Systematics and zoogeography of the deep‐sea hyperbenthic family Arietellidae (Copepoda: Calanoida) collected from the Sulu Sea

A faunistic survey of the deep‐sea hyperbenthic copepods in the Sulu Sea, which has a deep basin and is connected via shallow straits with the Pacific and the South China Sea, was carried out in November and December 2002, using beam trawls and a mid‐water trawl. Two new genera and four new species of the primitive calanoid family Arietellidae are described. A new genus Metacalanalis shows intermediate characteristics between those of Metacalanus Cleve, 1901 and Pilarella Alvarez, 1985. Another new genus, Protoparamisophria, is closely related to Paramisophria Scott, 1897, but exhibits more plesiomorphic states in the female genital system and appendages. Two new species of the genera Paraugaptiloides Ohtsuka, Boxshall and Roe, 1994 and Sarsarietellus Campaner, 1984 are the second and the third species, respectively, for these genera. This discovery enhances the validity of these rare genera, and sheds light on the geological history of the Sulu Sea and the evolution of the Arietellidae within it.


Introduction
Deep-sea hyperbenthic copepods have been poorly understood in comparison with pelagic forms, partly due to sampling difficulties. The systematics and ecology of these copepods have been studied using specially designed gears or deep-sea research vessels (Bradford 1969;Grice and Hulsemann 1970;Grice 1972;Wishner 1980;Fleminger 1983;Alvarez 1985;Wishner and Gowing 1987;Ohtsuka et al. 1991Ohtsuka et al. , 1994Ohtsuka et al. , 1998Ohtsuka et al. , 2003Nishida et al. 1999;Ferrari and Markhaseva 2000;Ohtsuka and Boxshall 2004). Considerable attention has been paid to these hyperbenthic copepods with regard to the evolution of copepods, because deep hyperbenthic waters have been suggested to have played an important role as Antennules asymmetrical, 21-segmented, left longer than right and reaching to pediger 3; posterior margin lacking long setules; first (I-IV) and last (XXIV-XXVIII) segments compound; segments II, IV, VI, VIII, and X lacking aesthetasc. Antenna with indistinctly three-segmented endopod; first endopodal segment bearing one inner seta, second and third segments bearing three and five setae, respectively; exopod indistinctly 10-segmented. Mandibular gnathobase without patch of long setules; four teeth present, dorsalmost tricuspid at tip; endopod represented by rudimentary segment with two unequal setae terminally; seta on first to fifth exopodal segments not reduced. Maxillule with one short and four long spines on praecoxal arthrite; coxal endite bearing single seta; coxal epipodite with six setae; endopod one-segmented, with two setae terminally. Maxilla with two setae and one vestigial element on first praecoxal endite; basal spine with short row of three or four minute spinules at mid-length. Maxilliped with innermost seta on fourth and fifth exopodal segments not reduced; setae a and b (see Ohtsuka et al., 1994) on sixth segment not reduced.
Leg 1 with third exopodal segment bearing only one outer spine. Some elements of legs 1-4 asymmetrical. Leg 5 symmetrical, with coxae and intercoxal sclerite separate; endopod lacking; exopod unisegmented, with two lateral and two terminal spines.

Remarks
The new genus is similar to the genera Metacalanus Cleve, 1901 and Pilarella Alvarez, 1985, and has mixed features of these two genera. The discriminative features between these are as follows (cf. Alvarez 1985;Ohtsuka et al. 1994) (Pilarella versus Metacalanus): (1) location and configuration of female gonopores and copulatory pores (medially located and paired versus posteriorly and paired or unpaired); (2) relative length of caudal rami (about five times longer than wide versus at most twice); (3) segmentation and relative length of antennules (21-segmented in both, left antennule reaching beyond urosomal end versus 16or 18-segmented in left and 18-or 20-segmented in right, left antennule not reaching prosomal end); (4) setal number on the second exopodal segments of antenna (three medial setae on the second segment versus only two); (5) armature of the praecoxal arthrite, coxal endite and endopod of maxillule (formula with 6, 1, 2 versus 0-2, 0-1, 0-1); (6) armature and ornamentation of elements on the maxillary endites (two distinct setae + one rudimentary element on the first praecoxal endite versus one + one). The new genus exhibits symplesiomorphies in (1), (3), (4), (5), and (6) with Pilarella. On the other hand, the relative shortness of the caudal rami and the antennules of the new genus are reminiscent of Metacalanus. The fifth leg of female Metacalanalis exhibits a more plesiomorphic state than those of Metacalanus and Pilarella, and is rather similar to that of another closely related genus Paramisophria Scott, 1897, in particular P. reducta Ohtsuka, Fosshagen and Iliffe, 1993 except for the absence of endopod.
Metacalanalis is distinguishable from Metacalanus and Pilarella by the combination of the following features: (1) caudal rami about 1.2 times as long as wide; (2) left antennule reaching at most to the posterior margin of pediger 3; (3) antenna showing the most primitive condition in the family Arietellidae, with an indistinctly three-segmented endopod and an indistinctly 10-segmented exopod with setal formula 0, 0, 1, 1, 1, 1, 1, 1, 0, 2; (4) praecoxal arthrite of the maxillule with one short and four long spines; (5) coxal epipodite of the maxillule with six setae; (6) basal spine of the maxilla with a short row of minute spinules medially; (7) fifth leg of the female with a unisegmented exopod bearing two lateral and two terminal spines.
The antenna of Metacalanalis exhibits a primitive condition, with an indistinctly threesegmented endopod and an indistinctly 10-segmented exopod. The endopod has a suture between the second and third segments clearly visible. Only the most primitive calanoid family Pseudocyclopidae (monotypic, Pseudocyclops Brady, 1872) is hitherto known to bear a distinctly three-segmented antennary endopod (Huys and Boxshall 1991). A 10-segmented exopod is also the most primitive condition in copepods (Huys and Boxshall 1991).
Asymmetry of legs 1-3 is unique to the new genus, although the asymmetrical antennules are common in the hyperbenthic arietellid genera Metacalanus and Paramisophria (cf. Ohtsuka et al. 1994). These are either an adaptation to its hyperbenthic life or aberrant (see Remarks for the new species).

Etymology
The generic name is derived from the closely related genus Metacalanus Cleve, 1901. Gender masculine.
Seta and spine formula of legs 1-4 ( Figure 3A-G, I-L) shown in Table I. Both rami of legs 1-3 and endopod of leg 4 three-segmented (exopods of leg 4 broken or aberrant). Asymmetry in legs 1-4. Leg 1 ( Figure 3A-E) with inner basal setae asymmetrical, left ( Figure 3D) thicker than right ( Figure 3E); outer distal corners of three endopodal segments acutely pointed ( Figure 3C). Leg 2 ( Figure 3F, G) with outer spines on third exopodal segments asymmetrical in number, size and location. Leg 3 ( Figure 3I, J) with outer spines on second and third exopodal segments asymmetrical in length and shape. Left leg 4 ( Figure 3K) with aberrant exopod, with first segment unarmed and second bearing outer long spine and six modified setae. Terminal exopodal segments of right leg 4 missing ( Figure 3L).
Leg 5 ( Figure 3M) nearly symmetrical; coxae and intercoxal sclerite separate; basis bearing thick, subterminal seta on posterior surface; endopod lacking; exopod one-segmented, with two lateral and two terminal spines; acute process present at base of inner terminal spine; outer terminal process more acutely pointed in right leg than in left. Table I. Spine and seta formula of legs 1-4 of Metacalanalis hakohoae n. gen., n. sp.
Asymmetry of legs 1-3 (maybe, leg 4 also) may be related to an as yet unknown swimming behaviour of the new genus as observed in Paramisophria spp. that swim with the left lateral side parallel to the bottom and the left antennule extended anteriorly (Fosshagen 1968;Ohtsuka and Mitsuzumi 1990). Bowman and González (1961) insisted that hyperbenthic calanoid copepods generally bear stouter outer spines on the exopods of legs in comparison with pelagic ones. Hence the longer spines on the second and third exopodal segments of left legs 2-4 suggest a possible peculiar swimming behaviour as found in Paramisophria spp. The inner basal seta of leg 1 is considered to play a role in grooming (Vaupel Klein 1972). The thicker inner basal seta of left leg 1 of the new species is likely to be adaptive for grooming the longer left antennule. Gut content analysis revealed that the new species fed upon copepods. Stalked cysts on legs (see Figure 3A, F, H, K) are possibly assignable to phoronts of apostome ciliates (cf. Grimes and Bradbury 1992;).

Etymology
The new specific name, hakuhoae, refers to the RV Hokuho-maru, University of Tokyo, that carried out the survey in the Sulu Sea in 2002.

Diagnosis
Female. Body compact. Prosome plump, about 2.6 times as long as urosome; prosomal ends almost symmetrical, with paired dorsolateral processes and lateral round lobes. Urosome with genital double-somite longer than wide; paired gonopores located anteriorly; paired copulatory pores located midway on ventral surface; seminal receptacles asymmetrical, tube-like; anal operculum lacking; caudal rami symmetrical, longer than wide.
Antennules asymmetrical, 22-segmented, left longer than right, and reaching beyond cephalosome; first (I-III) to eighth (X) segments with row of long setules along posterior margin; segments I-III and IV separate; segments II and IV of right antennule, and I, II, and IV of left antennule lacking aesthetasc. Antenna with two-segmented endopod; first endopodal segment bearing one inner seta, and second segment with three subterminal and six terminal setae; exopod indistinctly seven-segmented; terminal segment with one vestigial and two distinct setae. Mandibular gnathobase without patch of long setules; three teeth present, dorsalmost bicuspid; endopod represented by one rudimentary segment with two unequal setae terminally; seta on first to fifth exopodal segments not reduced. Maxillule with one process and five spines on praecoxal arthrite; coxal endite bearing single seta; coxal epipodite with eight setae; rudimentary basal seta present; endopod onesegmented, with three setae terminally. Maxilla with two setae and one vestigial element on first praecoxal endite; basal spine naked. Maxilliped with innermost seta on fourth and fifth exopodal segments not reduced; setae a and b (see Ohtsuka et al. 1994) on sixth segment not reduced.
Leg 1 with third exopodal segment bearing two outer spines. Leg 5 nearly symmetrical, with coxae and intercoxal sclerite partly fused; endopod represented by acutely pointed process bearing two plumose setae; exopod indistinctly three-segmented with suture subtly visible; first and second segments each with one outer spine; terminal segment bearing one spiniform seta and one subterminal and two terminal spines.

Remarks
The new genus is generally similar to Paramisophria, but exhibits more plesiomorphic states both in the female genital system and in some appendages. In the new genus, paired copulatory pores open at mid-length on the ventral side of the genital double-somite, while in Paramisophria a single copulatory pore is located midway or on either the left or right side (cf. Ohtsuka and Mitsuzumi 1990;Ohtsuka et al. 1991Ohtsuka et al. , 1993Ohtsuka et al. , 1994Heinrich 1996). The new genus displays paired, elongate seminal receptacles, each located between the corresponding copulatory pore and gonopore, with a short copulatory duct. In contrast, in some species of Paramisophria, the seminal receptacle is near the gonopore, with a relatively long, and in some cases, a highly curved copulatory duct (cf. Ohtsuka and Mitsuzumi 1990;Ohtsuka et al. 1993Ohtsuka et al. , 1994. These characteristics of the new genus seem to be relatively plesiomorphic with respect to the hypothetical calanoid ancestor as proposed by Huys and Boxshall (1991).
The exopod of the female fifth leg of the new genus displays the most primitive state in the family Arietellidae: indistinctly three-segmented; terminal segment with one outer, one subterminal and two terminal elements, all of which are separate at base from the segment. In some species of Paramisophria the exopod is indistinctly three-segmented, but the number of elements on the terminal segment is at most three elements and one process (in P. japonica Ohtsuka, Fosshagen and Go, 1991). In this case, the outer terminal spine of the new genus seems to be homologous to the terminal process of P. japonica. The new genus also has the following plesiomorphies, although some species of Paramisophria share some of them: (1) five spines and one process on the praecoxal arthrite of the maxillule; (2) the presence of a distinct seta on the coxal endite of the maxillule; (3) three setae on the maxillulary endopod; (4) two setae and one vestigial element on the first praecoxal endite of the maxilla; (5) separation of coxae and intercoxal sclerite in female leg 5; (6) two setae on the endopod of female leg 5.

Etymology
The new generic name, Protoparamisophria, is derived from the Greek proto, meaning primitive, plus the closely related genus Paramisophria. Gender feminine.
Seta and spine formula of legs 1-4 ( Figure 6A-F) shown in Table II. Legs 1-4 with threesegmented rami. Leg 1 ( Figure 6A-C) with basis bearing row of spinules along endopodal base; all endopodal segments with outer distal corners acutely pointed, and with marginal row of setules ( Figure 6A, B); first exopodal segment with row of minute spinules at distal margin; outer spines on all exopodal segments strongly serrated ( Figure 6C). Inner distal corner of basis of legs 2-4 ( Figure 6D-F) acutely pointed, but most sharply produced in leg 4; outer distal corner of second endopodal segments of legs 2-4 produced distally into sharply pointed process reaching beyond base of proximal outer seta on third endopodal segment.
Leg 5 ( Figure 6G) slightly asymmetrical; coxae and intercoxal sclerite only partly coalescent; basis bearing thick, plumose seta at outer distal corner (left seta missing); endopod represented by acutely pointed process with one terminal and one subterminal plumose setae; exopod indistinctly three-segmented, with suture visible on both sides; first and second exopodal segments each bearing serrate spine, near base of which two processes present; third exopodal segment with proximal spiniform seta, subterminal serrate spine, and two terminal serrate spines.

Remarks
The only specimen was collected with an IKMT originally deployed for collection of deepwater plankton and micronekton. However, the net must have accidentally touched the seabottom, because some sediment was found in the cod end after recovery. The asymmetrical antennules and stout outer spines on the legs of this new species strongly suggest that it is a truly hyperbenthic calanoid (cf. Bradford-Grieve 2002).

Etymology
The new specific name, biforaminis (Latin bi meaning two; Latin foraminis pore-bearing) alludes to the presence of paired copulatory pores on the female genital double-somite of the new taxon.

Diagnosis
Emended part only; see Ohtsuka et al. 1994. Male left geniculate antennule fringed with long setules posteriorly on compound segments I-IV only or segments I-IV to V; segment I with one or two setae. Second endopodal segment of antenna with two or three inner setae. Mandibular endopod one-segmented with two terminal setae of unequal length, or absent. Leg 4 with or without vestigial element on inner distal corner of coxa. Male fifth legs with basis and coxa of right leg separate or incompletely fused; right endopod unisegmented, rudimentary or well-developed; second exopodal segment of right leg expanded inwards or extremely elongated, third segment triangular, tapering distally, or lamellar; left endopod two-segmented, with second segment originating from inner base of first segment; second exopodal segment of left leg expanded inwards or outwards, third segment with two or three heavily chitinized, long processes.

Remarks
The monotypic genus Paraugaptiloides was established to accommodate Paraugaptilus magnus Bradford, 1974by Ohtsuka et al. (1994. This species was first collected off New Table II. Spine and seta formula of legs 1-4 of Protoparamisophria biforaminis n. gen., n. sp.
Although the fifth legs of the males appear to differ greatly in these two species, some homologous structures can be clearly traced on the basis of their number and position; namely the second endopodal segment of the left leg originates from the inner proximal part of the first segment; the third exopodal segment of the left leg bears four elements; the outer terminal corner of the second exopodal segment of the right leg bears a patch of fine setules and a triangular or round process with a minute spinule terminally. These differences in the fifth legs seem to be important at the specific level.
The genus seems to be associated with the sea-bottom (Bradford 1974;Heinrich 1993; present study), although it was previously regarded as pelagic (Ohtsuka et al. 1994
Mandible ( Figure 8G) with two mono-and one tricuspid teeth and two patches of setules along cutting edge; patch of long setules present near base of palp; endopod absent, but possibly represented by round prominence; exopod five-segmented, setal formula with 1, 1, 1, 1, 2. Maxillule ( Figure 8H) with praecoxal arthrite bearing five serrate spines and one process; coxal endite with one serrate seta; coxal epipodite with eight setae, two of which relatively short; basis with two rows of long setules and vestigial seta; endopod onesegmented, bulbous, with two serrate unequal setae terminally; exopod one-segmented, lobate, with three long setae terminally. Maxilla ( Figure 9A) with first praecoxal endite bearing two setae and one rudimentary element; second praecoxal to second coxal endites each with two serrate setae; basis bearing long spine ( Figure 9B) with longitudinal row of fine spinules; endopod indistinctly four-segmented, all setae on which well developed, with setal formula 1, 3, 2, 2. Maxilliped ( Figure 9C-E) with syncoxa bearing one middle and two subterminal serrate setae and subterminal patch of minute spinules; basis as long as syncoxa, with two serrate setae and longitudinal patch of fine spinules; first endopodal segment nearly incorporated into basis, with serrate seta; second to fifth endopodal segments bearing four, four, three and three setae, respectively; fifth endopodal segment slightly longer than preceding segment; terminal endopodal segment with setae a and b rudimentary.
Seta and spine formula of legs 1-4 ( Figure 9F-I) as in Protoparamisophria biforaminis. Legs 1-4 with both rami three-segmented. Leg 1 ( Figure 9F) with basis bearing anterodistal row of minute spinules near base of endopod; first endopodal segment with patch of fine setules anterodistally; outer spines on second and third exopodal segments weakly chitinized. Legs 2 ( Figure 9G) and 3 ( Figure 9H) similar except for shape of distal outer process on second endopodal segment. Leg 4 ( Figure 9I) with round prominence at inner distal corner of basis.
Leg 5 ( Figure 10) highly complex, enlarged, and heavily chitinized; coxae and intercoxal sclerite completely fused. Left leg with basis bearing plumose subterminal seta on posterior surface; endopod distinctly two-segmented, located near inner distal corner of basis, first segment diamond-shaped, second segment originating from inner midpoint of preceding segment; exopod three-segmented, first segment elongate, with minute element at outer distal corner; second segment massive, expanded outwards; third segment strongly curved, bearing proximal short seta, middle curved spine with spatulate tip (Figure 10C), and terminal bifurcate process. Right leg with basis bearing plumose seta on posterior surface; endopod one-segmented with broad base along inner margin of basis, furnished with fine setules subterminally on posterior surface; exopod indistinctly three-segmented, first segment curved inwards at mid-length, carrying two round processes on anterior surface and triangular outer distal process with fine seta; second segment extremely elongated, as long as basis and first exopodal segment combined, inserted on inner subterminal corner of preceding segment, and bearing short seta at mid-length of inner margin, distal patch of fine setules and subterminal round process with minute spinule at tip; third segment lamellar with three minute setae along distal margin.

Remarks
The new species is easily distinguishable from P. magnus (features in parentheses) as follows: (1) prosomal posterior corners irregularly pointed (not irregular); (2) three inner setae on the second endopodal segment of the antenna (only two); (3) absence of the mandibular endopod (one-segmented, with two terminal setae); (4) absence of a vestigial element on the inner distal corner of coxa of leg 4 (present); (5) third exopodal segment of the left fifth leg bearing three heavily chitinized processes (only two); (6) endopod of the right fifth leg well developed (relatively reduced); (7) proximal two exopodal segments of the right fifth leg extremely elongated (relatively compact).
Unidentified ciliates were attached near the distal patch of fine setules of the second exopodal segment of the right fifth leg (not illustrated in Figure 10A).

Etymology
The specific name of the new species, mirandipes (Latin mirandus, meaning surprising, plus Latin pes meaning leg), refers to the highly complex, enlarged fifth legs of the male.

Remarks
The genus Sarsarietellus comprises only three species: S. abyssalis (Sars, 1905), S. natalis Heinrich, 1993, and S. suluensis n. sp. The new species described herein adds the foregoing features to the generic diagnosis. The anterolateral cephalic papillae are known only in Sarsarietellus. Since they bear a pore on tip, this organ may have a secretory function.
The presence or absence of dorsolateral and ventrolateral processes on the prosomal end in species within a single genus is also recognized within congeners in other arietellid genera such as Arietellus Giesbrecht, 1892 and Paramisophria Scott, 1897. These processes seem to have evolved convergently many times within the family Arietellidae.
Segmentation and seta/spine formula of legs 1-4 ( Figure 13A-F) as in Protoparamisophria biforaminis. Leg 1 ( Figure 13A, B) with basis bearing anterodistal row of minute spinules near base of endopod; first and third endopodal segments with patch of fine setules on anterior surface; outer spines on second and third exopodal segments well chitinized. Legs 2 ( Figure 13C, D) and 3 ( Figure 13E) similar except for acuteness of process at inner distal corner of basis. Leg 4 ( Figure 13F) with short basal seta on posterior surface; basal process moderately pointed at inner distal corner.
Leg 5 ( Figure 13G) almost symmetrical, with coxae and intercoxal sclerite completely fused to form common base; basis bearing plumose seta near outer middle margin on posterior surface; endopod represented by process carrying four plumose setae, acutely pointed at tip; exopod unisegmented, with one long, terminal and four short, outer spines; terminal right process of exopod slightly longer than left one.

Remarks
The new species is readily distinguished from the other congeners based on: (1) welldeveloped dorsolateral and ventrolateral processes of pediger 5; (2) remarkably asymmetrical genital system of the genital double-somite; (3) structure of leg 5 (in particular, the non-expression of suture lines on the exopod).
Phoront of possibly apostome ciliates were found also on the mandibular palp ( Figure 12A, B) of this species as well as in Metacalanalis hakuhoae (see Figure 3A, F, H, K).

Etymology
The specific name of the new species, suluensis, refers to the type locality, the Sulu Sea.

Phylogenetic relationships among arietellids
The phylogenetic relationships among 10 arietellid genera were subject to a preliminary cladistic analysis, in which two clades were recognized: the Metacalanus-group (Crassarietellus-Paramisophria-Metacalanus-Pilarella) and the Arietellus-group (Campaneria-Sarsarietellus-Paraugaptiloides-Scutogerulus-Paraugaptilus-Arietellus) (Ohtsuka et al. 1994). The original data matrix of Ohtsuka et al. (1994) is reconstructed and the present four new taxa are added. One additional character is evaluated as character 45: seta on exopodal segment IV of antenna (present/absent: 0/1) (see Table III). This analysis is still tentative, because both sexes are known only in five genera.
A unique tree was generated by the new analysis: tree length5196; consistency index50.245 (Figure 14). The topology is essentially similar to that proposed by Ohtsuka et al. (1994, cf. Figures 44, 45), and suggests that the arietellid ancestor originated in deepsea hyperbenthic layers as supposed previously (Ohtsuka et al. 1994). The two new genera fall within the Metacalanus-group, with Metacalanalis as the sister group of Metacalanus, and Protoparamisophria of Paramisophria. This suggests that in the Metacalanus-group invasion of continental hyperbenthic layer may have occurred independently at least three times, once within Paramisophria, once in Pilarella, and once in Metacalanus. This is strongly supported by the analysis of Jaume et al. (2000) for the genus Paramisophria that is broadly distributed from the continental slope water to the lower intertidal zone. According to their analysis, it may have originated from the continental shelf edge, and then colonized both shallower and deeper waters.
In this clade, miniaturization seems to have been accompanied by invasion of shallow waters. For example, the bathyal Crassarietellus is the largest representative (ca 4 mm in body length), while the neritic Metacalanus is the smallest (less than 1 mm). The same tendency is recognizable within the genus Paramisophria: the offshore P. giselae is the largest (about 3 mm), whereas the intertidal P. platysoma is the smallest (ca 1 mm). In contrast, such miniaturization cannot be clearly detected in the other clade, the Arietellus-group, since it has never invaded the shallow hyperbenthic waters. Table III. Data matrix for analysis of relationships between 12 arietellid genera (see Ohtsuka et al. 1994 and text) Zoogeography and colonization of the Sulu Sea Bradford-Grieve (2002) speculated upon the evolutionary history and colonizing routes of calanoid copepods through geological time. According to her, the ancestral calanoids, perhaps the Pseudocyclopoidea and Epacteriscioidea, may have appeared in the late Palaeozoic as a shallow-water hyperbenthic form. The primitive superfamily Arietelloidea, including the family Arietellidae, may have originally invaded the water column in the Devonian, and then radiated into the deep-sea in the late Carboniferous-Early Permian, when the deep water was well ventilated. Some arietelloids such as the Arietellidae are considered to have re-invaded the deep hyperbenthic zone at that time. Bradford-Grieve (2002) assumed the presence of a deep-sea dysoxic layer for the possible survival of these deep hyperbenthic species through the anoxic midwater conditions of the Cretaceous. Recently, the survival of deep-sea benthic invertebrates during these anoxic episodes has been supported also by several authors (e.g. Horne 1999;Wilson 1999). However, this hypothesis, as Bradford-Grieve (2002) pointed out, must be confirmed by molecular techniques.
Taking the geological history of the Sulu Sea into account, the evolutionary pattern and process of arietellids including those in the Sulu Sea per se are supposed. The Sulu Sea is located approximately in the centre of the Indo-West Pacific Region, and is surrounded by many islands and sills shallower than ca 400 m. It has a deep (ca 5000 m) basin and is characterized by a homogenous meso-and bathypelagic zone (500-5000 m) with fairly high water temperature (ca 10uC) and low oxygen content (1.3-1.7 ml l 21 ) (Marumo 1975). On Figure 14. Cladogram of arietellid genera including the two new genera, Metacalanalis and Protoparamisophria. Main habitat and depth range provided only for hyperbenthic genera, excluding two truly pelagic genera, Arietellus and Paraugaptilus. the basis of biostratigraphic and sedimentary analyses, it is suggested that the Sulu Sea originated as a back-arc or intra-arc basin in the late early to early middle Miocene, and the isolation of the basin from deep waters of the Pacific occurred only 1.9 million years ago (Anonymous 1989). The occurrence of the present new taxa clearly indicates that the much higher water temperature and lower dissolved oxygen in the hyperbenthic waters of the Sulu Sea than in the other deep-sea habitats worldwide are not the limiting factors for the dispersion and survival of these copepods. While more extensive collections are needed to establish the endemism of the present taxa in the Sulu Sea, the time scale (.1 million years ago) and the extent of deep-water isolation appear to have resulted in the speciation and maintenance of a unique assemblage of hyperbenthic copepods.
If we regard depth as the most important limiting factor for colonization in the family Arietellidae, the following scenario can be proposed for the history of the family. First, the vertical distribution of the arietellid ancestors, those estimated parsimoniously from the present information, is superimposed on the branches of the foregoing cladogram ( Figure 15). In the Arietellus-group, all genera are deep hyperbenthic except the pelagic genera Arietellus and Paraugaptilus. So the habitat transition occurred once in the ancestral clade of genera Arietellus and Paraugaptilus. Two of the new species described in this study, namely Paraugaptiloides mirandipes and Sarsarietellus suluensis, are deep-water hyperbenthic taxa, so it can be assumed that their ancestor inhabited the Sulu Sea before the isolation of the deep water. It may be that, after the semi-closure of the Sulu Sea, their ancestral species populations were isolated from the main population and undertook speciation.
In contrast, the habitat transition in the Metacalanus-group is more complex to elucidate. Three genera, Metacalanalis, Protoparamisophria, and Crassarietellus, are deep-water hyperbenthic, while the rest of the genera are shallow-water hyperbenthic. Among them, Protoparamisophria and Crassarietellus are assumed to continue in the ancestral habitat. On the other hand, uncertainty still remains as to how Metacalanalis invaded the deep-sea hyperbenthos of the Sulu Sea. One way is to assume that Metacalanalis simply remained in the ancestral habitat of the Metacalanus-group, i.e. the deep-sea hyperbenthos. However, the sister genera of Metacalanalis (Metacalanus and Pilarella) are both shallow-water hyperbenthos. The possibility of a habitat shift of the ancestral species of Metacalanus, Metacalanalis, and Pilarella from deep to shallow hyperbenthic layers still remains. If we could find species of Metacalanus and Pilarella in deep hyperbenthic waters of regions other than the Sulu Sea, then this would provide further evidence that they have colonized the deep hyperbenthic layers of the Sulu Sea before the closure of its sills. This would support a deep hyperbenthic ancestral state for the clade. However, if we could collect species of Metacalanalis from shallow water, it could be assumed that its ancestor has invaded shallow water, and next colonized the deep hyperbenthos of the Sulu Sea, thus supporting a shallow hyperbenthic ancestral state for Metacalanalis. The latter scenario is less likely since no member of the genus has been found in shallow water despite intensive sampling in the Atlantic and the north-western Pacific. Based on the information available, we propose that the divergence of Metacalanus and Metacalanalis, and of Paramisophria and Protoparamisophri, occurred later than 1.9 million years ago ( Figure 15). Since Metacalanalis and Protoparamisophria occur only in the Sulu Sea, the closure of its sills must have occurred earlier than the divergence of Metacalanus and Metacalanalis, and of Paramisophria and Protoparamisophria. So we assume they diverged later than 1.9 million years ago (Anonymous 1989). In contrast, the ancestral species of Paraugaptiloides and Sarsarietellus must have invaded the Sulu Sea before the closure of the sills (Figure 15).
More precise inferences regarding endemism will depend on extensive sampling and genetic analyses in future studies.