Xenosinelobus balanocolus, Chim & Tong, 2019

Chim, C. K. & Tong, Samantha J. W., 2019, Xenosinelobus balanocolus, a new tanaidid genus and species (Crustacea: Peracarida: Tanaidacea) from barnacles on intertidal rocky shores and seawalls in the Singapore Strait, Zootaxa 4629 (3), pp. 413-427 : 415-426

Xenosinelobus balanocolus n. sp.

( Figs. 1–5 , Table 1 )

Material examined. Holotype, non-ovigerous female, dissected and mounted on 16 slides ( ZRC 2019.0582 ), 2.05 mm, Lazarus Island , Singapore Strait , Singapore (1°13.221’ N, 103°51.251’ E), intertidal, 0.9 m above chart datum, 12 September 2014 ; allotype, male, dissected and mounted on 21 slides ( ZRC 2019.0583 ), 2.00 mm, same collection data as holotype ; paratype, female with oostegites, dissected and mounted on 14 slides ( ZRC 2019.0584 ), 1.92 mm, same collection data as holotype ; one female with oostegites, ten non-ovigerous females and four mancae in 15 vials ( ZRC 2019.0585 ), same collection data as holotype ; one female, one male and one manca in three vials ( ZRC 2019.0586 ), Lazarus Island-St. John’s Island causeway (1°13.256’ N, 103°51.065’ E), seawall, intertidal, 0.6 m above chart datum, 20 May 2014 ; one female with oostegites and one non-ovigerous female in two vials ( ZRC 2019.0587 ), Tanjung Hakim , St. John’s Island (1°13.413’ N, 103°50.697’ E), rocky shore, intertidal, 0.5 m above chart datum, 19 June 2014 ; one female in one vial ( ZRC 2019.0588 ), Tanjung Hakim , rocky shore, intertidal, 0.9 m above chart datum, 12 March 2014 ; one female and one manca in two vials ( ZRC 2019.0589 ), Tanjung Lokos, St. John’s Island (1°12.856’ N, 103°51.051’ E), rocky shore, intertidal, 0.6 m above chart datum, 22 March 2014 ; all specimens were collected by the first author from the inside of intact tests of dead barnacles of the genus Tetraclita .

Diagnosis. As with characters of genus.

Description. Non-ovigerous female holotype ZRC 2019.0582 (note: right mandible based on paratype ZRC 2019. 0584).

Body ( Fig. 1A ) small, and five times as long as wide. Cephalothorax subtriangular and about 21% of body length; dorsal surface with one short seta posterior to each eye, two anterior setae posterior to the eyes, and one medial seta and one posterior seta on each lateral margin.

Pereon about 55% of body length; pereonite 1 shortest with two pairs of anterodorsal setae and one seta on each lateral side; pereonites 2 and 6 about 1.3 times as long as pereonite 1; pereonites 3 and 5 about 1.5 times as long as pereonite 1; pereonite 4 longest and 1.8 times as long as pereonite 1; pereonites 2 to 6 each bearing a pair of anterodorsal setae and two setae on each lateral side.

Pleon about 24% of body length; pleonite 1 longest; pleonites 2 and 3 subequal in length and each 0.75 times as long as pleonite 1; pleonite 4 0.5 times as long as pleonite 1; pleonite 5 and pleotelson fused and almost as long as pleonite 1; pleonites 1 and 2 each with dorsal transverse row of about 36 feather setae posteriorly; pleonites 1 to 3 each with about seven feather setae along lateral margin and simple seta at anterolateral margin; pleonite 4 with one short and one very long simple setae on mid-lateral margin and two simple setae on dorsal margin; pleotelson ( Fig. 1A , 4H ) with four setae on dorsal margin, two juxtaposed setae on anterolateral bulge, two setae on each posterolateral margin and eight simple and two plumose setae on posterior margin.

Antennule ( Fig. 2A ) about 0.5 times as long as cephalothorax; article-1 about 0.6 times as long as the total length of antennule, with two plumose setae proximally, four plumose setae and one long simple seta distally and some very fine setae; article-2 about 0.5 times as long as article-1, with some very fine setae proximally and four plumose setae and one long simple seta distally; article-3 about 0.6 times as long as article-2, with one plumose seta and two simple setae distally; article-4 very short, 0.2 times as long as article-3, with two aesthetascs and five simple setae.

Antenna ( Fig. 2B ) slightly shorter than antennule; articles-1 and -3 subequal in length and naked; articles-2 and 3 with dorsal flange; article-2 widest, about 1.5 times as long as article-1, with one simple seta at about mid-length and another at distal end; article-4 longest, about 2.2 times as long as article-3, with one plumose seta at mid-length and three plumose setae and two simple setae at distal end; articles-5 and -6 shortest and subequal in length; article-6 with five simple setae.

Labrum ( Fig. 3A ) hood-shaped, densely covered by long fine setae. Left mandible lacinia mobilis tooth-like, with undulating outer margin and long setulose seta at base (in holotype; simple seta in paratype ZRC.2019.0584, Fig. 3B ), incisor with undulate distal margin, molar process broad and corrugate. Right mandible (of holotype destroyed during dissection; description based on paratype ZRC.2019.0584, Fig. 3 B`) similar to left mandible but with smaller lacinia mobilis and naked seta. Labium ( Fig. 3C ) with inner and outer lobes covered by long fine setae at distal margins, outer lobes with fine setae on lateral margin. Epignath ( Fig. 3D ) kidney-shaped, with very long terminal feather seta. Maxillule ( Fig. 3E ) endite with eight distal spiniform setae and many long and short simple setae; palp with three long finely setulose setae. Maxilliped ( Fig. 3F ) coxa with one feather seta; basis with one simple seta at distal end; endite with some simple setae laterally, two spiniform setae subdistally and one short and one long circumplumose setae along the distal margin; palp article-1 with numerous fine setae at distal end; article- 2 outer margin with four simple setae, inner margin with three spiniform and two simple setae; article-3 with ten simple setae; article-4 with nine dorsal feather setae and one simple seta on mid-length of outer margin.

Cheliped ( Fig. 2C ) basis 1.2 times as long as wide, with one simple seta dorsodistally and one simple seta ventrodistally; merus triangular, with one simple seta proximally and two simple setae ventrodistally; carpus 1.4 times as long as wide, with one simple seta dorsomedially, two simple setae dorsodistally and two simple setae ventrodistally; propodus including fixed finger as long as carpus, 0.8 times as wide, with one simple seta ventrally and two outer simple setae and one plumose inner seta near dactylus insertion ( Fig. 5A ); fixed finger distally flattened into anvil-shaped process, outer margin with tubercle bearing four simple setae, inner margin with two simple setae; dactylus with one spiniform seta dorsomedially and one simple seta ventrodistally; unguis slightly larger than distal claw on fixed finger.

Pereopod 1 ( Fig. 4A ) longest, with very fine setae observed on all articles except unguis; coxa with one spiniform and one simple setae ( Fig. 5B ); basis four times as long as wide, with one simple and one plumose setae dorsoproximally and one simple seta at ventrodistal corner; merus 0.3 times as long as basis, with one simple seta at ventrodistal corner; carpus 0.4 times as long as basis, with one simple seta dorsally and one simple seta ventrodistally ( Fig. 5C ); propodus 0.5 times as long as basis, with one simple seta dorsodistally, three simple setae ventrodistally and one pinnate inner seta along distal margin ( Fig. 5D ); dactylus and unguis slender and almost straight, dactylus with one simple seta on inner margin ( Fig. 5D ), unguis slightly longer than dactylus.

Pereopod 2 ( Fig. 4B ) with very fine setae observed on all articles except coxa and unguis; coxa with one simple seta; basis 2.7 times as long as wide, with one simple and two plumose setae dorsomedially, one simple seta at ventrodistal corner; merus 0.4 times as long as basis, with one spiniform and one simple setae at ventrodistal corner; carpus 0.3 times as long as basis, with four outer and three inner spiniform setae along distal margin; propodus slightly longer than merus, with one simple seta at dorsodistal corner and one simple and two small spiniform setae at ventrodistal corner; dactylus and unguis stout and curved, dactylus with short seta at dorsodistal corner, unguis 0.4 times as long as dactylus.

Pereopod 3 ( Fig. 4C ) with very fine setae observed on all articles except coxa and unguis; coxa with one simple seta; basis 2.1 times as long as wide, with one large plumose seta dorsomedially and one simple seta at ventrodistal corner; merus 0.6 times as long as basis, with one simple and one spiniform setae at ventrodistal corner; carpus 0.3 times as long as basis, with four outer and three inner spiniform setae along distal margin; propodus slightly longer than merus, with one simple seta at dorsodistal corner and one simple and two small spiniform setae at ventrodistal corner; dactylus and unguis stout and curved, dactylus with short seta at dorsodistal corner, unguis 0.4 times as long as dactylus.

Pereopod 4 ( Fig. 4D ) with very fine setae observed on all articles except coxa; basis 2.5 times as long as wide, with three plumose setae dorso-proximally, one plumose seta ventromedially and one simple and one plumose setae at ventrodistal corner; merus 0.4 times as long as basis, with one simple seta dorsally and two setulose spiniform setae at ventrodistal corner; carpus slightly shorter than merus, with one simple seta dorsally and four outer and four inner setulose spiniform setae along distal margin; propodus 0.5 times as long as basis, with one plumose seta dorsodistally and one bipinnate outer seta and one bipinnate and one shorter simple inner setae along distal margin; dactylus and unguis fused to form a claw, with lateral row of fine setae at mid-length, two small spines ventrally and one short seta dorsodistally.

Pereopod 5 ( Fig. 4E , 5E ) identical to pereopod 4, except that the basis is slightly stouter, 2.2 times as long as wide.

Pereopod 6 ( Fig. 4F ) with very fine setae observed on all articles except coxa; basis 2.3 times as long as wide, with one plumose seta dorso-proximally and one simple seta at ventrodistal corner; merus 0.5 times as long as basis, with one simple seta dorsally and two setulose spiniform setae at ventrodistal corner; carpus 0.4 times as long as basis, with one simple seta dorsally and four outer and three inner setulose spiniform setae along distal margin ( Fig. 5F ); propodus slightly shorter than merus, with one plumose seta dorsodistally and one simple, one setulose and three flattened denticulate outer setae and two simple inner setae along distal margin ( Fig. 5G ); dactylus and unguis fused to form claw, with lateral row of fine setae at mid-length, two small spines ventrally and one setulose seta dorsally ( Fig. 5H ).

Pleopods 1–3 ( Fig. 4G ) Similar; basal article outer margin with one feather seta proximally, inner margin with one plumose seta proximally; exopod outer margin with 28 feather setae; endopod outer margin with 12–14 feather setae and one distal spiniform seta, inner margin with fine setae along proximal two-third and two proximal feather setae.

Uropod ( Fig. 4I ) basal article 1.9 times as long as wide, with three simple setae distally; endopod two-articled; article-1 with one simple seta distally; article-2 1.6 times as long as article-1, with three plumose setae subdistally and seven simple setae distally.

Female paratype ZRC 2019.0584 ( Fig. 1C ): Similar to female holotype except having (1) slightly more slender body at six times as long as wide, (2) simple seta on left mandible lacinia mobilis, (3) pleopod endopod inner margin with three setae. Pereopod 4 coxae with a pair of sac-like oostegites.

Male allotype ZRC 2019.0583 ( Fig 1B ): Similar to female paratype except antennule longer (0.8 times as long as cephalothorax; Fig. 2A ), antenna with relatively longer article-4 (three times as long as article-3; Fig. 2B ), cheliped dactylus cutting edge with several small denticles ( Fig. 2C ).

Etymology. The specific epithet refers to the microhabitat where the species was found, living (Latin = - colus) inside barnacles (Greek = balano).

Type locality. Lazarus Island, Singapore.

Distribution and habitat. Lazarus and St. John’s Islands, Singapore, intertidal rocky shores and seawalls, from inside Tetraclita barnacle tests.

Associated fauna. An acotylean flatworm, nematodes, mytilid bivalve Brachidontes sp., littorinid gastropod Peasiella patula Reid & Mak , haminoeid gastropod Smaragdinella sp., gastropod veligers, syllid polychaetes, mites, sesarmid crab Nanosesarma sp., sphaeromatid isopod Dynamenella ptychura Harrison & Holdich , copepods, ostracods and insect nymphs and larvae.

Remarks. Xenosinelobus balanocolus n. sp. is most similar to Tanais dulongii (Audouin) if based on conspicuous morphological features and these include the (1) four free pleonites, (2) complete dorso-transverse row of setae on pleonites 1 and 2, (3) plumose inner seta near dactylus insertion on cheliped propodus, (4) presence of a seta on pereopod 1 dactylus, (5) claw-like terminal articles on pereopods 2 and 3, (6) comb-like row of flattened denticulate and setulose setae on distal margin of pereopod 6. However, upon examination of dissected mouth parts and appendages under high magnification, it is apparent that X. balanocolus belongs to the subfamily Sinelobinae and is more closely related to the genus Sinelobus . The characters that Xenosinelobus share with Sinelobus include (1) three or fewer aesthetascs on the terminal article of antennule, (2) antenna with six articles but without setal tuft on articles-2 and -4, (3) no terminal processes on the labium outer lobes, (4) three or fewer setae on maxillule palp, (5) three or fewer ventral setae on pereopod 1 propodus, (6) eight or fewer spiniform setae on pereopods 2–6 carpus, (7) six or fewer outer setae and one inner seta on pleopod basal article, (8) 28 or fewer outer setae on pleopod exopod, (9) 14 or fewer outer setae and three or fewer inner setae on pleopod endopod.

Xenosinelobus balanocolus possesses characters that are not only unique within the subfamily Sinelobinae but also the family Tanaididae . One of these is the unusually short fifth antennal article, which is as short as the sixth antennal article. In other tanaidids, the fifth antennal article is much longer, and of similar length to the fourth antennal article. It is common for the lacinia mobilis to be tooth-like on the left mandible and of a less-developed form (e.g. absent, thorn-like, peg-like) among tanaidids, but it is only in the present species that the lacinia mobilis of both left and right mandibles are tooth-like. The epignath of X. balanocolus has a terminal seta that is almost similar in length while that of other tanaidids has a terminal seta that is barely a quarter as long. Even though simple setae are present on the pereopod 1 coxa of many tanaidids, X. balanocolus is the only species with a spiniform seta on the same article. The presence of a dorsal seta on the claws of pereopods 4–6 in the present species is unusual because it is absent in other tanaidids. Three-articled uropods (basal article included) are rare among tanaidids, and are present only in the present species and Tanais dulongii (Audouin) . The more peculiar condition is the relative length of the uropod final article to the preceding article. In X. balanocolus , this proportion is 1.6 times whereas the uropod final article is either shorter or of the same length as the preceding article in other tanaidids. There are also many fine setae on the pereopods of X. balanocolus , and they can be easily overlooked due to their thinness. These fine setae are not as short and regularly arranged as microtrichia that are present in other tanaidids.

Xenosinelobus balanocolus is a small tanaid that is similar in size to other sinelobines, with the exception of Parasinelobus chevreuxi (Dollfus, 1898) and Sinelobus pinkenba , which are 3.5 and 2.0 times as long, respectively. While information on Parasinelobus males remain unavailable, the males of almost all Sinelobus species are known to exhibit strong sexual dimorphism and can be readily distinguished from their conspecific females by their much longer antennules and antennae, conspicuous pear-shaped cephalothorax and more robust and/or dimorphic chelipeds. Bamber (2008) proposed that the low level of sexual dimorphism in S. pinkenba is due to the males not being fully mature. This explanation is plausible and can also be applied on the present species. Mancae of X. balanocolus have none or under-developed pereopods 6 and body length of 0.5–0.9 mm.

Figures

FIGURE 1. Xenosinelobus balanocolus n. sp. Lazarus Island, Singapore. A, non-ovigerous female holotype (ZRC 2019.0582), habitus, dorsal view; B, male allotype (ZRC 2019.0583), habitus, dorsal view; C, female with oostegites paratype (ZRC 2019.0584), habitus, lateral view. Scale bar 0.5 mm.

FIGURE 2. Xenosinelobus balanocolus n. sp. Holotype female and allotype male. A, antennules; B, antennae; C, chelipeds. Scale bars 100 µm.

FIGURE 3. Xenosinelobus balanocolus n. sp. Holotype female (A, C–G) and paratype female (ZRC 2019.0584) (B). A, labrum; B, left and B`, right mandibles; C, labium; D, epignath; E, maxillule; F, maxilliped. Scale bar 100 µm.

FIGURE 4. Xenosinelobus balanocolus n. sp. Holotype female. A–F, pereopods 1–6; G, pleopod 3; H, pleotelson; I, uropod. Scale bar 100 µm.

FIGURE 5. Xenosinelobus balanocolus n. sp. Holotype female. A, inner seta near dactylus insertion on cheliped propodus; B, spiniform seta on pereopod 1 coxa; C, very fine setae on pereopod 1 carpus; D, inner setae on pereopod 1 propodus and dactylus; E, dorsal seta on pereopod 5 claw; F, spiniform setae on pereopod 6 carpus; G, setal row at distal margin of pereopod 6 propodus; H, dorsal seta on pereopod 6 claw. Scale bars 50 μm.

Tables

TABLE 1. Comparison of eight species in the three genera Parasinelobus, Sinelobus and Xenosinelobus n. gen. belonging to the subfamily Sinelobinae.

Species	P. chevreuxi	S. stanfordi (Rich-	S. pinkenba	S. barretti	S. bathykolpos	S. vanhaareni	S. stromatoliticus	X. balanocolus
	(Dollfus, 1898)	ardson, 1901)	Bamber, 2008	Edgar, 2008	Bamber, 2014	Bamber, 2014	Rishworth, Perissino-	n. gen. n. sp.
							tto & Błażewicz, 2018
Body length	7.3 mm	2.8 mm	Female: 4.1 mm	Female: 2.3 mm	Female: 1.6 mm	Both sexes: 2.7	Female: 2.2 mm	Female: 2.1
			Male: 3.3 mm	Male: 2.4 mm	Male: 1.8 mm	mm	Male: 1.8 mm	mm
Sexual dimorphism	Unknown	Strong	Weak	Strong	Strong	Strong	Strong	Male. 2.0 mm Weak
Antennule terminal	3	Female: 2	3	Female: 1	1	1	2	Female: 2
article aesthetascs Antenna articles-2	Present on	Male: 3 Absent	Present on both	Male: 2 Present on	Present on both	Absent	Present on both	Male: 3 Present on both
and -3 flange Antenna articles-2	article-2 only Present	Absent	articles Absent	article-2 only Absent	articles Absent	Absent	articles Absent	articles Absent
and -4 setal tuft Antenna article-5	As long as	0.7 times as long	0.6 times as long	As long as	As long as	0.9 times as	0.6 times as long as	Less than 0.1
	article-4	as article-4	as article-4	article-4	article-4	long as article-	article-4	times as long
Left mandible	Tooth-like, with	Tooth-like, with	Tooth-like, with-	Tooth-like,	Tooth-like, with	4 Tooth-like,	Tooth-like, with one	as article-4 Tooth-like,
lacinia mobilis Right mandible	two setae Peg-like, with	one seta Peg-like, with one	out seta Peg-like, with-	without seta Peg-like, with-	one seta Peg-like, with	with one seta Peg-like, with	seta Peg-like, with one seta	with one seta Tooth-like,
lacinia mobilis Maxillule endite	one seta 9	seta 8	out seta 7	out seta 7	one seta 7	one seta 8	8	with one seta 8
distal spiniform
setae Maxillule palp setae	13	2	2	2	2	2	2	3
Epignath terminal	Short	Short	Short	Short	Short	Short	Short	Long
seta Cheliped propodus	One simple seta	One simple seta	Two simple	One simple seta	Two simple	Two simple	Naked	One plumose
inner setation near			setae		setae	setae		seta
dactylus insertion Pereopod 1 coxa	Absent	Absent	Absent	Absent	Absent	Absent	Absent	Present
spiniform seta Pereopod 1 propo-	2, 10, 0	1, 3, 0	2, 1, 0	1, 1, 0	2, 1, 0	2, 1, 0	2, 1, 0	1, 3, 1
dus dorsal, ventral
and inner setae