Synallactesmcdanieli sp. nov., a new species of sea cucumber from British Columbia, Canada and the Gulf of Alaska, USA (Holothuroidea, Synallactida)

Abstract Background The family Synallactidae comprises mostly deep-sea forms and is the least-studied large taxon amongst deep-sea cucumbers. They are part of the abyssal megafauna and play an important role in modifying the sediment landscape and structuring the communities that live within it. The family embraces the genus Synallactes, which contains approximately twenty-five species from the Pacific, Atlantic (six species), Indian (seven species) and Antarctic Oceans (one species). New information Synallactesmcdanieli sp. nov. is described from the Northeast Pacific, Knight Inlet, British Columbia, Canada to Kodiak Island, Gulf of Alaska, USA, at depths from 21 to 438 m. This new species is unique amongst the species of the genus Synallactes because of the number and arrangement of dorsal papillae, number of Polian vesicles, together with the entire ossicle arrangement. In addition, this species has the shallowest bathymetric distribution ever recorded for this genus.


Introduction
The family Synallactidae Ludwig, 1894 comprises mostly deep-sea forms and is the leaststudied large taxon amongst deep-sea cucumbers (Solis-Marin 2005).Synallactids are one of the most characteristic animals of the deep ocean.They often appear in photographic collections of abyssal megafauna (Bluhm andGebruk 1999, Smet et al. 2021).Many of these photographs show their characteristic tracks and faecal remains (Young et al. 1985, Bluhm and Gebruk 1999, Bribiesca-Contreras et al. 2022) providing evidence of their important role in modifying the sediment landscape and in structuring the communities that live within it (Roberts et al. 2001).The majority of synallactids appear to spend their life on the sediment surface and some species are capable of active swimming, such as Bathyplotes natans (Sars, 1868) and Paelopatides confundens Théel, 1886 ( Miller and Pawson 1990).The epibenthic species traverse the seabed, feeding on the uppermost layer of sediment, for example, Mesothuria verrilli (Theel, 1886).
The family Synallactidae formerly belonged in the order Aspidochirotida Grube, 1840, but was later transferred to the order Synallactida Miller, Kerr, Paulay, Reich, Wilson, Carvajal and Rouse, 2017.The order Synallactida includes the families Deimatidae Theel, 1882, Stichopodidae Haeckel, 1896 and Synallactidae Ludwig, 1894.The last family embraces the genus Synallactes Ludwig, 1894, which contains approximately twenty-five species.As far as we know, eleven of these species occur in the Pacific Ocean: Synallactes aenigma, S. alexandri, S. chuni, S. discoidalis, S. gilberti, S. horridus, S. multivesiculatus, S. nozawai, S. sagamiensis, S. triradiata and S. virgulasolida.The remaining species inhabit the Atlantic Ocean (six species), the Indian Ocean (seven species) and the Antarctic Ocean (one species).The purpose of this paper is to describe a new species of Synallactes from the Northeast Pacific.

Materials and methods
Specimens are housed in the Royal British Columbia Museum, Invertebrate Zoology Collection, Victoria, B.C., Canada.Ossicles were extracted from the body wall (anterior, middle and posterior regions), dorsal papillae, ventral tube feet, tentacles and gonads.The tissue was dissolved in fresh household bleach (5-6.5%).After centrifugation at 1000 rpm for 10 min, bleach was pipetted off and the ossicles were rinsed and centrifuged with distilled water that was subsequently pipetted off.The same process was done with 70, 80 and 95% ethanol.Absolute ethanol was added to the ossicles and a small aliquot was placed to dry on a cylindrical double-coated conductive carbon tape stub.Then, it was sputter-coated with gold 2.5 kV in the ioniser JEOL JFC-1100 for 3 min and photographed using a JEOL JSM-6360LV scanning electron microscope (SEM) at the ICML, UNAM.

Abbreviations used in the text:
ICML, UNAM Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México; RBCM, Royal British Columbia Museum, Victoria, British Columbia, Canada.TL, total length.

Description
Holotype description.Specimen 310 mm long; firm, slightly rough skin.Colour in alcohol light violet, the dorsal side more colourful than the ventral area where the prevailing colour is whitish-beige.Body subcylindrical, slightly flattened, more tapering posteriorly than anteriorly.Mouth ventral, anus terminal, both surrounded by small papillae (1.0-1.7 mm long).Peltate tentacles 20, each with 9-10 distal digitations.Subcylindrical tube feet ventrally (0.8-3 mm long), restricted to the ventral ambulacra.Distal end of feet with supporting sucking discs.The odd ambulacrum has two zigzag rows of about 62 tube feet each, ventrolateral ambulacra each with a zigzag row of 45 tube feet along the margin of ventral side.On the dorsal side are long papillae, 13 mm long and 4 mm across at base, most situated on conical warts.They form four parallel rows, each consisting of about 25-30 papillae.Papillae of the central dorsum are larger than those of the rest.Much smaller papillae belonging to ventrolateral ambulacra form a marginal fringe around the mouth and anus.
Calcareous ring composed of five radial and five interradial plates.Small interradial pieces with one central anterior process (Fig. 1) and massive radial pieces with a posterior notch.The stone canal is fixed dorsally to the skin by the madreporic plate.
There is one Polian vesicle.There are two well-developed respiratory trees, branched, occupying almost the entire length of the body.They consist of a long common stem which bifurcates into two short vessels.Gonad branched, disposed in two tufts.The longitudinal muscles are not divided.
Ossicles.There are few ossicles in the dorsal and ventral skin.Most are in the dorsal papillae, the ventral tube feet and the tentacles.Internally there are very scarce ossicles in the gonads.
The body wall contains small (40-100 µm in diameter) and large (250-316 µm in diameter) tri-or quadri-radiate tables (Fig. 2D and E1).The end of each arm is bifurcated several times or perforated and spatulate in shape, some of them forming a brief lattice-like network.Centrally, there is one pillar (50-60 mm tall in the small tables and 70-100 mm tall in the large tables) which may be terminally divided in a single point or in 2-4 spines or perforated.
The tube feet contain rods (Fig. 2C and 2E2), quadri-and pentaradiate tables (Fig. 2B) and an end plate (Fig. 2A).The rods, which are straight or curved and sometimes forked, have perforated ends.They are 300-690 mm long.These rods are spiny, the lateral spines sometimes branched (Fig. 2C).The end plate reaches 1 mm in diameter and is composed of a single perforated plate (Fig. 2A).
The dorsal papillae contain rods and tri-, quadri-and pentaradiate tables which are particularly densely packed at the tip of the papillae (Fig. 3A), some of them forming a brief lattice-like network.The rods at the base of the papillae (Fig. 3B and 3C) are similar to those of the tube feet, whereas the rods at the tip of the papillae are long (700-900 mm), thin and smooth with perforated ends (Fig. 3C).The quadri-radiate tables (Fig. 3A) are numerous and smaller (120-130 mm in diameter) than in the body wall.
The tentacles contain only rods which are straight (Fig. 2F1) or curved, forked and sometimes branched (Fig. 2F2).They are spiny and measure 400-700 mm long.Gonads with irregular calcareous bodies 10-20 mm (Fig. 4) branched and unbranched rods with pointed ends.Some rods with a single knobbed centre.Respiratory trees devoid of any ossicles.
Colour of live specimens is pale pink violet on the dorsum and same colour, but lighter on the ventrum (Fig. 5).Preserved specimens can retain some violet colour on the dorsal area, but normally they are completely beige.
Paratype variations: Specimens range from 84-130 mm in length.Ossicles: The body wall contains abundant tri-, quadri-or pentaradiate tables, with spatulated arm ends.The end of each arm is bifurcated several times or perforated, sometimes there are lateral processes which may unite some arms.The spire consists of a single pillar, which may be divided or perforated, or both, at the terminal end.One or two pairs of small, short, and robust spines project on the lateral sides of the upper end of the spire.There are tables, robust supporting spiny rods, and terminal disks in the tube feet.Papillae contain massive rods (smooth or branched), delicate rods and tables which are particularly densely packed at the tip of the papillae.Tri-, quadri-and pentaradiate tables are present.Tentacles with curved or straight spiny rods.Gonads with irregular calcareous bodies.Respiratory trees devoid of any ossicles.

Etymology
This species is named after Neil McDaniel, long-time Canadian marine naturalist, photographer and videographer, in recognition of his many contributions to marine sciences.The epithet is a noun in the genitive case.

Ecology
Synallactes mcdanieli sp.nov.was collected at 18 different stations between 21 and 380 m depth.Edwards (1907) mentions that the maximum depth of the specimens obtained in Alaska (as S. challengeri) was 438 m.The species occurs mainly on sandygravelly bottoms and amongst boulders and cobble substratum (Fig. 5).This species feeds on bottom sediments with its peltate tentacles like its congeners.
S. nozawai possesses an external morphology very similar to S. mcdanieli sp.nov., but differs in the number of dorsal papillae, six and four rows, respectively.Furthermore, ossicles in S. nozawai are nearly all quadri-radiate tables, very rarely tri-radiate, while in S. mcdanieli sp.nov., the body wall contains tri-and quadri-radiate tables.The table spires of S. nozawai can have from one to three holes at the tip and more spinelets at the top than those in S. mcdanieli sp.nov.
In addition to the more northern geographical distribution of S. nozawai (Bering Strait) in the Northeast Pacific, its bathymetric range (108-787 m) is deeper than in S. mcdanieli sp.nov.as currently known.
Synallactes triradiata is also very similar in external appearance to S. mcdanieli sp.nov., but has six longitudinal rows of dorsal papillae instead of four.Internally, S. triradiata differs from S. mcdanieli sp.nov. in having a variable number of polian vesicles (1-3) and the calcareous deposits are tri-radiate tables (arms of which stand 120˚ apart) with the spire terminating in several points.In addition to the above characteristics, S. triradiata inhabits Sagami Bay and Sagami Sea (Mitsukuri 1912) and the Northeast Pacific Ocean: Bering Sea, Alaska, Aleutian Islands, Fox Islands, Unalaska Bay, at depths from 640-1092 m (Solis-Marin 2003).Edwards (1907) mentioned the existence of S. challengeri in the Gulf of Alaska based on six specimens collected between 87 to 438 m depth, on green mud, fine sand; this was followed by Lambert (1997) and Lambert and Boutillier (2011).As stated by Solis-Marin (2003) and Massin and Hendrickx (2010), the presence of S. challengeri along the west coast of North America up to the coast of California was putative and was in need of review.Indeed, S. challengeri is known from sub-Antarctic islands (HMS Challenger St. 148a, 46º 53' S, 51º 52' E, 990 m depth) (Théel 1886, Massin 1992); thus, we consider the specimens described by Edwards (1907) and Lambert (1997)
to be Synal, Massin 1992)li sp.nov.andnot S. challengeri.S. mcdanieli sp.nov.hasdorsalconicalpapillaearranged in four longitudinal series confined to ambulacra at almost regular intervals.The number of Polian vesicles is also remarkable: S. mcdanieli sp.nov.hasonlyonevesicleand, in S. challengeri, that number is variable from two to five(Théel 1886, Massin 1992).Quadriradiate ossicles from the papillae vary from 20-350 mm in S. challengeri, while table sizes of S. mcdanieli sp.nov.areonlylarge,from 300-320 mm.The new species has tri-, quadri-and pentaradiate tables densely packed at the tip the papillae and also massive (smooth or branched), delicate rods.The body wall of the two species has triand quadri-radiate tables with similar disc diameter (35-100 µm) and spire 40-80 µm tall.The main difference between the two species is that S. mcdanieli sp.nov.also has larger ossicles from 250-280 µm in diameter and a central pillar with a spire from 50-60 mm tall in the small tables and 70-100 mm tall in the big tables.Species of the genusSynallactes are mostly found in deep water.Only three previously described species have their shallow bathymetric distribution limits at depths less than 200 m: S. multivesiculatus (194 m), S. sagamiensis (180 m) and S. nozawai (108 m).Only S. mcdanieli sp.nov.rangesfrom shallow (21 m) to deep water (438 m).Synallactes mcdanieli sp.nov. is unique amongst the species of the genus Synallactes because of the number and arrangement of dorsal papillae and polian vesicles, together with the entire ossicle arrangement.• Smet BD, Simon-Lledo E, Mevenkamp P, Pasotti F, Jones D, Vanreusel A (2021) The megafauna community from an abyssal area of interest for mining of polymetallic nodules.Deep Sea Research Part I: Oceanographic Research Papers 172.• Solis-Marin FA (2003) Systematics and phylogeny of the holothurian family Synallactidae.University of Southampton • Solis-Marin FA (2005) Synallactes laguardai, a new species of sea cucumber from South Africa (Echinodermata: Holothuroidea: Aspidochirotida: Synallactidae).Proceedings of the Biological Society of Washington 118 (3): 570-575.https://doi.org/10.2988/0006-324X(2005)118[570:SLANSO]2.0.CO;2 • Thandar AS, Rambaran R (2015) On some sea cucumbers (Echinodermata: Holothuroidea) from off the south and west coasts of South Africa collected by the South African Environmental and Observation Network (SAEON).Zootaxa 3999: 41-61.• Théel H (1886) Report on the Holothuroidea dredged by the HMS Challenger during the years 18873-1876.Part II.Report of the Scientific Results of the Voyage of H.M.S. Challenger 1873-1876.Zoology 14: 1-290.• Young DK, Jahn WH, Richardson MD, Lohanick AW (1985) Photographs of the deepsea Lebensspuren: A comparison of sedimentary provinces in the Venezuela Basin, Caribbean Sea.Marine Geology 68: 269-301.https://doi.org/10.1016/0025-3227(85)90016-7 Synallactes challengeri has a total length that varies from 69 to 160 mm (Théel 1886, Massin 1992, Thandar and Rambaran 2015), whereas S. mcdanieli sp.nov. is larger, ranging from 84 to 310 mm.As Théel mentioned in 1886, S. challengeri specimens have dispersed papillae on the ambulacral and interambulacral areas with visible rows laterally, while