Four New Species of Zosimeidae (Copepoda: Harpacticoida) from the Southwestern Gulf of Mexico

As a part of ongoing efforts for monitoring benthic ecosystem in the Gulf of Mexico, Harpacticoid copepods were collected from the southwestern Gulf of Mexico. Among them we report three new species of Zosime, and a new species of Peresime for the study area. Zosime is the most specious-rich genus in the family. Three species of Zosime were morphologically similar to Z. paratypica Becker and Schriever, 1979, Z. atlantica Bodin, 1968, and Z. destituta Kim J.G., Jung and Yoon, 2016, respectively. However, all three species have unique characteristics that distinguish them from similar species such as setal formulae and shape of female P5 and caudal rami. The new species of Peresime has similar morphological characters with P. reducta (Becker and Schriever, 1979). These two species can be distinguished by differences in mouth parts and the length of the setae on P2. This is the first report on the genera Zosime and Peresime from the Gulf of Mexico. We also discuss the global diversity and distribution of Zosimeidae and provide a key to the genera of the family and species of each genus, including the four new species from the Gulf of Mexico.


Introduction
The Gulf of Mexico has undergone two major oil spills [1] and has been the subject of several environmental studies focused on planktonic and benthic organisms [2][3][4][5][6].
The number of harpacticoid copepods reported in the Gulf of Mexico is small, with only 71 species listed [4]. However, a total of 696 harpacticoid species were identified during a survey in the northern Gulf of Mexico [3] implying they have a high diversity in the deep sea. Species of Zosimeidae are dominant in the harpacticoid community of the northern Gulf of Mexico. Zosimeidae are responsible for 32.98% of contribution of harpacticoid families to total harpacticoid abundance [3]. Although Baguley et al. [3] reported that "Tisbidae" has the highest contribution in the northern Gulf of Mexico, most of the species in their study actually belong to Zosimeidae (personal observation by W.L.).
The family Zosimeidae is a small family harboring five genera, Zosime Boeck, 1873, Peresime Dinet, 1974, Pseudozosime Scott T., 1912, Acritozosime Kim, JG and J Lee, 2021, and Heterozosime Kim, JG and J Lee, 2021. Currently each genus consists of 23, 2, 1, 1, 1 valid species, respectively. Koller and George [7] summarized the distribution of 15 species of Zosime, and Kim et al. [8] provided a key to the species of the genus known in year 2016. In this study, we provide a key to the genera of the family and to the species of each genus, along with the descriptions of four new species of two genera. In addition, the contents related to molecular research and distribution of Zosimeidae, and the morphological features of species within Zosime were summarized (Table S1). This is the first report on the genera Zosime and Peresime from the Gulf of Mexico.

Morphological Examination
Specimens were dissected in lactic acid and dissected parts were mounted on slides in lactophenol as a mounting medium. Preparations were sealed with transparent nail varnish. All drawings were prepared using a drawing tube attached to a Zeiss Axioskop phase-interference compound microscope and Olympus BX51 differential interference contrast microscope.
The descriptive terminology of Huys et al. [10] was adopted. Abbreviations used in the text are as follows: A1, antennule; A2, antenna; ae, aesthetasc; exp, exopod; enp, endopod; P1-P6, first to sixth thoracopod; exp (enp)-1 (2,3) to denote the proximal (middle, distal) segment of a three-segmented ramus. Specimens were deposited in the National Marine Biodiversity Institute of Korea (MABIK). Scale bars in figures are in µm. Sediments were sampled with a multiple corer. Three replicate cores with an inner diameter of 9.5 cm were collected at each station. The sediment samples were fixed with 7% formalin buffered with Borax©. Copepods were extracted from the sediment samples by sieving the samples using 45 µm mesh sieve and then removed by hand and preserved in 70% ethanol.

Morphological Examination
Specimens were dissected in lactic acid and dissected parts were mounted on slides in lactophenol as a mounting medium. Preparations were sealed with transparent nail varnish. All drawings were prepared using a drawing tube attached to a Zeiss Axioskop phase-interference compound microscope and Olympus BX51 differential interference contrast microscope.

Phylogenetic Analysis
The molecular analysis involved 21 nucleotide sequences. All sequences were downloaded from GenBank (Table S2). Obtained sequences were aligned by the ClustalW algorithm [11] in MEGA version 7.0 [12]. Phylogenetic analyses were performed using Maximum Likelihood (ML) approaches. ML analysis used the K2 (Kimura 2-parameter Diversity 2022, 14,198 3 of 29 model, [13]) +G+I model based on the model test result in MEGA. A discrete Gamma distribution was used to model evolutionary rate differences among sites (5 categories (+G, parameter = 0.5681)). The rate variation model allowed for some sites to be evolutionarily invariable ((+I), 38.47% sites). One thousand bootstrap replicates were performed to obtain a relative measure of node support for the resulting trees. Tree was rooted with Tigriopus (Harpacticidae) sequences.  Differential Diagnosis. Rostrum triangular, slightly pointed apically. Each somite armed with serrated posterior margin. Antennule seven-segmented in both sexes. Genital somite and succeeding urosomal segments with lateral expansions in the female. P3-and P4-bearing somites with lateral expansion on the male. Caudal rami three times longer than wide in both sexes. Female P5 with six setae, without seta on exp surface. Male P2  and two naked setae; distal endite with stout serrate spine and two pinnate setae. Allobasis produced into strong curved claw, and pinnate curved spine with short slender seta basally; accessory armature consisting of three slender lateral setae proximally, and close to base of endopod. Endopod one-segmented with five slender distal setae.  Differential Diagnosis. Rostrum triangular, slightly pointed apically. Each somite armed with serrated posterior margin. Antennule seven-segmented in both sexes. Genital somite and succeeding urosomal segments with lateral expansions in the female. P3-and P4bearing somites with lateral expansion on the male. Caudal rami three times longer than wide in both sexes. Female P5 with six setae, without seta on exp surface. Male P2 endopod two-segmented, and enp-2 modified into hook shaped apophysis with two pinnate setae. Maxilliped ( Figure 4D). Syncoxa elongate and cylindrical without ornamentations. Basis with row of outer spinules distally, with pinnate seta distally. Endopod small with two long, sparsely pinnate seta (one seta broken in Figure 4D); accessory armature consisting of two naked setae.   Description of Female. Total body length of holotype 535 µm (measured from anterior margin of rostrum to posterior margin of caudal rami). Largest width (175 µm) measured at posterior margin of cephalic shield. Urosome distinctly narrower than prosome (Figure 2A).

Subclass
Cephalothorax triangular with serrate posterior margin; dorsal surface smooth with few sensilla posteriorly. Prosomites (Figure 2A) with smooth dorsal surface, serrated posterior margins, and few sensilla posteriorly as figured.
Urosomites (Figure 2A) with serrated posterior margin as illustrated. Dorsal surface and posterior margin of P5-bearing somite ornamented as preceding somites. Genital doublesomite with smooth dorsal surface. Genital somite and succeeding urosomal segments produced laterally. Each urosomite with several posterior sensilla as illustrated (Figure 2A).
Caudal rami (Figure 2A,B) twice longer than anal somite, and three times longer than wide; setae I and II arising laterally halfway along outer margin, seta II longer than seta I; seta III longer than setae II; seta IV basally fused to seta V; seta V longest, much longer than all urosomites combined; seta VI as long as seta I and located on distal inner corner; seta VII bare, located dorsally, triarticulated.
Antenna ( Figure 3B) four-segmented, comprising coxa (not figured), basis, and free two-segmented endopod. Basis with several rows of spinules along inner margin, with pinnate abexopodal seta subdistally. First endopodal segment with a subdistal seta missed during dissection and sample preparation (the scar indicates the position of a seta); segment two with row of stout spinules on apical margin; apical armature consisting of two stout pinnate spines and naked spine, slender distal pinnate seta and two distal geniculate spines (damaged during dissection); and two inner lateral pinnate spines. Exopod threesegmented, with one, one, and three setae, respectively; all setae and spines pinnate; second segment shortest; first segment twice as long as second; last segment much longer than preceding two segments combined, with one lateral and two apical pinnate setae.
Mandible ( Figure 4A) well-developed, gnathobasis with three teeth and pinnate seta at distal corner. Basis with three pinnate setae. Exopod one-segmented with one lateral and two apical pinnate setae. Endopod one-segmented, with one lateral and three apical pinnate setae.
Maxillule ( Figure 4B) with praecoxa without spinular ornamentation along outer lateral margin. Arthrite strongly developed, with two surface setae (one naked, one pinnate), seven elements around distal margin. Coxa with cylindrical endite bearing four setae apically, and epipodite on outer lateral margin. Basis with three pinnate and three naked setae. Endopod one-segmented with one pinnate, and five naked setae. Exopod one-segmented, smaller than endopod with three pinnate setae (inner most one missing in Figure 4B). Maxilla ( Figure 4C). Syncoxa with spinular row on anterior surface and three endites; proximal endite bilobate, proximal lobe with pinnate spine, pinnate seta, and naked seta, and distal lobate with one naked and two pinnate setae; middle endite with pinnate spine and two naked setae; distal endite with stout serrate spine and two pinnate setae. Allobasis produced into strong curved claw, and pinnate curved spine with short slender seta basally; accessory armature consisting of three slender lateral setae proximally, and close to base of endopod. Endopod one-segmented with five slender distal setae. Maxilliped ( Figure 4D). Syncoxa elongate and cylindrical without ornamentations. Basis with row of outer spinules distally, with pinnate seta distally. Endopod small with two long, sparsely pinnate seta (one seta broken in Figure 4D); accessory armature consisting of two naked setae.
P4 ( Figure 5D). Coxa trapezoid, distal margin longer than proximal margin. Basis with row of long spinules along inner distal margin, and with outer pinnate seta, and several spinules along outer margin. Exopod longer than endopod; all exopod segments with row of inner spinules; exp-3 longest, and exp-2 shortest. Each endopod segments with row of spinules along outer and inner lateral margins; all segments subequal in length; enp-3 with well-developed an apical tube pore between two apical setae; endopod reached to proximal region of exp-3. Armature formulae as in Table 1.  Figure 5E). Outer basal seta long and pinnate set on cylindrical setophore. Exopod fused to baseoendopod. Endopodal lobe trapezoid with pore near apical region, and two long pinnate apical setae of which the inner is longer than the outer; with few scattered anterior spinules along outer lateral margin. Exopod rectangular, slightly longer than endopodal lobe, with two lateral and two apical pinnate setae; outer proximal seta shortest, innermost longest; with some anterior spinules near inner margin. P6 ( Figure 5F) represented by single plate bearing three naked setae of which the outer is the longest, and the middle is the shortest. Copulatory pore large, crescentic, and located at slightly distal region from median line of genital double somite.
Description of Male. Total body length 450 µm (measured from anterior margin of rostrum to posterior margin of caudal rami). Largest width (150 µm) measured at posterior margin of cephalic shield. Body surface smooth without ornamentations ( Figure 2D). Each body somites with serrated posterior margin, and few sensillae as in female. Cephalothorax slightly depressed with parallel lateral margins. Rostrum bell-shaped with pointed apical margin, Diversity 2022, 14, 198 9 of 29 and pair of sensilla as in female ( Figure 2D). P3-and P4-bearing somites slightly depressed showing lateral expansion as in Figure 2D. Pseudoperculum well developed and covering anal somite as in female. Caudal rami 1.5 times longer than anal somite, and three times longer than wide. Sexual dimorphism expressed in A1, P2, P5, P6, and segmentation of urosome.
P2 endopod ( Figure 5G) two-segmented, with enp-2 with hook-shaped apophysis and with 2 pinnate setae. P5 ( Figure 5H) with baseoendopod forming a shallow lobe with two small pinnate setae fused to exopod, and with pore present between baseoendopod and exopod. Exopod fused to baseoendopod forming a rectangular lobe with inner seta and two apical pinnate setae. Single surface seta isolated along outer side.
P6 (not figured) represented on both sides by ventral plate close to posterior margin of somite; each plate bearing one naked and two apical spines.
Remarks. Based on the keys to species of Zosime [8,14], Z. montagnai sp. nov. is morphologically similar to Z. paratypica Becker and Schriever, 1979. These species share the morphological characters of female A1 seven-segmented P1 enp-2 with four setae, P3 enp distal segment with four setae, P2-P4 exp-3 with seven setae, and length:width ratio of caudal rami. However, Z. montagnai sp. nov. and Z. paratypica can be distinguished from each other by the combination of the following morphological characteristics. Z. montagnai sp. nov. has less setae on A2 exp-3 (three setae in the new species), P4 enp distal segment, and female P5. Z. paratypica has one more seta, respectively. In addition, the length:width ratio of the second antennulary segment in the female of Z. montagnai sp. nov. is bigger than in Z. paratypica. Considering most of the morphological differences in terms of oligomerization, Z. montagnai sp. nov. is deemed to be comparatively more derived than Z. paratypica.
Etymology. The species was named in honor to Dr. David Thistle (Florida State University) who first introduced the world of Copepods to Melissa Rohal Lupher. It is a noun in the genitive case, gender masculine.
Differential Diagnosis. Rostrum bell-shaped, with round apical margin and A1 sevensegmented in both sexes. Caudal rami slightly longer than wide. Female P5 with seven setae, without seta on exp surface. Male P2 endopod three-segmented, enp-1 and enp-2 incompletely separated, each with inner pinnate seta, and enp-3 with hook-shaped apophysis and with one pinnate seta.
Description of Female. Total body length of holotype 328 µm (measured from anterior margin of rostrum to posterior margin of caudal rami). Largest width (184 µm) measured at posterior margin of cephalic shield. Urosome distinctly narrower than prosome ( Figure 6A).
Cephalothorax trapezoid with serrate posterior margin; dorsal surface smooth with few sensilla posteriorly. Prosomites ( Figure 6A) with smooth dorsal surface, serrated posterior margins and few sensilla posteriorly as figured. Each somite with pointed lateral posterior end.
Rostrum fused to cephalothorax, with rounded and shallow apical margin, bell-shaped, and round at apical margin and bearing two sensilla ( Figure 6A). Urosomites ( Figure 6A) with serrated posterior margin as illustrated. Dorsal surface and posterior margin of P5 bearing somite ornamented as preceding somites. Genital double-somite with smooth dorsal surface. Posterior margins of each urosomite finely serrated. Each urosomite with several posterior sensilla as illustrated.
Anterior half of anal somite ( Figure 6A) covered with well-developed pseudoperculum, unornamented except for few spinules along lateral distal corner and median distal margin.
Caudal rami (Figures 6A and 7C) as long as anal somite, and slightly longer than wide; setae I and II arising laterally halfway along outer margin, seta I smallest; seta III as long as seta II; seta IV basally fused to seta V; seta V longest and slightly longer than all urosomites combined; seta VI shorter than seta III and located on distal inner corner; seta VII bare, located dorsally, triarticulated.
Cephalothorax trapezoid with serrate posterior margin; dorsal surface smooth with few sensilla posteriorly. Prosomites ( Figure 6A) with smooth dorsal surface, serrated pos- Antenna ( Figure 7B) four-segmented, comprising coxa (not figured), basis, and free two-segmented endopod. Basis with row of long inner spinules, and with subdistal abexopodal seta. Endopodal segment-1 naked with no surface ornamentations; enp-2 with row of inner spinules proximally; apical armature consisting of stout pinnate spine and three geniculate spines, and strong pinnate innermost spine basally fused to laterally with short pinnate seta; short pinnate spine and two pinnate stout spines. Exopod threesegmented, with one, one, and four setae, respectively; exp-1 and exp-2 with pinnate spines; second segment shortest; first segment twice as long as second; last segment much longer than preceding two segments combined, with lateral spine and three apical pinnate spines. Anterior half of anal somite ( Figure 6A) covered with well-developed pseudoperculum, unornamented except for few spinules along lateral distal corner and median distal margin.
Caudal rami (Figures 6A and 7C) as long as anal somite, and slightly longer than wide; setae I and II arising laterally halfway along outer margin, seta I smallest; seta III as  Mandible, maxillule, maxilla, maxilliped as in Z. montagnai with no significant discrepancies. Swimming legs 1-4 ( Figure 8A-D) biramous, with three-segmented exopods, P1 endopod two-segmented, P2-P4 endopods three-segmented. Coxae and bases with anterior rows of surface spinules as illustrated. Praecoxae triangular. Coxa rectangular. P1 ( Figure 8A). Basis with strong pinnate inner seta and with spinules along inner margin, and with pinnate outer seta and several spinules along outer margin; with row of spinules near endopod, and on median anterior surface. Exopod three-segmented; exp-1 and exp-2 with pinnate outer spine, respectively; exp-2 with inner pinnate seta; exp-3 with three pinnate outer spines, two pinnate distal setae and inner pinnate seta. Endopod two-segmented; enp-1 unarmed and ornamented as shown, enp-2 slightly longer than enp-1, and with inner seta and two apical spines of which the inner apical spine is two times longer than the outer one.
P4 ( Figure 8D). Basis with row of long inner spinules, and with bare outer seta. Exopod longer than endopod; exp-1 and exp-2 with row of inner spinules; exp-1 longest, and exp-2 shortest. Each endopodal segments with row of outer spinules; enp-1 slightly longer than other two segments; enp-2 and enp-3 subequal in length; endopod not reaching to distal margin of exp-2. Armature formula as in Table 2.  Figure 8E). Outer basal seta long and pinnate, set on cylindrical setophore bearing few spinules on anterior surface. Exopod fused to baseoendopod. Endopodal lobe forming a shallow lobe with four long pinnate setae, outermost one shortest, outer middle one longest, and the other two inner ones subequal in length; with rows of outer spinules anteriorly. Exopod also forming a shallow lobe with three pinnate setae, of which the outer is the shortest, and the innermost is the longest.
P6 damaged (not figured). Description of Male. Total body length 329 µm (measured from anterior margin of rostrum to posterior margin of caudal rami). Largest width (132 µm) measured at posterior margin of cephalic shield. Body surface smooth, without ornamentations. Each body somites with serrated posterior margin, and few sensillae as in female. Cephalothorax as long as wide including rostrum. Rostrum more prominent than in female, bell-shaped with round apical margin, and pair of sensilla as in female ( Figure 6B). Pseudoperculum well-developed and covering proximal half of anal somite as in female. Caudal rami as long as anal somite, and slightly longer than wide. Sexual dimorphism expressed in A1, P2, P5, P6, and segmentation of urosome.
Remarks. Based on keys to the species of Zosime [8,14], Z. thistlei sp. nov. is morphologically similar to Z. atlantica Bodin, 1968. Z. thistlei sp. nov. and Z. atlantica share the characters of female A1 seven-segmented, P1 enp-2 with three setae, P4 enp-3 with four setae, P2-P4 exp-3 with seven setae. However, Z. thistlei sp. nov. and Z. atlantica can be distinguished from each other by the combination of the following morphological characteristics: (1) Z. thistlei sp. nov. has less setae on female P5. Z. atlantica has one more seta, (2) The cleft between the baseoendopod and exopod of the female P5 is less pronounced in Z. thistlei than in Z. atlantica, (3) armature of the A2 exp-3 (with seta in Z. thistlei, unarmed in Z. atlantica), (4) length of distal segments of P4 enp and exp are similar with mid-segments, but, those of Z. atlantica are longer than mid-segments, (5) length/width ratio of the caudal rami (2 in Z. thistlei, but 3 in Z. atlantica).
Zosime tunnelli sp. nov. (Figures 9 and 10)  Differential Diagnosis. Based on female. Body slightly flattened. Prosome 1.4 times longer than urosome, and distinctly wider than urosome Rostrum bell-shaped, and smoothly round apically. Antennule six-segmented. Genital somite and succeeding urosomal segments with lateral expansions. Caudal rami 1.4 times longer than anal somite, and 1.5 times longer than wide. P5 with nine setae, without seta on exp surface.
Description of Female. Total body length of holotype 452 µm (measured from anterior P1 ( Figure 10A). Basis with strong inner pinnate seta, and with spinules along inner margin, and with outer pinnate seta, and several spinules along outer margin; several Differential Diagnosis. Based on female. Body slightly flattened. Prosome 1.4 times longer than urosome, and distinctly wider than urosome Rostrum bell-shaped, and smoothly round apically. Antennule six-segmented. Genital somite and succeeding urosomal segments with lateral expansions. Caudal rami 1.4 times longer than anal somite, and 1.5 times longer than wide. P5 with nine setae, without seta on exp surface.
Description of Female. Total body length of holotype 452 µm (measured from anterior margin of rostrum to posterior margin of caudal rami). Largest width (223 µm) measured at posterior margin of cephalic shield ( Figure 9A). Body slightly flattened. Prosome 1.4 times longer than urosome, and distinctly wider than urosome. Cephalothorax 1.3 times shorter than length, with serrate posterior margin; dorsal surface smooth with few sensilla posteriorly. Prosomites ( Figure 9A) with smooth dorsal surface, serrated posterior margins and few sensilla posteriorly as figured.
Urosomites ( Figure 9A) with serrated posterior margin as illustrated. Dorsal surface and posterior margin of P5 bearing somite ornamented as preceding somites. Genital double-somite with smooth dorsal surface. Posterior margins of each urosomite serrated with rough denticles. Genital somite and succeeding urosomal segments produced laterally. Each urosomite with several posterior sensilla as illustrated.
Caudal rami ( Figure 9A,D) 1.4 times longer than anal somite, and 1.5 times longer than wide; setae I and II arising laterally halfway along outer margin, seta II longer than seta I; seta III pinnate and much longer than setae II; seta IV and seta V pinnate; seta V longest, but much shorter than all urosomites combined; seta VI located on distal inner corner; seta VII bare, located dorsally, triarticulated.
Antenna ( Figure 9C) four-segmented, comprising coxa, basis, and free two-segmented endopod. Coxa small without ornamentations. Basis with row of spinules along inner margin with pinnate abexopodal seta. First endopodal segment as long as second segment with abexopodal seta; segment 2 with row of stout spinules on apical margin and row of spinules around inner lateral margin; apical armature consisting of two stout pinnate and three geniculate spines; pinnate apical spine basally fused to naked small seta; two pinnate spines laterally. Exopod three-segmented, with one, one, and three setae, respectively; all setae and spines pinnate; second segment shortest; first segment twice as long as second; last segment much longer than preceding two segments combined with lateral seta and two apical pinnate setae.
Mandible, maxillule, maxilla, and maxilliped are the same as in Z. montagnai sp. nov. without significant discrepancies.
P4 ( Figure 10D). Basis with row of long spinules along inner distal margin, and with outer seta, and several spinules along outer margin. Exopod much longer than endopod; all exopod segments with row of spinules along inner lateral margin; exp-1 longest, and exp-2 shortest. Each endopod segments with row of spinules along outer lateral margins; enp-1 and enp-2 subequal in length; enp-3 longest; enp-3 only reached to middle of exp-2. Armature formulae as in Table 3.  Figure 10E). Baseoendopod forming a shallow lobe with four long pinnate setae and fused to exopod. Exopod rectangular shape with outer seta and three apical setae, and isolate naked seta located between exopod and basal cylindrical process. Basal seta long and pinnate arising from elongated cylindrical process. P6 ( Figure 10F) represented by single plate bearing three naked setae; middle shortest. Male. Unknown. Remarks. Z. tunnelli sp. nov. is morphologically similar to Z. destituta Kim J.G., Jung and Yoon, 2016. These species share the morphological characters of P1 enp-2 with three setae, P3 enp distal segment with four setae, P4 enp distal segment with three setae, P2-P4 exp-3 with seven setae. However, Z. tunnelli sp. nov. and Z. destituta can be distinguished from each other by the combination of the following morphological characteristics: (1) Z. tunnelli sp. nov. has less setae on A2 exp-3 (one more in Z. destituta); (2) Z. tunnelli sp. nov. has one more seta on female P5 than Z. destituta; (3) distance of female P5 baseoendopod and exp is closer in Z. tunnelli sp. nov.; (4) P6 setae of Z. tunnelli sp. nov. are all long and naked, while one of them is relatively short and pinnate in Z. destituta; (5) location of inner seta on P4 enp-3 is more distal in Z. destituta; (6) seta type and length of caudal rami seta II is shorter and naked in Z. destituta, while the seta II is longer and pinnate in Z. tunnelli sp. nov.
Description of female. Total body length of holotype 376 µm (measured from anterior margin of rostrum to posterior margin of caudal rami). Largest width (126 µm) measured at posterior margin of cephalic shield ( Figure 11A). Urosome distinctly narrower than prosome.
Cephalothorax somewhat triangular with smooth posterior margin; dorsal surface smooth with few sensilla posteriorly. Prosomites ( Figure 11A) with smooth dorsal surface, row of spinules along posterior margins and few sensilla posteriorly as figured.
Rostrum fused to cephalothorax, triangular, and with round apical margin and bearing two sensilla ( Figure 11A,B).
Anal somite ( Figure 11A) completely covered with well-developed operculum with serrated distal margin.
Urosomites ( Figure 11A) with few sensilla, and row of posterior spinules as illustrated. Dorsal surface and posterior margin of P5 bearing somite ornamented as preceding somites. Genital double-somite with smooth dorsal surface. Succeeding urosomal somite after genital somite, with smooth posterior margin and few spinules along both lateral posterior corner; posterior margins of all other urosomites deeply serrate and with several sensilla as illustrated.
Caudal rami (Figures 11A and 12G) slightly longer than anal somite, and 1.1 times longer than wide; setae I and II arising laterally along outer margin, seta II longer than seta I; seta III longer than setae II; seta IV basally fused to seta V; seta V longest, much longer than all urosomites combined; seta VI shorter than seta III and located on distal inner corner; seta VII bare, located dorsally, triarticulated.
Antenna ( Figure 11C) four-segmented, comprising coxa, basis, and free two-segmented endopod. Basis with row of anterior spinules, with pinnate abexopodal seta. Endopodal segment 1 smooth with seta; segment 2 with row of stout apical spinules; apical armature consisting of pinnate spine, four pinnate setae, and naked seta; three pinnate spines and naked spine laterally. Exopod three-segmented, with one, one, and four setae, respectively; second segment shortest; first segment about 1.3 times longer than second; last segment much longer than preceding two segments combined with lateral seta and three apical pinnate setae.
Mandible ( Figure 11D). Well-developed with about six multiple faced sharp teeth and naked seta at distal corner. Basis with pinnate seta. Endopod fused to basis forming one plate, with one lateral and four apical setae. Exopod one-segmented with lateral seta and two apical pinnate setae.
Maxilla ( Figure 11F). Syncoxa with three endites. Proximal endite bilobate; basal lobate with two pinnate setae; distal lobate with naked seta and two pinnate setae. Medial and distal endite with pinnate spine and two naked setae, and two pinnate spines, respectively. Allobasis produced into strong curved claw and overlapped curved spine with short slender seta basally; accessory armature consisting of two slender lateral setae proximally, and close to base of endopod. Endopod 1-segmented with two slender distal setae.
Maxilliped ( Figure 11G). Syncoxa elongate and cylindrical with seta. Basis with row of outer spinules. Endopod small with long sparsely pinnate seta, short apical seta, and two naked lateral setae.
P4 ( Figure 12D). Basis with rows of long inner spinules distally on anterior surface, with pinnate outer seta and several outer spinules. Exopod much longer than endopod; exp-1 longest, and exp-2 shortest; exp-1 and exp-2 with pinnate outer spine and long inner pinnate seta, respectively; exp-3 with two inner pinnate setae and two distal pinnate setae, and with three outer spines. Each endopodal segments with row of outer spinules; enp-1 with inner pinnate seta; enp-2 clearly longer than enp-1, with two pinnate setae; endopod reaching to middle region of exp-2. Armature formulae as in Table 4. P5 ( Figure 12E). Outer basal seta long and pinnate set on cylindrical setophore. Exopod fused to baseoendopod. Endopodal lobe trapezoid with three long pinnate apical setae; innermost one shortest, other two subequal in length; with few scattered inner spinules anteriorly. Exopod rectangular, slightly higher than the endopodal lobe, with three apical pinnate setae; inner seta shortest, and middle apical seta longest. P6 ( Figure 12F) represented by single plate bearing three elements; outer is the longest, inner is the shortest, small denticle shaped spine. Copulatory pore large, crescentic, and located at slightly distal region from median line of genital double somite.
Remarks. Based on Well's (2007) key to the species of Zosimeidae [14], P. pryorae sp. nov. is significantly similar to P. reducta (Becker and Schriever, 1979). They share the setal formulae of P1-P5, length:width ratio of caudal rami, and length of the body. However, P. pryorae sp. nov. and P. reducta can be distinguished from each other by the combination of the following morphological characteristics: (1) P. pryorae sp. nov. has less setae on mandibular palp; P. reducta has one more seta and all setae are naked (four setae in P. reducta), (2) P. pryorae sp. nov. has more setae on the coxa and basoendite of the maxillule (nine setae in P. pryorae sp. nov. but seven setae in P. reducta), (3) P. pryorae sp. nov. has more setae on distal lobate of maxilla (three setae in P. pryorae sp. nov. but two setae in P. reducta), and maxilliped (five setae in P. pryorae sp. nov. but three setae in P. reducta), (4) P. pryorae sp. nov. has more segments in female A1, and (5) the length of setae of P2 exp-1 and enp-3 are shorter in P. pryorae sp. nov.

Key of Zosimeidae
Kim et al. [8] proposed three groups of species in Zosime according to morphological characteristics. However, Pointner [15] refuted the defining the group by the number of A1 segments. In addition, species that do not meet the existing criteria have also been reported (e.g., Z. carsteni Pointner, 2017 and Z. eliasi Pointner, 2017).
Referring to Wells [14], a new classification key of the family Zosimeidae was prepared with 27 species including the new species described here. The three species-groups of Zosime were judged to be controversial, so the classification key did not include any related content.

Key to the Genera of Zosimeidae
Lee et al. [21] have argued that many harpacticoids found in the Gulf of Mexico tend to have a smaller size than their congeners from other regions. It has also been reported that the size of the species inhabiting this area is rather small because of the low availability of food in the area [2]. A recent study [22] summarized the composition and type of sediment by depth in the southern Gulf of Mexico area, and it was confirmed that the rate of sand was lower, and the rate of clay was higher in the deep sea than in the shallow sea. Since the grain size of the deep-sea sediments is small and the habitat will be narrowed, it can be inferred that the size of the copepods living there will be relatively small.
As mentioned in the introduction, according to the results of ecological studies conducted in this area, it is reported that Tisibidae is the dominant family [3]. Because many species of Zosimeidae previously belonged to Tisibidae, although small in body size, this taxon may be an important taxon in the Gulf of Mexico benthic ecosystem.

Status of Gene Data and Molecular Phylogenetic Analysis with Related Taxa
The gene sequence studies conducted so far in the family Zosimeidae are summarized in the Table S3. The Zosimeidae sequences uploaded to the NCBI database are from three studies resulting in 47 sequences, the gene sequence information corresponds to a total of five markers, of five to seven species of two genera (Zosime, Pseudozosime). Only six of the uploaded Zosimeidae sequences have been identified to the species level, all are mtCOI sequences of Zosime atlantica. The marker with the most data is 18S rRNA, and all sequences are relatively short fragment sequence information of 740 bp, except one.
In this study, an attempt was made to secure the gene sequences of the new species, but it was difficult because they were from formalin-fixed samples. Although the phylogenetic analysis using the data uploaded to NCBI is very limited, we tried to confirm the molecular phylogenetic relationship based on the commonly uploaded 18S rRNA fragment sequences for each family. In addition, based on the recorded taxonomic shifts in the Zosimeidae species, the families determined to be morphologically related (Pseudotachidiidae, Idyanthidae, Tisibidae, Ectinosomatidae, and Aegisthidae) were intensively included. To increase the accuracy of the results, only data longer than 1000 bp among uploaded 18S rRNA sequences were used.
As a result (Figure 13), it was confirmed that Ectinosomatidae, Idyanthidae, and Tisbidae showed a relatively close relationship with Zosimeidae in the order. In Easton and Thistle [23], a phylogenetic tree was created based on short fragment sequences (740 bp), and the results also confirmed a close phylogenetic relation between Zosimeidae, Ectinosomatidae, and Idyanthidae. Seifried [16] defined Idyanthidae and Zosimeidae together as Idyanthidimorpha, having a close morphological relationship with Ectinosomatidae, and claimed that three subfamilies of the previously known family Tisbidae show a distant phylogenetic relationship.
The phylogenetic tree of this study included three species belonging to subfamily Tisbinae Stebbing, 1910, and one species each from Idyanthidae and Zosimeidae. Unlike the results of Seifried [16], the results of this study showed that they were closely related. However, since the number of species included in this study was very small, more gene sequence studies from more species are needed to make more accurate claims and judgments, which should be able to fill the gap in the phylogenetic tree and increase resolution. bp), and the results also confirmed a close phylogenetic relation between Zosimeidae, Ectinosomatidae, and Idyanthidae. Seifried [16] defined Idyanthidae and Zosimeidae together as Idyanthidimorpha, having a close morphological relationship with Ectinosomatidae, and claimed that three subfamilies of the previously known family Tisbidae show a distant phylogenetic relationship. The phylogenetic tree of this study included three species belonging to subfamily Tisbinae Stebbing, 1910, and one species each from Idyanthidae and Zosimeidae. Unlike the results of Seifried [16], the results of this study showed that they were closely related. However, since the number of species included in this study was very small, more gene sequence studies from more species are needed to make more accurate claims and judgments, which should be able to fill the gap in the phylogenetic tree and increase resolution.

Conclusions
Based on the morphological characters we report three new species of Zosime and a new species of Peresime from the Gulf of Mexico. The present study is the first report of Zosimeidae from the study area and suggests high zosimeid biodiversity in the Gulf of Mexico.

Conclusions
Based on the morphological characters we report three new species of Zosime and a new species of Peresime from the Gulf of Mexico. The present study is the first report of Zosimeidae from the study area and suggests high zosimeid biodiversity in the Gulf of Mexico.
A key for the genera of the family Zosimeidae and keys to the species to Zosime and Peresime were provided, and the distribution and molecular research status of each species were summarized.
To infer the molecular phylogenetic relationship between Zosimeidae and the morphologically and taxonomically related families, NCBI uploaded data were used, and as a result, it was confirmed that they were closely related to Ectinosomatidae, Idyanthidae, and Tisbidae. For accurate molecular phylogenetic studies, future molecular studies with more molecular markers and various species will be needed.
As such, this study comprehensively summarizes the morphology, DNA studies, and distribution of Zosimeidae, and these contents will be used as basic data for future Zosimeidae studies or deep-sea copepods research in the Gulf of Mexico.