Heart urchins from the depths: Corparva lyrida gen. et sp. nov. (Palaeotropidae), and new records for the southwestern Atlantic Ocean

Introduction: Sea urchins in the order Spatangoida are the most diverse group of extant echinoids. Objective: Describe a new genus and species of Spatangoida from abyssal depths, and add new records for known species. Methods: Specimens were collected during several cruises at different areas of the southwestern Atlantic Ocean (SWAO), among 37-55° S latitude at depths ranging from 55 to 3 000 m. We present morphological and ultrastructure analyses. Results: Corparva lyrida gen. et sp. nov. (Palaeotropidae) is described from the Mar del Plata Canyon on the Argentine continental slope (2 950 m depth), the first record of this family from Argentina. Corparva gen. nov. differs in having an apical system semi-ethmolytic, and labrum reaching to rear part of second adjacent ambulacral plate. We also report the northernmost distribution and deepest record for Brisaster moseleyi (38° S latitude, 2 212 m depth), the northward extension of the distribution range of Tripylus excavatus (39° S latitude, 74 m depth), and the first record of Abatus philippii and Abatus agassizii at the Burdwood Bank/ MPA Namuncurá. Conclusions: The present work brings novel and updated data about the diversity and distribution of spatangoids from the SWAO, including the description of C. lyrida gen. et sp. nov., and new records of species. This shows how much remains to be known about the diversity and distribution of heart urchins in the SWAO, especially from the deep-sea.

considered important components of benthic communities. Spatangoids are deposit-feeders, they ingest a large amount of detritus from the sediment and cause bioturbation due to their locomotion and feeding mode (De Ridder, Jangoux & De Vos, 1987;Lohrer, Thrush, Hunt, Hancock & Lundquist, 2005). Concerning their reproductive aspects, spatangoids show broadcasting or brooding spawning modes, and indirect or direct developmental strategy, these life-history traits affects particularly the early larval dispersion and the distribution patterns of species (Poulin & Féral, 1996, 1998. The southwestern Atlantic Ocean (SWAO) receives waters formed in remotes areas of the world, generating one of the most energetic oceanic regions of the world (Piola & Matano, 2001). Along the Argentine continental shelf, the upper ocean is mainly dominated by the Malvinas Current, a branch of the Antarctic Circumpolar Current that flows offshore to north, the Brazil Current, a branch of the subtropical gyre that flows south along the continental slope of South America, and by the Brazil/Malvinas Confluence, the collision of these two opposite currents near 38° S latitude (Matano, Palma & Piola, 2010). Also, the deep layers from the continental slope off Argentina are characterized by a variety of water masses with origin in deep and bottom waters from the North Atlantic, South Pacific, and Antarctic regions (Piola & Matano, 2001). According with several authors, two biogeographic provinces are recognized on the Argentine continental shelf by the patterns of distribution of different macroinvertebrates taxa: the Argentine Biogeographic Province (ABP), and the Magellan Biogeographic Province (MBP) (Bernasconi, 1964;Bastida, Roux & Martínez, 1992;Doti, Roccatagliata & López Gappa, 2014). The ABP extends from 23° S (Cabo Frio, Brazil) to 43° S (Península Valdés, Argentina), while the MBP is present on both sides of South America, from the southeastern Pacific Ocean at 41° S (Puerto Montt, Chile) reaching around the tip of South America into the SWAO at 43° S latitude, where the MBP turns aside from shallow shelf waters following a northeastern direction to more deep shelf waters, as far as 35° S latitude (López Gappa, Alonso & Landoni, 2006;Doti et al., 2014).
This paper aims to contribute to improving the knowledge of the diversity and distribution of the Spatangoida from the SWAO, through the study of specimens recently collected.
Here we describe a new genus and species of Palaeotropidae from the Mar del Plata Canyon, which represents the first report for this family in Argentina. Besides, we inform new records in the SWAO for other known species.

MATERIALS AND METHODS
The study area is located at the southern extreme of the SWAO, from 37° to 55° S latitude at depths encompassing from the shallow waters of the continental shelf to abyssal waters of the slope off the Argentine coast (Fig. 1A).
Specimens were collected using an adapted towed dredge or a bottom trawl net. The towed dredge used consisted of a rigid body structure 150 cm long and 100 cm wide, with a 60 cm wide by 40 cm high mouth frame and carrying two joint fishing nets with two different pore-size (an external one of 3x3 cm, with another internal one of 1x1 cm).
Once onboard, specimens were fixed in 96 % ethanol. All the specimens are now housed Gross morphology of specimens was studied under naked eye and using a stereomicroscope, while the ultrastructure of several appendages was analyzed by scanning electron microscopy (SEM). Test measures were made with a digital caliper. To be able to reveal the arrangement of the plates, some specimens were carefully cleaned. Appendages and soft tissue were removed from the test with tweezers, and a small brush soaked with a diluted solution of sodium hypochlorite was used to remove the organic remains. When the plates and sutures were exposed and visible, the whole specimen was rinsed in distilled water. Plates were labelled according to the standard nomenclature for the test plating provided by Lovén (1874).
For SEM images, appendages (spines, pedicellariae, sphaeridia, and tube feet) were removed from different parts of the test and placed into distilled water. Organic remains were cleared with a diluted solution of sodium  hypochlorite for a few minutes. The appendages were rinsed in distilled water twice, followed by a third rinse with 96 % ethanol to allow rapid air drying. Finally, appendages were transferred to aluminium stubs, sputtered with gold-palladium, and examined and photographed with a Philips XL30 scanning microscope at the Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" (MACN). Complete specimens were photographed using a Nikon D800 camera with a Micro-Nikkor 60 mm f/2.8 lens, and details of the plates from the test were photographed with a Zeiss Discovery V20 stereoscopic microscope. This article is registered in ZooBank under ID: urn:lsid:zoobank.org:pub:2FBDDA8F-DB8A-4266-B606-553AF5C2F366.

RESULTS
In all, 299 specimens of spatangoids were collected and studied, identifying eight species distributed in two families, and six genera. In the present study, we report a new genus and species Corparva lyrida (Spatangoida: Palaetropidae) along with the occurrence and new records in the SWAO of known species.
In the Taxonomic List are listed the species identified following the classification of Kroh (2020) for higher-level taxa,  for the family Schizasteridae and its lower-level taxa, and Kroh & Mooi (2020) for the Palaeotropidae family.
As Néraudeau, David & Madon (1998) pointed out, the arrangement of the plates in the apical system should not be used as a morphological character to differentiate at family level, due to changes in the structure of the apical system occurred convergently in many different groups of spatangoids. Thus, we consider the semi-ethmolytic pattern of Corparva gen. nov. and the labral plate extending to the rear part of second adjacent ambulacral plate as features to separates it from the other genera of Palaeotropidae.
Palaeobrissus A. Agassiz, 1883 differs by its ethmolytic apical system with fused genital plates, anterior, two gonopores in the posterior genital plates surrounded by a rim and sometimes two smaller anterior gonopores in large specimens, paired ambulacra flush, subpetaloid with ambulacral pores double, peristome not sunken and transversally oval, labral plate extending to first adjacent ambulacral plate, slightly projecting over peristome, and rounded subanal fasciole present in juveniles (lost in adults) (Agassiz, 1883;1950;Kroh & Mooi, 2020).
Etymology: lyrida refers to the April Lyrids, the meteor shower that occurs each year in the constellation Lyra. The specific name is a noun in apposition (feminine).
Zoobank ID: urn:lsid:zoobank. o rg : a c t : 9 B 9 3 6 F 7 D -8 6 0 D -4 1 3 8 -A 9 9 1 -673821C3AF48 Remarks: C. lyrida sp. nov. is based on a single complete specimen (holotype MACN-In 43280). Its test is fragile and small (18.2 mm long, 14.8 mm wide, and 6.0 mm high), despite this we consider that the specimen studied is an adult, due to the presence of open gonopores. Of course, this supposition is not conclusive but it is in accordance with the characteristic small form of adult specimens among the family Palaeotropidae (Lovén 1871;Koehler, 1914;Mortensen, 1950;Allison, Duncan & Mintz, 1967;Baker & Rowe, 1990;Mironov, 2006). Mortensen (1950) report the presence of mature gonads containing a few ripe eggs in a specimen of Palaeotropus josephinae of 14 mm length.
In some spatangoids, fascioles may be present during all stage of development, in others they are present in juveniles and then maybe secondarily lost in adults, and in some taxa there is no evidence that fascioles were developed during ontogeny Stockley et al., 2005). Due to the holotype of C. lyrida sp. nov. does not present any trace of fascioles, and it is the only specimen that we have, we assumed that fascioles are absent at least in adults. Additional specimens in different stages of development are needed to test if fascioles develop during ontogeny or it is completely lost in C. lyrida sp. nov.
Corparva lyrida sp. nov. shows various secondary morphological simplifications that suggest an epibenthic lifestyle. As is observed in many others deep-sea spatangoids (Larrain, 1985;Stockley et al. 2005;Saitoh & Kanazawa, 2012), C. lyrida sp. nov. has features related with the building and maintenance of sanitary funnels poorly developed (absence of fascioles and distinctive tufts of spines on the aboral side or at the posterior end of the test, and the inframarginal position of the anus). Besides, others ecological traits present in deep-sea taxa and C. lyrida are the test thinner than shallow waters spatangoids, and the poorly developed (or completely lost in some groups) of petals and specialized tube feet, due to in the deep-sea the respiratory demanding is less than in shallow waters .
Distribution: Tripylus excavatus occurs northward in the SWAO at 39° S (this study), and southward occurs along the Strait of Magellan, Tierra del Fuego, Isla de los Estados, and Cape Horn (Fig. 1C) (Fabri-Ruiz et al., 2017). Also recorded at South Georgia Islands .
Depth: From the littoral to 110 m depth .
Remarks: Here we report the northernmost record for T. excavatus at 39°05'49'' S & 58°02'09'' W at 74 m depth (Table 1, Fig.  1C), resulting in a great extension in its range of distribution. Despite its broad wide range, T. excavatus seems to be an unusual species. Among 299 spatangoids collected we found only three specimens, this peculiarity also was highlighted by Bernasconi (1953Bernasconi ( , 1966. In this line, is to be expected that the disjunct distribution shown in figure 1C became continuous as the sampling increasing in both effort and quality for this area. (Koehler, 1912) Examined material: MACN-In 43285, MACN-In 43286, MACN-In 43297, MACN-In 43299 (Table 1).

Tripylus reductus
Distribution: T. reductus is restricted to the southern region of the SWAO (Tierra del Fuego, Isla de los Estados) (Fig. 1D), and the northern part of the Antarctic Peninsula, also South Shetland Islands, and South Orkney Islands .
Depth: From 50 to 761 m depth .

Abatus philippii
Distribution: Widely distributed along the SWAO from 35° S to 55° S, including the Strait of Magellan, Isla de los Estados, Malvinas Islands (Bernasconi, 1964;Bernasconi,1966;Fabri-Ruiz et al., 2017), and the MPA-N/BB (this study). In the southeastern Pacific Ocean at 52° S latitude. In the Southern Ocean recorded at South Georgia Islands, South Shetland Islands, South Orkney Island, Sabrina Coast, and Ross Sea .
Depth: From 25 to 800 m depth .
Remarks: Here we report the first record for A. philippi at the MPA-N/BB (Table 1). Due to the occurrence of the species in surrounding areas, its presence in the AMP-N/BB was expected (Fig. 1F). Mortensen, 1910 (  (Table 1).

Abatus agassizii
Distribution: A. agassizii occurs in the southern region of the SWAO, including Tierra del Fuego, Isla de los Estados, Malvinas Islands (Bernasconi, 1964;Saucéde et al., 2020), and the AMP-N/BB (this study) (Fig. 1G). Also recorded in the Southern Ocean at South Georgia Islands, South Shetland Island, the northern end of the Antarctic Peninsula, and in the eastern Weddell Sea .
Depth: From the littoral to 600 m depth .
Remarks: Here we report the first record for A. agassizii at the MPA-N/BB (Table). The range of distribution known for A. agassizii seems to have a north limit at Malvinas Islands. These new records for the AMP-N/BB are in accord with the distribution area known for the species (Fig. 1G).
Distribution: B. moseleyi is recorded on both side of South America, in the southeastern Pacific Ocean off Chile from 45° S to 54° S, the Straits of Magellan, and in the SWAO recorded at Malvinas Islands . We report the first record of B. moseleyi at both the northwest slope surrounding the MPA-N/BB, and the Mar del Plata Canyon (Fig. 1H). In the Southern Ocean recorded at South Orkney Islands .
Remarks: According to Saucède et al. (2020) the known geographic and bathymetric limit for B. moseleyi was the Malvinas (Falkland) Islands in the SWAO and 1 400 m depth, respectively. Here we report the northernmost, as well as the deepest distribution for B. moseleyi so far known. The former is the Mar del Plata Canyon on the Argentine continental slope (38°01'23'' S & 53°51'00'' W), and the second 2 212 m depth (MACN-In 43281) (Table 1). In addition, we report its occurrence at the northwest slope of the MPA-N/ BB at 642 m depth (MACN-In 43309) (Fig.  1H). While B. moseleyi is a typical deep-sea species, it should be mentioned that also was found in the shallow waters from the Strait of Magellan by Larraín et al. (1999). Thus, the records here presented confirm for the first time the presence of B. moseleyi in deep waters off Argentina, an extraordinary extension to its geographic and bathymetric range. Hood and Mooi (1998) pointed out that examined specimens labelled as B. moseleyi from Chile (southeastern Pacific Ocean) were distinct from other B. moseleyi from the Malvinas Islands and Patagonia (SWAO) and considered them as two distinct taxa termed "B. moseleyi (north)" and "B. moseleyi (south)" respectively. Also, their phylogenetic analysis based in morphological characters revealed that B. moseleyi (north) is more closely related to those species of Brisaster from the west coast of the Americas, while B. moseleyi (south) is a sister taxon of B. antarcticus (Döderlein, 1906). Brisaster moseleyi (south) and B. antarcticus differ from B. moseleyi (north) by possessing narrow petals IV and V, and long and narrow plastron (Hood & Mooi, 1998), these features were also present in the specimens treated in this study (Fig. 6E). Besides, following the identification key provided by Saucède et al. (2020) the specimens collected match well with their B. moseleyi entity, differing from B. antarcticus by having the apical system slightly displaced posteriorly, and the posterior petals short, slightly more than 1/3 the length of the anterior paired petals (Fig. 6E). Due to the uncertainty in taxonomic distinctiveness and to avoid possible futures nomenclature troubles, the specimens treated in this study are provisionally assigned to B. moseleyi. Genetic studies could help reveal the specific validation of the north/south groups and possible synonyms, enriched through the inclusion of samples from the northernmost population of B. moseleyi found at the Mar del Plata Canyon on the Argentine continental slope SWAO, and to reconstruct phylogenetic relationships among Brisaster species.
Distribution: T. philippii occurs at the SWAO from 35° S up to the southeastern Pacific Ocean at 41° S, including the Strait of Magellan, Tierra del Fuego, Isla de los Estados, and Malvinas Islands (Fig. 1I) (Bernasconi, 1964;Fabri-Ruiz et al., 2017). Also recorded at the South Georgia Islands, and close to Prince Edward Island .
Depth: From the littoral to 600 m depth .

DISCUSSION
The family Palaeotropidae comprises six genera including the new genus here described. Comparing structural features and shape of the test, development of petals, the shape of peristome, and aboral tuberculation, Corparva gen. nov. conform within the Palaetropidae. Besides, Corparva gen. nov. can be differentiated from the rest Palaeotropidae by its apical system semi-ethmolytic and its labral plate reaching to rear part of second adjacent ambulacral plate. C. lyrida sp. nov. is known only from abyssal depths at the Mar del Plata Canyon on the Argentine continental slope, and it is sympatric with others spatangoids herein treated. This new genus and species represent the first report on the family Palaeotropidae from Argentina.
Many authors have pointed out the high incidence of benthic marine invertebrates with non-pelagic development and high rates of parental care in the SWAO and the Southern Ocean (Poulin & Féral, 1996, 1998Pearse, Mooi, Lockhart & Brandt, 2009), and echinoids are not an exception. Although species of the genera Brisaster, and Tripylaster are broadcast spawners with indirect development through planktotrophic larvae, members of the genera Abatus, Tripylus, Parapneustes and Delopatagus are brooders and present a lecithotrophic development (Mortensen, 1951;Pearse & Cameron, 1991;Poulin & Féral, 1996;Gil, Zaixso & Tolosano, 2009). Also, the distinction between sexes in echinoids is frequently expressed by sexual dimorphism, such as the form of genital papillae and the size of genital pores (among others). Mostly in brooding spatangoids, the females present extreme secondary sexual features associated with this reproduction strategy, such as the modification of the aboral region of the test into marsupial brood pouches that give protection for the development of embryos and juveniles (David & Mooi, 1990;Gil, Zaixso & Tolosano, 2020). The depressed ambulacra as an aboral brooding system were widely described for different spatangoids taxa (Mortensen, 1951;Philip & Foster, 1971;Schinner & McClintock, 1993). Corparva lyrida sp. nov presents large gonopores and the paired ambulacra are deeply depressed forming rounded pouches. A lecithotrophic developmental mode for the species can be suspected, due to large gonopores is associated with the production of large yolky eggs (Pearse & Cameron, 1991). The apical system of C. lyrida sp. nov is sunken with respect to the extreme of the interambulacral areas and the ambulacral pouches are slightly deeper than the apical system as well, forming a wide sunken cruciform area (Fig. 3A). This area is cover by long primary spines that emerge leaned over towards the center of the animal body from the upper external edges of the ambulacral plates and surrounding interambulacral plates, and by pedicellariae ( Fig. 2A), resulting in a protected chamber. Although no embryos, nor juveniles were found into the chamber, the morphological features previously mentioned suggesting that the specimen could be a female with marsupial brood chamber Although is necessary to increase the sampling effort and use a proper experimental design to conclude on biogeographical issues, from this study some exploratory trends can be observed in the distribution of the spatangoids here studied (Fig. 1). The species composition in the SWAO has sub-Antarctic and Antarctic affinities. This could be explained in part by the presence of water masses formed in these regions of the world and they are carried to the SWAO by the MC, which could be acting as a dispersal vector of propagules and larvae. Recently studies about biogeographical patterns and benthic eco regionalization based on echinoids from the Southern Ocean, highlights the strongly faunal affinities between South helpful comments by four anonymous reviewers. We thank Fabián Tricárico for his assistance with the SEM and to Daniel Lauretta for facilitating the camera and lens. We acknowledge the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Argentina, to which JNF belong as a PhD fellow, and PEP and MIB as members of the "Carrera del Investigador Científico y Tecnológico". This work is the contribution N° 44 to the Área Marina Protegida Namuncurá (MPA-N/BB, Ley 26.875).
Palabras clave: Atelostomata; erizos de mar; mar profundo; diversidad; Argentina; AMP-N/BB. America and sub-Antarctic islands, possibly due to the connectivity by the flow of the Antarctic Circumpolar Current (Pierrat, Saucède, Brayard & David, 2013;Fabri-Ruiz, Danis, Navarro, Koubbi, Laffont & Saucède, 2020). However, is interesting to highlight that C. lyrida sp. nov. was found at 2 950 m depth at the Mar del Plata Canyon on the Argentine continental slope, and in this region between 2000 to 3000 m there is present the North Atlantic Deep Water (Voigt et al., 2013), which is originated at the North Atlantic Ocean, and flush southward along the continental slope of the American continent (Piola & Matano, 2001) The present work brings novel and updated data about the diversity and distribution of spatangoid sea urchins from the SWAO, including the description of C. lyrida a new genus and species from abyssal waters off Argentina, new records, and occurrence of species previously reported. This reveals that there is still much to know about the diversity and distribution of heart urchins in the SWAO, especially from the deep-sea. Moreover, the knowledge in these aspects will contribute to clarify the matted phylogenetic and evolutionary relations among the Spatangoida taxa.
Ethical statement: authors declare that they all agree with this publication and made significant contributions; that there is no conflict of interest of any kind; and that we followed all pertinent ethical and legal procedures and requirements. All financial sources are fully and clearly stated in the acknowledgements section. A signed document has been filed in the journal archives.

ACKNOWLEDGMENTS
We are grateful to the different crews of the R/V Puerto Deseado, and the scientific staff involved in each cruise. JNF would like to thank Carlos Conejeros-Vargas for his observations that enriched an early version of this work. The final version greatly benefitted from