CLASSIFICATION AND IDENTIFICATION OF HADZIID AMPHIPODS
Hadziidae is a large, widespread, predominately
marine/brackish water family complex, which contains significantly more genera
with stygobionts than any other amphipod group. Many genera are exclusively
stygobiont, whereas others are epigean. A few contain both stygobionts and epigean
species. At present, there is considerable taxonomic instability in this group,
and phylogenetic relationships among many of the genera remain unresolved. The
most important genera with respect to number of stygobiont species include Metaniphargus,
Psammogammarus, Bahadzia, Hadzia
(including Liagoceradocus) and Metahadzia.
However, approximately 25 genera are monotypic and nine others contain only
two or three species. Virtually all of these genera are either exclusively stygobiont
or contain at least one or more stygobionts. Pending changes to the classification
system of the family, the large, predominately epigean, marine genus Melita
may also be assigned to the Hadziidae.
As presently understood, the hadziid family complex contains approximately 70 genera and 300 species, of which about 110 are stygobionts. The family is cosmopolitan but strongly circumtropical in distribution. Many stygobionts inhabit anchialine caves in coastal areas or inland subterranean freshwaters in areas that have been subjected to marine transgressions since the early Cretaceous.
Texiweckelia texensis Holsinger (ca. 6 mm long) from artesian well, San Marcos, Texas, USA
Because of their morphological variability and taxonomic diversity, hadziids are a bit difficult to characterize, but they can generally be distinguished by the following combination of characters:
Antennae and pereopods 5, 6 and 7 typically elongate, often fragile; length of antenna 1 and pereopod 6 or 7 sometimes exceeding that of the body;
Propodus of gnathopod 1 usually much smaller than that of gnathopod 2, sometimes mitten-form, palm short;
Propodus of gnathopod 2 proportionately elongate and usually much larger (and longer) than that of gnathopod 1, generally larger in male; carpus (segment 5) of gnathopod 2 subrectangular in shape and often as long as or longer than adjoining propodus;
Urosomites with dorsal spines (sometimes inconspicuous);
Uropod 3 well developed and distinctly biramous, rami prominent and often elongate (in comparison to those of the crangonyctids, which are proportionately much shorter);
Telson of variable length, apical margin cleft, often to base.
Line drawing of Bahadzia williamsi Holsinger (female ca. 5 mm long) from anchialine cave in the Bahamas
Line drawing of Tuluweckelia cernua Holsinger (6.8 mm female), a morphologically unique stygobiont hadziid amphipod with a remarkable bent head. This species inhabits subterranean groundwater habitats in the northern part of the Yucatan Peninsula of southern Mexico, where it is recorded from a number of cenotes.