A new species of Streblospio (Polychaeta: Spionidae) from the northern Adriatic Sea (Mediterranean Sea)

The present study describes a new species of spionid polychaete, Streblospio eridani n. sp., from the Italian coasts of the Northern Adriatic Sea (Mediterranean Sea). This new spionid species was recorded from shallow lagoon and marine habitats: the soft bottoms of the Sacca di Goro (Po River Delta), on October 2017, and the Lido di Dante (Emilia Romagna), between 2016 and 2017. The re-assessment of spionid specimens from other lagoon systems (the Valli di Comacchio and the Valle di Gorino) led us to recognize the presence of this species in the northern Adriatic Sea since 2009. Streblospio eridani n. sp. had low occurrence and density at the marine site Lido di Dante and at the lagoon sites Valli di Comacchio and Valle di Gorino; whereas it reached the highest density of 21,213.3 ind. m -2 at the Sacca di Goro. Morphologically this new species is characterized by dorsal (occipital) papilla on the first chaetiger, sabre chaetae and hooded hooks first appearing on chaetigers 7, hooks with 4-5 pairs of small secondary teeth, pygidium with ventral lappets, and brooding branchiate structures between chaetigers 13 and 28. In the phylogenetic reconstruction, based on the mitochondrial cytochrome c oxidase subunit I (COI) gene, the Adriatic taxon results clearly indicate that this is distinct from the other species of the genus and formed a well-supported clade with other Streblospio sp. specimens from India. Nucleotide divergences calculated between the Adriatic specimens and the other Streblospio species are higher than the intraspecific range reported for the genus and support the description of a new species. Morphological characters important for differentiation of the new species herein described from congeneric species are discussed and an updated key for Streblospio species is provided.


Introduction
Spionidae Grube, 1850 is among the most diverse of polychaete families: more than 500 species belonging to about 40 genera are known globally (Meiβner et al. 2014). Spionids are ubiquitous polychaetes, and commonly found, often in large numbers, in intertidal or subtidal soft sediments (Blake & Kudenov 1978;Meiβner & Götting 2015;Glasby et al. 2000;Rouse & Pleijel 2001) in most marine habitats. Among them, the genus Streblospio Webster, 1879 contains a low number of species, with few distinguishing morphological characters (Mahon et al. 2009).

Description of sampling sites
The Sacca di Goro (Emilia Romagna, northern Adriatic Sea) is a wide (26 km 2 ) microtidal, and polyhaline, lagoon located in the southernmost Po Delta area (Fig. 1a, b). The lagoon has a maximum depth of 2.0 m. It receives nutrient-rich freshwater, primarily from the Po di Volano. The neighbouring Valle di Gorino (8 km 2 ) is a cul-de-sac of the Sacca di Goro (Fig. 1b), with a maximum depth of 1.5 m that receives freshwaters from the Po di Goro through a gate. The Sacca di Goro is spatially enclosed by a long natural sandbank and is characterized by limited water circulation. Being eutrophic, this ecosystem is very suitable for shellfish farming, and it is one of the largest clamfarming grounds in Europe.
The Valli di Comacchio (Fig. 1a,c), are a complex of brackish and hypereutrophized lagoons (width over 100 km 2 ) in the southern part of the Po River Delta (Emilia Romagna), whose depth ranges from 0.5 to 1.5 m and bottoms are typically muddy. The ecosystem is characterized by limited water renewal, being almost completely surrounded by earthen dikes. This lagoonal system is connected with the Adriatic Sea by 2 marine channels, and it also receives a small amount of continental waters. Occasional marine water inflow may come through the opening of Bellochio drain.
Lido di Dante is a flat dissipative sandy beach 12 km south of the port of Ravenna (Emilia Romagna) ( Fig. 1a,d), between the mouths of the Fiumi Uniti (0.9 km North) and Bevano rivers (2.6 km South). The seabed has a gentle slope of about 6 m/km, with a tidal amplitude ranging between 0.30 m and 0.85 m. Bottom sediments ranges from well-sorted fine to medium sand. During summer it is subject to periods of oxygen deficiency, and development of algae and mucilage.

Materials and methods
Following the review of the genus Streblospio provided by Rice & and Levin (1998), and the recent identification key to Streblospio species provided by Delgado-Blas et al. (2018), specimens of the genus Streblospio from the northern Adriatic Sea are herein described as a new species. The terminology used to describe morphological features is derived from Delgado-Blas et al. (2018) and the previous review of spionid polychaetes provided by Radashevsky (2012).
Specimens of the new species of Streblospio were collected during different monitoring surveys of benthic macroinvetebrates carried out along the coasts of the Emilia Romagna (north-eastern Italy), in the Northern Adriatic Sea (Fig. 1a): ten stations (P1-P5 and C1-C5) were sampled in the Po Delta lagoon Sacca di Goro (Fig. 1b) on October 2017, within the LIFE13 NAT/IT/000115 project; two stations, ID and OD (Fig. 1d), were sampled at the seaside resort Lido di Dante on August and September 2016, and 2017. The finding of a new species of Streblospio from these two sites, led us to examine specimens previously identified as Streblospio sp. (Munari and Mistri, unpublished data), all of which were collected at several sampling stations along the coast of the Emilia Romagna, i.e., Valle di Gorino (Fig. 1b)  In order to investigate the community of benthic macroinvertebrates (i.e., organisms > 500 µm), at each stations (and at each sampling time), three replicate samples of sediments were taken with a Van Veen grab (area: 0.027 m 2 ; volume: 4 l); the contents of the grab were sieved through a 0.5 mm sieve. Material retained on the sieve was fixed in 5 % buffered formalin, and then transferred to alcohol. In the laboratory, macroinvertebrates were stained with Rose Bengal to facilitate sorting and identified to species level where possible. Specimens of Streblospio eridani n. sp. were measured using a compound microscope Nikon Eclipse (E200) equipped with a digital camera to achieve images and morphometric measures. Several chosen specimens were dehydrated and gold-coated for Scanning Electron Microscope (SEM) study following standard procedure reported by Munari (2014). Selected specimens of the new species are deposited at the Museum of Natural History of Ferrara (MNHF). A saturated solution of Methyl Green in ethanol was used on some specimens for temporary staining in order to highlight distinct staining color pattern.
During the sampling campaigns water parameters were measured in situ with a probe. Total genomic DNA of 14 specimens from station P2 was extracted using the DNeasy Blood & Tissue kit (Qiagen) according to the manufacturer's protocol. The mitochondrial cytochrome c oxidase subunit I gene (COI) was amplified using the primer pair LCO1490 and HC02198 (Folmer et al. 1994), following the PCR conditions listed in Schulze et al. (2000). The obtained PCR products (643 bp long) were cleaned using the HT ExoSAP-IT (Applied Biosystems™) and sequencing was carried out at the BMR Genomics Sequencing Service (University of Padova, Italy) with the same primers employed in the amplification reaction. As the COI sequences obtained for the 14 samples of Streblospio sp. were identical, only one was deposited in the International Nucleotide Sequence Database Collaboration (INSDC) repositories with the following GenBank accession number: LR597481. The identity of the obtained sequence was checked by using the BLAST program available at the USA National Center for Biotechnology Information (NCBI) web server (http://www. ncbi.nlm.nih.gov).
To infer the phylogenetic position of the collected specimens, a dataset of 40 COI sequences of samples belonging to S. benedicti Webster, 1879, S. gynobranchiata Rice & Levin, 1998, S. shrubsolii (Buchanan, 1890 and Streblospio sp. was constructed. Prionospio steenstrupi Malmgren, 1867 (AF138955) was chosen as outgroup according to Mahon et al. (2009). Sequences for comparison were downloaded from the USA National Center for Biotechnology Information (NCBI) web server (http://www. ncbi.nlm.nih.gov (Schwarz 1978), was HKY + G. Non-parametric bootstrap re-sampling (Felsenstein 1985) was performed to test the robustness of the tree topology (1000 replicates).

results
We identified a total of 7250 individuals, 7227 out of which from the Sacca di Goro, 13 from the Lido di Dante, 6 from the Valli di Comacchio, 4 from the Valle di Gorino. In all the sampled stations of the 4 sites the specimens belonging to the new species were counted in order to obtain also knowledges on the spatial distribution of the individuals. In particular, were collected: 1) from the Sacca di Goro (in October 2017) 1367 specimens at station C1, 454 specimens at station C2, 305 specimens at station C3, 837 specimens at station C4, 1720 specimens at station C5, 1117 specimens at station P1, 942 specimens at station P2, 281 specimens at station P3, 124 specimens at station P4, 80 specimens at station P5; 2) from the Lido di Dante 3 specimens at station ID and 1 specimen at station OD in August 2016, 7 specimens at station ID in September 2016, 1 specimen at station ID in August 2017, 1 specimen at station ID in September 2017; 3) from the Valle di Gorino 4 specimens at station G2 in June 2009; 4) from the Valli di Comacchio 1 specimen at station C2 in June 2013, and 5 specimens at station B1 in May 2014.
Not all of them were used as type materials because they did not add further information to those obtained from specimens selected as types. Best preserved and complete specimens (representative of the variability among individuals) were measured, thoroughly analyzed, and deposited at the Museum of Natural History of Ferrara (MNHF), as holotype and paratypes.
The main environmental data (e.g., sediment characteristics, depth) of the sampling stations at each sampling site are provided in Table 1. Sampling stations in the Sacca di Goro and Valle di Gorino were characterised by muddy (silty clay) sediments, those in the Valli di Comacchio by clay and clayey silt, whereas those in the Lido di Dante by sandy sediments (Table 1).
Prostomium elongate and anteriorly rounded (Fig. 2c), partially enveloped by peristomial extensions; presence of prostomial peaks (sensory knobs similar to papillae) along the fronto-lateral edge of the prostomium, on the dorsal and ventral surface (Fig. 2c), and easily visible even with a light microscope (Fig. 2d). Two pairs of brownish, small eyes in a trapezoidal arrangement (Fig. 2d); a few specimens with 5 eyes. Eyes no longer visible in some specimens preserved in alcohol. One pair of palps situated on midlateral side of prostomium (Fig. 3a), ciliated on frontal surfaces (Fig. 3b), extending to chaetigers 6 in the holotype, and to chaetigers 5-10 in paratypes. Palps have three groups of cilia (i.e., lateral cilia, frontal cilia and latero-frontal cirri; Fig. 3c, d). Peristomium with short dorsolateral wings (Fig. 2b), surrounding base of palps ( Fig. 2c), and fused with the first chaetiger.
One pair of thick, ciliated branchiae inserted just posterior to palps (on chaetiger 1), extending back to chaetigers 5 in the holotype, and to chaetigers 5-11 in paratypes, with distal digitiform appendage (Fig. 4a, d); slightly thicker than palps. Branchiae with central axis and two laterally flattened surfaces ( Fig. 4a, b, c); frontal surface densely ciliated on inner part (oriented towards midline of body) and sparsely ciliated on its outer part; abfrontal surface sparsely ciliated with two main tracts of cilia along long axis of branchia (Fig. 4b). A few specimens having branchiae with transverse bands of dark pigment disappearing after fixation over time. Length of palps and branchiae depends on state of contraction or expansion. A small conical dorsal papilla (like a small occipital antenna) on chaetiger 1 between branchiae (Fig. 2c), difficult to see without removing them.
Type locality. Sacca di Goro, Po Delta lagoon, Emilia Romagna, Italy, Adriatic Sea. Habitat and ecology. Adults of Streblospio eridani n. sp. were collected between 0.5 and 3.5 m depth in sand, silty clay, clayey silt and clay sediments (Table 1). In the eutrophic Sacca di Goro, this species became numerically dominant in the community in October 2017, and it occurred in silty clay sediments to a depth of 1.7 m, at sampling stations characterized by wide salinity range (4.5-33.7 PSU). In that period, its density ranged from 986.7 ind. m -2 (Station P2) to 21,213.3 ind. m -2 (Station C5), and it accounted for 52-97.8 % of the total abundance of the benthic macroinvertebrates (Table 1). Streblospio eridani n. sp. was associated with Chironomus salinarius Kieffer, 1915, indicator of organic enrichment and stagnant waters; other abundant species were tolerant or opportunistic, such as spionid polychaetes, Capitella Blainville, 1828 species complex, Hydroides dyanthus (Verrill, 1873) and the bivalve Arcuatula senhousia (Benson, 1842).
Re-examination of old material from the Valli di Comacchio (dating back to 2013, in clayey sediments, 1.5 m depth) and the Valle di Gorino (dating back to 2009), revealed that some specimens (6 and 4, respectively), originally identified as Streblospio sp., in fact belong to S. eridani n. sp., and that some of them (i.e., two specimens from the Valle di Gorino, and one from the Valli di Comacchio) had oocytes. In the Valle di Gorino and Valli di Comacchio the new species occurred in low density (Table 1). The community of the Valli di Comacchio in which the new species occurred was extremely similar to that of the Sacca di Goro, and characterized by opportunistic (e.g., spionid polychaetes, oligochaetes and chironomids) and tolerant species (i.e., the bivalve Cerastoderma galucum (Bruguière, 1789)). The community of the Valle di Gorino was characterized by stress tolerant species such as the amphipod Corophium orientale Schellenberg, 1928, the gastropod Ecrobia ventrosa (Montagu, 1803), and the polychaete Hediste diversicolor (O. F. Müller, 1776). In the marine site, Lido di Dante, S. eridani n. sp. occurred in sandy sediments in low density, representing only from 0.1% (station ID August and September 2017) to 0.5% (station ID September 2016) of the total abundance of the benthic macroinvertebrate community. Its density ranged from 12.3 ind. m -2 (stations: OD August 2016, ID August 2017, ID September 2017) to 86 ind m -2 (station ID September 2016). These sampling stations were characterized during the study periods by salinities of 33.2-37.3 PSU. Also at the Lido di Dante, Streblospio eridani n. sp. occurred in association with other opportunistic spionids belonging to the genera Minuspio Foster, 1971, Prionospio Malmgren, 1867 and Polydora Bosc, 1802.  Imajima, 1990 to full species level as S. japonica Imajima, 1990 on the basis of several morphological characters (i.e., number of segments, segment on which sabre chaetae occur, number of accessory teeth on neuropodial hooks, and presence of ventral lappets on pygidium). Thus, with the addition of S. eridani n. sp., the number of described species in Streblospio would increase to seven. Among the Streblospio species previously described, four seem to be native to European waters: the new species herein described, S. shrubsolii (Buchanan, 1890) (Carlton 1979;Fonseca-Genevois & Cazaux 1987), it has not been reported from the Mediterranean Sea. Interestingly, S. japonica has never been found outside its native biogeographic region. The species placed within the genus Streblospio display slight morphological differences although there are considerable differences in their reproductive biology (Mahon et al. 2009). For example, males of S. gynobranchiata Rice & Levin, 1998 are morphologically similar to S. benedicti, but females differ in reproductive structures, that is for the lateral body wall extensions resembling branchiae in S. gynobranchiata and for the specialized dorsal brooding pouches in S. benedicti (Rice & Levin 1998). Schulze et al. (2000) on the basis of genetic and morphological analyses even hypothesized a historical hybridization between S. benedicti and S. gynobranchiata followed by repeated backcrossing of the hybrids to S. benedicti, resulting in the transfer of S. gynobranchiata haplotype(s) to an S. benedicti morphology. Moreover, a wide variability in the morphological characters was found in the species of the genus Streblospio, and with considerable overlap in ranges. For example, a certain overlap is reported in the first and last gametogenic chaetiger in females and for this reason Schulze et al. (2000) suggested using caution when using the reproductive morphology alone to identify the species. Likewise, some morphological characteristics of S. eridani n. sp. displayed an intraspecific variability, such as the beginning of gametogenic chaetigers, the number of sabre and capillary chaetae, the number of hooded hooks. In specimens of S. eridani n. sp. such a variability of morphological characters was also found between body regions. Further difficulties in the morphological identification derive from the differences in the descriptions of specimens and/or populations from different geographical areas of S. benedicti (Webster 1879;Hartman 1936;Foster 1971), S. gynobranchiata (Rice 1984) and S. shrubsolii (Buchanan 1890;Horst 1909;Fauvel 1927). For example, Horst (1909) described S. dekhuyzeni Horst, 1909 with two types of capillary notochaetae and more hooded hooks per ramus than S. shrubsolii. However, these differences were minor enough and S. dekhuyzeni was considered synonymous with S. shrubsolii by Fonseca-Genevois & Cazaux 1987. Hartman (1936) described Streblospio lutincola Hartman, 1936 with much smaller ventral peristomial fold, and much larger prostomium than S. benedicti, and S. lutincola was later synonymized with S. benedicti (Hartman 1944).
Streblospio eridani n. sp. is similar to the original description (Rice & Levin 1998) of S. gynobranchiata in having branchiae with distal digitiform appendage, branchiate brooding structures, dorsal collar entire on chaetiger 2, presence of a dorsal papilla between branchiae, and beginning from chaetiger 7 of both sabre chaetae and hooded hooks. However, S. eridani n. sp. can be distinguished by the shape of posterior dorsal appendages on female, be-ginning (i.e., first branchiate structures at chaetiger 13 in the new species and chaetiger 20 in S. gynobranchiata) and number of paired branchiate structures (i.e., for about 10-12 chaetigers in S. eridani n. sp. and for about 15 chaetigers in S. gynobranchiata), as well as in that the new species has: 4-5 pairs of teeth in the neuropodial hooks rather than 3-4 rows of teeth, absence of postchaetal lamellae on posterior chaetigers rather than digitiform ones, and pygidium with lappets rather than simple, as specified in the original description of S. gynobranchiata by Rice & Levin (1998) and in the subsequent description by Çinar et al. (2005a). Besides the presence of branchiate brooding structures (rather than transverse, mid-segmental ridges), the new species is easily distinguished from the other congeneric species from the Mediterranean, that is S. shrubsolii, by several characters: shape of pygidium (with lappets rather than simple, that is without such lobes, as in the latter species), beginning of neuropodial hooks from chaetiger 7 rather than from chaetigers 8-10, number of small teeth in the neuropodial hooks, the first gametogenic chaetiger (19 in the latter species, rather than in chaetigers 7-8), presence of dorsal papilla and sabre chaetae, which are both absent in S. shrubsolii even though the latter species presents a low raised elevation on the first chaetiger. Finally, S. eridani n. sp. differs from S. shrubsolii also in the distribution of prostomial peaks: it has such sensory knobs (like papillae) restricted to the fronto-lateral edge of the prostomium, rather than being widely scattered on all surfaces of the prostomium as observed in S. shrubsolii by Dauer et al. (2003). Streblospio eridani n. sp. differs from S. padventralis in that in the former species sabre chaetae begin from chaetiger 7 rather than from chaetiger 3, and dorsal (occipital) papilla is present, rather being absent. The shape of prostomium is a further character useful to differentiate this new species from S. padventralis, in that the latter species has a subdistal pad surrounding the ventral region. Streblospio eridani n. sp. is easily distinguished from the description of S. eunateae (Martínez & Adarraga 2019) by its smaller size (4.7 -14.9 mm, compared to 10 -25 mm in S. eunateae), and the staining pattern with methyl green, in that the latter species shows uniform staining thoroughout the body, rather than a defined pattern (as in the new species herein described). Streblospio eridani n. sp. differs from S. eunateae also in that the latter has: neuropodial hooded hooks from chaetigers 9-10, rather than from chaetiger 7; oogenesis beginning from chaetiger 20, rather than 7-8; presence of incubator chambers without extensions resembling branchiae, rather than with such structures (as in this new species).
About the term nuchal antenna (or papilla) used by Rice and Levin (1998), in agreement with Delgado-Blas et al. (2018) and Radashevsky (2012) we believe that it is unsuitable as this appendage is located dorsally on the first chaetiger between the two branchiae. Following these authors (Radashevsky 2012;Delgado-Blas et al. 2018) we referred to this structure to as dorsal papilla or occipital antenna. On the basis of the revision of the genus Streblospio provided by Rice & Levin (1998) and the recent descriptions of two new species of Streblospio by Delgado-Blas et al. (2018), and Martínez & Adarraga (2019), we summarize in Table 2 the main morphological, reproductive and ecological characteristics of the currently known species of Streblospio, providing differences between the new species of Streblospio and the previously described ones. Females of the S. eridani n. sp. also reveal differences from S. benedicti (planktotrophs and lecithotrophs), S. gynobranchiata and S. shrubsolii in egg diameter, location of the first gametogenic chaetiger, as well as brood structures (Table 2).  Moreover, in the phylogenetic reconstruction based on the COI marker ( Fig. 8) the sequence obtained from the Adriatic S. eridani n. sp. specimens resulted clearly separated from the other species of the genus and was included in a well-supported clade (100NJ/100MP/99ML) with nine sequences of Streblospio sp. from India. The range of nucleotide divergence within the clade was 0-1. 20%. S. gynobranchiata resulted sister taxon to this group (100NJ/ 100MP/99ML). The nucleotide divergence calculated between the Adriatic specimens and the other Streblospio species ranged from 16.43% to 17.84% (S. eridani n. sp. vs S. benedicti), from 9.02% to 9.82% (S. eridani n. sp. vs S. gynobranchiata) and from 23.25% to 23.85% (S. eridani n. sp. vs S. shrubsolii). These values are significantly higher than the intraspecific range and comparable with the interspecific one reported for the genus by Mahon et al. (2009) (0.44-2.60% and 15.51-22.50%, respectively). These molecular results strongly support the erection of a new species, S. eridani n. sp., for the Adriatic and the Indian Streblospio sp. specimens. etymology. The specific name (S. eridani) refers to Eridanus, the ancient Latin name of the Po River, which ones flooded in the vicinity of Ravenna town, and whose current course ends creating a wide delta projected into the northern Adriatic Sea and including the Sacca di Goro, the Valle di Gorino, and the Valli di Comacchio from which the specimens of the new species come from. Levin (1984) described the occurrence of multiple patterns of development (i.e., poecilogony) in S. benedicti Webster, 1879, and the associated variations in egg size, fecundity, and length of planktonic larval life, and stated that in S. benedicti oogenesis begins in anterior chaetigers (chaetigers 7-11 in planktotrophic populations and chetigers 12-14 in lecithotrophic populations). Oocytes develop within paired ovaries attached to genital blood vessels which extend into the coelomic space; when oocytes are fully developed they move into posterior segments, are fertilized by sperm stored in spermatophores, and enter brood structures (Levin 1984). Similarly, in S. eridani n. sp. oogenesis seems to begin in anterior chaetigers (i.e., chaetigers 7-8) and oocytes seem to move out into the branchiate structures as they become more developed. In specimens of S. eridani n. sp. oocytes were variable in size (25.8 -135.6 µm); maximum diameter size of the oocytes was large compared to those reported by Rice & Levin (1998) for S. gynobranchiata Rice & Levin, 1998 (70-90 µm) and plaktotrophic forms of S. benedicti (diameter is 70-90 µm in), and to those reported by Mccain (2008) for obligates planktotrophic populations of both S. gynobranchiata and S. benedicti (80-95 μm), as well as compared to those described by Çinar et al. (2005a), and Radashevsky & Selifonova (2013) for specimens of S. gynobranchiata (75-88 µm in diameter) from the eastern Mediterranean Sea. However, oocytes of the new species herein described were small compared to those of lecitotrophic forms of S. benedicti (100-220 µm) and S. shrubsolii (Buchanan, 1890) (200-230 µm in diameter) reported by Rice & Levin (1998), to those found by Mccain (2008) for the facultative planktotrophic S. benedicti (180-190 μm) and to those of S. eunateae Martínez & Aderraga, 2019 (125-250 µm in diameter) described by Martínez & Aderraga (2019).

Discussion
Therefore, the occurrence of small oocytes in S. eridani n. sp. suggests that they are probably immature or not fertilized. Dorsal appendages occurr on posterior chaetigers on females of S. gynobranchiata (Rice & Levin 1998) and, in the same way, of S. eridani n. sp., however in the latter such structures do not seem to be as ciliated as in the former.
The Mediterranean Sea is a hotspot of biodiversity with a high rate of endemism (Bianchi & Morri 2000;Coll et al. 2010). The current estimated number of polychaete species in the Mediterranean is about 1100, which contribute to 10% of worldwide biodiversity, and count 210 endemisms (Coll et al. 2010). However, the species richness of invertebrates is still underestimated (Coll et al. 2010), as demonstrated by the current increasing number of new species described (see Çinar et al. 2011b, 2015Çinar & Dagli 2013;Lezzi 2017). The risk is that undescribed invertebrate species may become extinct before we even become aware of their existence. In fact, Mediterranean marine biodiversity is undergoing rapid alteration (Bianchi 2007) due to habitat loss and degradation, anthropogenic impacts, climate change, eutrophication, and the establishment of alien species (Coll et al. 2010;Micheli et al. 2013). The description of S. eridani n. sp. contributes to improve our knowledge on the diversity of Mediterranean invertebrates, although much further efforts remain to be done in order to obtain a realistic estimate of the number of taxa occurring in such a threatened marine system.
Identification key to Streblospio species (adapted from Delgado-blas et al. 2018)