meganyctiphanes, characterized by the absence of a grasping foot on
any of the thoracic limbs ; and the Nematoscelinae, which included the
genera Nematoscelis, Thysanoessa, Tessarabrachion, Stylocheiron, and
Nematobrachion, all with a grasping foot on one of the thoracic limbs.
This classification would not appear to be well founded because some
species of Thysanoessa show no evidence, a t any stage of their life, of
possessing a grasping foot. Colosi (1917b) regarded the group as an
Order, the Euphausiacea, and divided it into two families, the
Bentheuphausidae, with Bentheuphausia, and the Euphausidae, which
was further subdivided into four sub-families. These were as follows :
Euphausinae
Euphausia, Pseudeuphausia
Thysanopodinae Thysanopoda, Nyctiphanes, Meganyctiphanes
Nematoscelinae
Nematoscelis, Thysanoessa, Tessarabrachion
Stylocheirinae
Stylocheiron, Nematobrachion
Zimmer and Griiner (1956) kept the same two families, but called
them Bentheuphausiidae and Thysanopodidae. They did not, however,
propose any division of the latter into sub-families. Instead they listed
the ten genera in that order which they considered best to represent
their affinities one with another, namely :
Thysanopoda
with 14 species
Meganyctiphanes
Nyctiphanes
with 4 species
Euphausia
with 30 species
Pseudeuphausia
with 2 species
with 2 species (norvegica and calmani)