Biacanthus, Tang, Danny & Izawa, Kunihiko, 2005

Tang, Danny & Izawa, Kunihiko, 2005, Biacanthus pleuronichthydis (Yamaguti, 1939) gen. n., comb. n. (Copepoda: Taeniacanthidae), an ectoparasite of flatfishes from Japanese waters, Zootaxa 1071, pp. 47-60 : 56-59

Biacanthus gen. n.

Diagnosis. Female: Cephalothorax formed from fusion of cephalosome with first pedigerous somite. Abdomen 4­segmented. Caudal ramus with 7 setae. Rostral area protuberant with horseshoe­shaped structure on ventral surface. Antennule 7­segmented. Antenna 3­segmented. Uncinate process posterior to antennule base present. Postantennal process present. Labrum spinulated on posterior margin. Mandible with 2 spinulated blades. Paragnath present. Maxillule lobate, bearing 5 setae. Maxilla armed with 1 terminal spinulated process and 2 spinulated setae. Maxilliped 3­segmented, last segment with sigmoid claw bearing an accessory tooth. Legs 1–4 biramous. Leg 1 exopod trimerous and endopod bimerous; legs 2–4 trimerous. Inner coxal seta present on legs 2 and 3. Terminal segment of leg 4 endopod with 4 elements. Leg 5 with 3 spines and 1 seta on second segment. Leg 6 vestigial, represented by 3 setae.

Male: Body tagmosis similar to that in female except with 3­segmented abdomen. Maxilliped 4­segmented; second segment ornamented with denticles; last segment forming long recurved claw, denticulated along inner margin. Leg 6 absent.

Type species. Biacanthus pleuronichthydis ( Yamaguti, 1939) comb. n.

Etymology. The generic name is a composite of the Latin bi (= two) and acanthus (= a common suffix in the Taeniacanthidae , meaning spine), alluding to the powerful uncinate process posterior to the antennule bases.

Morphologic fea­ Yamaguti (1939) Izawa (1986) Present Study ture

Ψ Anal somite Naked Naked Ornamented

Antennule segmen­ 7­segmented 6­segmented 7­segmented tation

Antennule arma­ 5, 13, 4, 3, 4, 3, 8 5, 14, 8, 2, 2, 7 5, 15, 5, 3, 4, 2+aes, ture formula 7+aes

Maxillule 4 setae 5 setae 5 setae

Maxilla, terminal 3 elements 2 elements 3 elements segment

Maxilliped claw 1 basal seta 1 basal seta 2 basal setae

Leg 1 exopod 3­segmented 2­segmented 3­segmented

Legs 2–4 rami Spinules absent Spinules absent Spinules present

Leg 4 endopod, 1st Armed with Armed with inner Armed with intermedi­ and 2nd segments inner seta seta ate spine

Leg 5, first segment Naked Naked Ornamented

ɗ Antennule arma­ ­ 5, 16, 4, 4, 4, 5, 15, 5, 3, 4, 2+aes, ture formula 2+aes, 7+aes 7+aes

Maxilliped, 2nd ­ Spinule patch Spinule patch present segment (posterior) absent

Maxilliped claw ­ 1 seta 2 setae (anterior)

Character B. pleuronich­ Irodes Phagus Pseudotae­ Scolecicara

thydis niacanthus

Remarks. The presence of a horseshoe­shaped, sclerotised structure on the ventral surface of the rostrum and a robust uncinate process posterior to each antennule base are the two most distinctive characters of adult B. pleuronichthydis . According to Izawa (1986), the ventral side of the rostrum of B. pleuronichthydis undergoes considerable morphologic changes throughout the copepodite stages. For instance, a large median hook develops in the first copepodite, but is absent in the next copepodite stage. In the third to fifth male copepodites, as well as the third and fourth female copepodites, the rostrum bears a small anteroventral sclerotised projection. The projection is lost and the horseshoeshaped structure develops in the adult stages.

In contrast to the rostral area, the uncinate processes are present in all copepodite stages, except for the first copepodite ( Izawa, 1986). It should be noted that male Taeniacanthodes haakeri Ho, 1972 and female Anchistrotos caligiformis ( Gurney, 1927) have one and two pairs, respectively, of small spiniform processes situated posterior to the antennule. However, these structures are not homologous with the uncinate processes of B. pleuronichthydis .