Study site

The samples were collected from San Diego Trough, the southern-most basin in the California Continental Borderland. The study site was shaped like an equilateral triangle 500 m on each side, which was centered at 32°35.75′N and 117°29.00′W (Thistle 1978). Depth varied from 1218 to 1223 m. The sediment at the study site consisted of 3% sand, 52.7% silt, and 44.3% clay. Mean grain size of 0.004 mm. The temperature of the overlying water was 3.5°C, and the oxygen concentration was 0.71 ml per liter. Maps were obtained from Quantum GIS Geographic information System and final figure plates were prepared in Adobe Photoshop CS6.

Sample collection

The samples were taken with an Ekman-style grab modified to be used by a remotely operated vehicle. The grab contained four 10-cm by 10-cm subcores. The animals we studied came from a subcore from grab E12 and from a subcore from grab E47 (Thistle 1978). The grabs were separated by 285 m. The 0–1 cm layer of sediment and the overly water from each subcore were fixed with formalin on board. Subsequently, each sample was washed on a 0.062-mm aperture sieve, transferred to ethanol, and stained with rose bengal. The harpacticoids were removed from the sediment with the aid of a dissecting microscope.

Specimens preparation and illustrations

Observations and drawings were made from whole and dissected specimens mounted in lactophenol with an Olympus BX51 microscope equipped with a drawing tube. Permanent slides were sealed using transparent nail varnish. The morphological terminology follows Huys & Boxshall (1991).

Abbreviations

A1, antennule; Enp, endopod; exp, exopod; exp (enp)-1 (2, 3) to denote the proximal (middle, distal) segments of exopods and endopods; P1–P6 (first to sixth thoracopod). Specimens were deposited in the National Institute of Biological Resources (NIBR). This work has been registered in ZooBank with the registration number [LSID]: urn:lsid:zoobank.org:pub:4130BD 08-8A02-4188-A873-4D884E59D7B2

Type locality

San Diego Trough, Pacific Ocean, North America (32°35.75′N, 117°29.00′W Fig. 1, 1218 to 1223 m).

Fig. 1.

A–B, map with dark dots showing the type localities of both new species are collected.

Material examined

Holotype: 1 female, dissected, mounted on 3 slides (NIBRIV0000787786).

Description of female (based on holotype)

Habitus cylindrical in shape, total body length 706 μm, and width 126 μm (Fig. 2A); rostrum very small, with 2 sensilla terminally; cephalothorax (Fig. 2A) bell shaped in dorsal view, about 0.8 times as long as wide, comprising 24.4% of total body length, dorsal side with ridges, additional ornamentation consisting of few paired sensilla, posterior margin serrate and with several sensilla. Prosome 4-segmented comprising cephalothorax and 3 free pedigerous somites. Urosome comprising first urosomite, genital double-somite and 3 free urosomites. Boundary between prosome and urosome clearly visible, urosome/prosome length ratio 0.9 times (Fig. 2A). Genital double-somite fused in both dorsal and ventral view, 2.3 times as wide as long in dorsal view. Distal margins of prosomites, first urosomite and genital double somite with single row of spinules; distal margin of last urosomite without sensilla. Anal somite almost square in dorsal and triangular in lateral views, (Figs. 2A and 5B), with two dorsal sensilla, denticulated in lateral view, distal margin of operculum smooth (Figs. 2B and 5B), and not reaching to end of segment.

Fig. 2.

Mesocletodes tetrasetosus. Female, holotype. A, habitus, dorsal; B, habitus, lateral.

Antennule (Fig. 3A) 7-segmented; second, third and seventh segments subequal in length, and first segment with few spinules. Armature formula as follows: 1-[0], 2-[8], 3-[6], 4-[2+aes], 5[1], 6[2], 7[6+acrothek]. Aesthetasc located on fourth segment lost during preparation of holotype. Apical acrothek, and another setae located near acrothek broken.

Fig. 3.

Mesocletodes tetrasetosus. Female, holotype. A, antennule, dorsal; B, antenna; C, mandible; D, maxillula; E, maxilla; F, maxilliped.

Antenna (Fig. 3B) coxa short and unarmed. Basis smooth, twice longer than width. Exopod 1-segmented, twice longer than width, with 2 terminal sub equal naked setae. Endopod 2-segmented, enp-1 with 6 outer spinules, enp-2 with 2 outer and 3 inner spinules, distally with 5 pinnate setae (2 geniculated and 3 normal type) and 2 outer medial pinnate seta.

Mandible (Fig. 3C) with broad gnathobasis, single row of spinules near articulation of basis, cutting edge composed of grinding face with 1 naked seta at distal corner, and 6 major overlapping teeth. Mandibular palp 2-segmented with spinules. Basis with 1 inner bipinnate seta. Endopod 1-segmented, with 5 distal bipinnate setae. Exopod with 2 bipinnate setae.

Maxillule (Fig. 3D) with well developed praecoxal arthrite, ornamented with row of spinules, armed with 2 slender and naked surface setae and 8 stout elements with spinules. Coxal endite forming 1 strong spinular element ornamented with long spinules and with 3 long slender naked setae. Basis with 6 long slender naked setae (5 at terminal margin and 1 at sub proximal region).

Maxilla (Fig. 3E) with large syncoxa, ornamented with rows of spinules, bearing 2 endites. Proximal endite with 1 naked seta, distal endite with 1 strong pinnate claw accompanying 2 naked setae. Basis with 2 strong pinnate claws, and 1 naked seta. Endopod 1-segmented, very small, with 2 naked setae.

Maxilliped (Fig. 3F) subchelate, 3-segmented. Syncoxa slightly shorter than basis, with 1 bipinnate seta, exceeded distal margin of basis. Basis twice longer than width, with row of spinules along inner and outer margins. Endopod 1-segmented, fused to strong claw, ornamented with small spinules.

P1–P4 (Fig. 4A–D) with smooth and bow-like intercoxal sclerite (not illustrated). Coxa with 1 (P2 and P3), with 2 (P1) spinular rows on anterior surface, with 1 (P2–P4) spinular row along outer margin. Basis with 2 (P3) spinules along outer margin near insertion of exopod, with 1 (P2 and P3) or 2 (P1) rows of spinules along distal margin near insertion of endopod; outer spine (P1–P2) and seta (P3–P4) pinnate, inner pinnate spine present on P1. Exopod 3-segmented, with robust spinules along outer distal margins (P1 exp-2 to -3, P2–P4 exp-1 to -3), with row of spinules along inner margins (P1 exp-1 to -3), and row of setules along inner margins (P2 exp-1 to -2, P3–P4 exp-1 to -3), with serrate outer spines (P1–P4 exp-1 to -3); all exopodal segments with bipinnate spine (P1–P4). Endopod 2-segmented (P1–P2), or 1-segmented (P3–P4); with robust spinules along inner to outer distal margins (P1 enp-1 to -2), row of outer spinules (P2 enp-2), with 2 serrate spines and 1 pinnate seta (P1 enp-2), with 3 pinnate spines or setae (P2 enp-2), with 3 pinnate spines or setae along distal margins and 1 inner sub apical pinnate seta (P3 endopod), with 2 pinnate spines or setae and 1 naked setae (P4 endopod). Setal formula as given below:

Fig. 4.

Mesocletodes tetrasetosus. Female, holotype. A, P1; B, P2; C, P3; D, P4.

Fig. 5.

Mesocletodes tetrasetosus. Female, holotype. A. P5, anterior; B, Caudal rami (lateral view).

P5 (Fig. 5A) baseoendopod with 1 long naked seta. Endopodal lobe with 3 elements (2 long bipinnate setae and 1 naked seta). Exopod articulating with baseoendopod, rectangular in shape and about 4 longer than width, with 3 spinules along inner margin, bearing 5 elements [1 innermost seta naked, 2 distal bipinnate setae (innermost seta twice longer than outer seta), 2 outer setae (1 pinnate and 1 naked)].

P6 difficult to observe due to several dust particles.

Caudal rami (Fig. 5B) long, slender, and smooth. Almost 6.5 times as long as wide. Seta I (bipinnate) located on lateral side close to seta II (naked). Seta III (bipinnate) located medially on outer dorsal side. Seta III and VII inserted close to each other. Setae IV–VI located terminally [setae IV and V bipinnate, seta VI small naked], seta V twice as long as caudal rami. Seta VII triarticulate located on inner dorsal side.

Etymology

The specific name refers to the numbers of setae in endopod of P3.

Remarks

As explained by Menzel & George (2009) the presence of Subapical Tubulate Extensions (STEs) in P1 exp-3 outer spines are normal in Mesocletodes. However, such characteristic in M. tetrasetosus is invisible. Since this specific character is extremely small, it is difficult to make a judgment on their absence under the microscope, whether it is naturally not present or broken by physical forces. Caudal rami spinular ornamentation was difficult to observe from ventral view, due to this reason it was hard to depict in the Fig. 5B.

Type locality

San Diego Trough, Pacific Ocean, North America (32°35.75′N, 117°29.00′W; Fig. 1, 1218 to 1223 m).

Material examined

Holotype: 1 male, mounted on 1 slide (NIBRIV000 0787787). Paratypes: 2 males mounted on 2 slides (NIBRIV0000787788, NIBRI V0000787792).

Description of male (based on holotype and paratypes)

Habitus slender and dorsoventrally flattened, total body length 540 μm, and width 151 μm (Fig. 6A); rostrum very small, with 2 sensilla terminally; cephalothorax (Fig. 6A) almost rectangular in shape, about 0.78 times as long as width, comprising 35% of total body length, dorsal surface with strong ridges, additional ornamentation consisting of few paired sensilla. Prosome 4-segmented comprising cephalothorax and 3 free pedigerous somites. Urosome comprising P5-bearing somite, genital somite and 4 free abdominal somites. Boundary between prosome and urosome clearly visible, urosome/prosome length ratio 0.9 (Fig. 6A). Distal margins of first to fourth urosomites with few spinules in dorsal view (Fig. 6A). Distal margins of third, fourth and fifth urosomites with several spinules in ventral view (Fig. 7C). Anal somite with 2 pairs of dorsal sensilla (Fig. 6A), with very small operculum, with 3 long spinules raised on small projections on either side in ventral view (Fig. 7C).

Fig. 6.

Mesocletodes nudus. Male, holotype. A, habitus, dorsal; B, habitus, lateral.

Fig. 7.

Mesocletodes nudus. Male, holotype A–B, E–F, and paratype C–D. A, antennule, dorsal; B, antenna; C, genital double-somite, P6 (indicated by arrow), and abnormal caudal rami (left side); D, P6 (enlarged view); E, P5; F, Caudal rami.

Antennule (Fig. 7A) 6-segmented; second, fourth and sixth segments subequal in length; first segment with few spinules (lateral view). Armature formula as follows: 1-[0], 2-[6], 3-[5], 4-[7+aes], 5[1], 6[9 + acrothek]. Aesthetasc located on fourth segment 0.75 times as long as antennule, and robust. A second aesthetasc located on sixth segment 0.28 times as long as antennule. Fourth segment with 3 unique pinnate setae (over all leaf like shaped, arrowed in Fig. 7A).

Antenna (Fig. 7B) coxa short and unarmed. Basis smooth, twice longer than width. Endopod 2-segmented, enp-1 with 5 outer spinules, enp-2 with 4 outer and 4 spinules distally, distal margin with 2 pinnate and 2 naked setae, and 1 medial pinnate seta.

Mouthparts are atrophied, could not be traced due to their strong reduction.

P1–P4 (Fig. 8A–D) with smooth and bow-like intercoxal sclerite (not illustrated). Surface ornamentations of coxa and basis unclear. Basis with outer bipinnate setae (P1–P4), and inner small naked spine present on P1. Exopod 3-segmented, with few spinules along outer margins (P1–P4 exp-1 to -3, P4 exp-1), with row of setules along inner margins (P1–P3 exp-1), with serrate outer spines (P1–P4 exp-1 to -3); with naked setae (P1 exp-3), and bipinnate setae (P2–P4 exp-1 to -3). Endopod 2-segmented (P1–P4), with spinules along outer margins (P1–P4 enp-1 to -2), with several setules along inner margin (P1–P3 enp-1 and P1 enp-2), with bipinnate setae (P1–P4 enp-2). Setal formula as follows:

Fig. 8.

Mesocletodes nudus. Male, holotype. A, P1; B, P2; C, P3; D, P4.

P5 (Fig. 7E) baseoendopod with 1 long pinnate seta. Endopodal lobe with 2 naked setae (1 long and 1 small). Exopod articulating with baseoendopod, rectangular in shape and about twice longer than width, without surface ornamentation, bearing 2 distal long naked setae, and 1 small seta fused to segment located on distal margin just above 2 long naked setae.

P6 (Fig. 7C–D) integrated into genital field, reduced, and with 1 very small naked seta.

Caudal rami (Fig. 7F) long, slender, without surface ornamentation, almost 11 times as long as wide. Seta I (naked) located on dorsal side, and inserted close to seta II. Seta II (naked) located on dorsal side. Seta III (naked) located on dorsal side subterminally. Caudal ramus extremely elongated between setae III and VII. Setae IV–VI located terminally [seta IV (broken), seta V (0.7 times as long as caudal rami and bipinnate), seta VI (very small and naked)]. Seta VII triarticulate located on dorsal side. Abnormal caudal rami described based on paratype (see Fig. 7C).

Etymology

The specific name refers to the presence of only bare setae in P5 exopod and endopod.

Remarks

The presence of Subapical Tubulate Extensions (STEs) in P1 exp-3 outer spines of M. nudus cannot be traced. Caudal rami surface ornamentation could not be observed.