The new species derives its name from the Malagasy word ‘sponjy’, which translates to ‘sponge’. The term is used as a noun in opposition.

Holotype : MADAGASCAR • 1 male (CL 3.6 mm); Nosy Be; 13°25’9.7”S, 48°15’37.94”E; 17 May 2008; Bakary G, Bruggermann H, Michonneau F, Paulay G, Werner T leg.; 6-9 m, from cryptic sponge; FLMNH-UF-14409. — Paratypes: MADAGASCAR • 19 males (CL ranging from 1.99 to 3.6 mm); Nosy Vorona; 15 May 2008; Paulay G leg.; 4 m, from cryptic sponge; FLMNH-UF-14256 • 2 males (CL 3.1-3.2 mm); same data as for preceding; FLMNH-UF-14228 • 1 male (CL 3.35 mm); same data as for preceding; Anker A, Bakary G, Boissin E, Bruggemann H, Horeau T, Michonneau F, Paulay G, Werner T leg.; FLMNH-UF-14313 • 1 male (CL 3.23 mm); same data as for preceding; FLMNH-UF-14316.

Small-sized species of Synalpheus. Carapace (Fig. 2A, B) glabrous, unarmed; frontal margin between rostrum and orbital teeth relatively deep, U-shaped, with moderately developed rostrum; rostrum triangular in dorsal view, subacute at tip, about twice as long as wide, reaching to about proximal third of visible part of first antennular article, slightly shorter than orbital teeth, orbitorostral process present; orbital teeth relatively stout, subtriangular, distally subacute, as long as wide, about three times as broad as rostrum at base; pterygostomial angle bluntly protruding; cardiac notch deep.

Figure 2. 

Synalpheus sponjy sp. nov., holotype (FLMNH-UF-14409). A anterior region, dorsal view B same, lateral view, C pleon, D telson and uropods, E third maxilliped.

Male pleon (Fig. 2C) with first pleuron bearing small ventrally-pointing hook-shaped projection posteroventrally; third pleuron rounded anteroventrally and angled posteroventrally; fourth pleuron rounded anteroventrally and subrounded posteroventrally; fifth pleuron rounded antero- and posteroventrally; sixth pleuron with blunt projection on posterolateral margin.

Telson (Fig. 2D) subrectangular, tapering distally, slightly shorter than endopod, flat, without depression on mid-line; dorsal margin with two pairs of relatively strong spiniform setae located at 0.3 and 0.6 telson length, respectively; posterolateral angle with no projection; posterior margin broad, slightly convex, furnished with about 10 setae, with two pairs of spiniform setae, mesial spiniform setae slightly longer than lateral ones.

Antennula (Fig. 2A, B) with peduncle relatively stout; first antennular article almost twice as long as broad; stylocerite relatively short, subacute, falling short of distal margin of first antennular article; second antennular article slightly shorter than first one, slightly longer than broad; third antennular article slightly longer than second one, subequal to first one, slightly longer than broad; lateral flagellum biramous, fused portion consisting of five subdivisions, secondary ramus with three subdivisions, aesthetascs extending from third subdivision of fused portion to end of secondary ramus.

Antenna (Fig. 2A, B) with inferior tooth of basicerite subacute, slightly overreaching first antennular article, superior tooth small, subacute; scaphocerite relatively slender, lateral tooth overreaching end of second antennular article, lateral margin straight, blade fully absent; carpocerite somewhat slender, extending beyond antennular peduncle.

Mouthparts not dissected. Third maxilliped (Fig. 2E) pediform; lateral plate with small acute hooked-shaped projection pointing laterally; antepenultimate article about 4 times as long as widest part, mesial margin furnished with setae; penultimate article short, about a sixth of antepenultimate length; ultimate article relatively stout, about 0.6 times as long as antepenultimate article, slightly tapering distally, with crown of six spiniform setae at tip; exopod slender, reaching to about 0.8 of antepenultimate article length.

Major cheliped (Fig. 3A–D) with coxa bearing small spiniform seta on dorsodistal margin; basis and ischium short; merus robust, dorsal margin convex, about twice as long as broadest part, dorsodistal margin with blunt projection; carpus short, cup-shaped; palm swollen, ovoid, about 1.5 times as long as wide, dorsodistal margin with subrounded, prominent tubercle; fingers 0.4 times as long as palm, pollex subequal to dactylus. Minor cheliped (Fig. 3E, F) with coxa bearing small spiniform seta on dorsodistal margin; basis and ischium short; merus relatively robust, ventral margin concave, about 3 times as long as broadest part; carpus vase-shaped; palm ovoid, nearly 0.7 times as long as merus, about 0.7 times as long as wide; fingers subequal, about 0.8 times as long as palm; pollex with several tufts of setae situated on lateral side, simple at tip; dactylus bearing densely-situated tufts of distally-curved setae (known as gambarelloides setae), simple at tip.

Figure 3. 

Synalpheus sponjy sp. nov., holotype (FLMNH-UF-14409). A major cheliped, B same, chela, C lateral view same, D mesial view same, E fingers minor cheliped, F mesial view same, lateral view.

Second pereiopod (Fig. 4A) with coxa bearing small spiniform seta dorso-distally; basis short; ischium relatively robust, approximately 3.5 times as long as broad; merus relatively robust, 3.5 times as long as broad, about 1.3 times as long as ischium; carpus slightly longer than merus, composed of five articles, proximal article longest, as long as sum of three medial articles, second to fourth articles subequal, distal article twice as long as fourth one; palm subequal to fingers.

Figure 4. 

Synalpheus sponjy sp. nov., holotype (FLMNH-UF-14409). A second pereiopod, B third pereiopod, C lateral view same, mesial view, D fourth pereiopod, mesial view, E fifth pereiopod, lateral view.

Third pereiopod (Fig. 4B, C) robust, coxa with small spiniform setae on dorsodistal margin; ischium vase-shaped, 1.5 times as long as broadest part; merus robust, about 3 times as long as broad, about 2.8 times as long as ischium, distal half of ventral margin concave, mesial margin of concavity furnished with thick setae and lateral margin widened; carpus slightly widening distally, slightly longer than ischium, with spiniform seta on distoventral margin; propodus with five spiniform setae on ventral margin in addition to distal pair, about 1.7 times as long as carpus; dactylus biunguiculate, inferior unguis slightly wider at base than superior one, latter one slightly longer than former. Fourth pereiopod (Fig. 4D) generally similar to third pereiopod, less robust; coxa lacking spiniform setae on dorso-distal margin; merus concavity less prominent and mesial margin lacking thick setae. Fifth pereiopod (Fig. 4E) generally similar to third pereiopod, less robust; merus without concavity; carpus about 0.7 times as long as merus, without spiniform setae on distoventral margin; propodus subequal to merus, bearing four transverse rows of microserrulate setae on distal half.

Uropods (Fig. 2D) with protopods bearing two subacute teeth; exopod with small distolateral tooth; diaeresis inconspicuous except for lateral margin terminating to small tooth; distolateral spiniform setae next to diaresis stout, not overreaching posterior margin of exopod.

North of Madagascar: Nosy Be and Nosy Vorona.

All the specimens were collected from cryptic sponges situated among dead coral and coral rubbles.

The new species exhibits similarities to several rare species found in the Indo-West Pacific region, characterized by a dense brush of setae on the dactylus of the minor cheliped, known as gambarelloides setae. These species include S. sladeni Coutière, 1908; S. spongicola Banner and Banner, 1981; S. crosnieri Banner and Banner, 1983; and S. gambarelloides (Nardo, 1847) [as reported in Banner and Banner (1983)]. Originally described from Cargados Carajos Shoals (Mauritius) and reported in the Gulf of Aqaba [Banner and Banner (1981)], S. sladeni, each time based on a single specimen, can be distinguished from the new species by several notable characteristics: slender orbital teeth, presence of a scaphocerite blade, less prominent inferior tooth and more prominent superior tooth of the basicerite, highly robust exopods of the second and third maxillipeds, a subacute tooth on the dorso-distal margin of the major chela palm, and complete presence of diaeresis.

According to Banner and Banner (1983), 45 males and 6 females, suggesting a potential communal lifestyle for the species, from Tuléar, Madagascar were reported and identified as S. gambarelloides, (for further details, see Banner and Banner 1983: page 97). However, no descriptions or illustrations were provided for these specimens, except for one distinguishing characteristic of S. gambarelloides, namely the absence of an orbitorostral process. This particular trait can be utilized to differentiate the new species from S. gambarelloides.

Synalpheus sponjy sp. nov. is closely related to the other two species, S. crosnieri and S. spongicola, both of which have not been reported since their original descriptions. The new species can be distinguished from S. crosnieri (found northwest of Madagascar) by the overall shape of the major cheliped fingers. In S. sponjy sp. nov., the major cheliped is normal and straight, whereas in S. crosnieri, it appears twisted when viewed dorsally. Additionally, the shape of the meri in the third and fourth pereiopods sets the new species apart. In the new species, these meri are concave on the distal half, and the third pereiopod merus bears a row of stiff setae along the mesial margin of the concavity. However, caution should be exercised when using the latter characteristic as the drawings provided by Banner and Banner (1983) are schematic, and the description lacks precise information regarding the exact shape of the meri in the third and fourth pereiopods.

One of the authors (HA) had the opportunity to examine the type series of S. spongicola deposited in the Naturalis Biodiversity Center, Leiden, Netherlands. As mentioned by Banner and Banner (1981), S. spongicola is a relatively large species of Synalpheus. In addition to differences in size, S. sponjy sp. nov. can be distinguished from S. spongicola based on the following characters: 1) the stylocerite falls short of reaching the end of the first antennular article in the new species, whereas in S. spongicola, it extends to the end; 2) the blade of the scaphocerite is absent in the new species, while it is complete in S. spongicola; 3) the palm of the major cheliped terminates in an obtuse, robust tubercle that points forward on the dorso-distal margin in S. sponjy sp. nov., whereas in S. spongicola, it ends in an upturned subacute tooth; 4) the last three pereiopods are more robust in the new species, and the meri of the third and fourth pereiopods are ventrally concave (lacking concavity in S. spongicola); and 5) in the new species, the diaeresis is confined to the lateral tooth, whereas in S. spongicola, the diaeresis is complete. Unfortunately, as these types were very old (date), they could not be included in the phylogenetic tree. Limited information is available regarding the ecology of S. spongicola, as it has been recorded only once with 9 females and 1 male from a single collection, suggesting a potential communal lifestyle for the species.

In the phylogenetic tree (Fig. 1), S. sponjy sp. nov. forms a clade with S. aff. somalia (Banner and Banner, 1979) and S. thai (Banner and Banner, 1966), along with S. sladeni. The new species can be easily distinguished from both S. somalia and S. thai by the densely-located (gambarelloides) setae on the dactylus of the minor cheliped. Additionally, S. thai stands out within the entire genus due to its distinctive shape of the telson (see Banner and Banner 1966: fig. 19L). Synalpheus somalia can be further differentiated from the new species by the dactylus of the major cheliped being much longer than the pollex (Banner and Banner 1979: fig. 3c, e).

The new species is named after Gustav Paulay (FLMNH), who collected the majority of the specimens that contributed to the present study, and in recognition of his generous help and warm hospitality during the authors’ visit to the FLMNH collection.

Holotype : MADAGASCAR • 1 male (CL 3.4 mm); Nosy Vorona; 15 May 2008; Paulay G leg.; 4m, in cryptic sponge; FLMNH-UF-71394. — Allotype: MADAGASCAR • 1 female (CL 3.6 mm); same data as for holotype; FLMNH-UF-71395. — Paratypes: MADAGASCAR • 15 males (CL ranging from 1.99 to 2.98 mm); from the allotype colony; FLMNH-UF-14259 • 4 males (CL ranging from 1.73 to 2.98 mm); from the holotype colony; FLMNH UF-14255 • 19 males and 0 females (CL ranging from 1.37 to 3.35 mm); same data as for holotype; FLMNH-UF-14257 • 1 female and 3 males (CL ranging from 1.73 to 2.85 mm); same data as for holotype; FLMNH-UF-14231 • 1 female and 3 males (CL ranging from 2.86 to 3.35 mm); same data as for holotype; FLMNH-UF-14230 • 1 male (CL 2.23 mm); Nosy Vorona; 15 May 2008; Anker A leg.; cryptic sponge; FLMNH-UF-14915 • 2 males (CL 2.85-2.98 mm); same data as for holotype; FLMNH-UF-14229 • 5 males (CL ranging from 2.36-3.48); same data as for holotype; FLMNH-UF-14236.

Small-sized species of Synalpheus. Carapace (Fig. 5A, B) glabrous, smooth; frontal margin between rostrum and orbital teeth shallow, broadly U-shaped, with very small rostrum; rostrum triangular in dorsal view, subacute at tip, approximately as long as wide, slightly surpassing orbital teeth, orbitorostral process present; orbital teeth shallow, broad, distally subrounded, about four times as broad as long, about three times as broad as rostrum at base; pterygostomial angle bluntly protruding; cardiac notch relatively deep.

Figure 5. 

Synalpheus gustavi sp. nov., holotype (FLMNH-UF-71394). A anterior region, B dorsal view same, C lateral view pleon, D same, allotype, E telson and uropods, F third maxilliped.

Pleon only showing sexual dimorphism in first pleonite. Male pleon (Fig. 5C) with first pleuron angulate or with tiny projection posteroventrally; third to fifth pleuron rounded anteroventrally and posteroventrally. Female pleon (Fig. 5D) with first pleuron rounded posteroventrally.

Telson (Fig. 5E) subrectangular, tapering posteriorly, remarkably shorter than endopod, approximately as long as broadest part, dorsal margin with two pairs of relatively strong spiniform setae at 0.5 and 0.8 telson length (right pairs in holotype), respectively, without depression on mid-line; distolateral angle with no projection; posterior margin broad, straight, furnished with 8 setae, with two pairs of spiniform setae, mesial spiniform setae about twice as long as lateral ones and subequal to dorsal spiniform setae.

Antennula (Fig. 5A, B) with peduncle relatively stout; first antennular article about 1.7 times as long as broad; stylocerite relatively short, subacute at tip, falling short of reaching distal margin of first antennular article; second antennular article subequal to first one, slightly longer than broad; third antennular article shortest, about 0.8 times as long as first antennular article, slightly longer than broad; lateral flagellum biramous, fused portion consisting of five subdivisions, aesthetascs extending from third subdivision of fused portion to end of secondary ramus.

Antenna (Fig. 5A, B) with inferior tooth of basicerite subacute, reaching to about middle of second antennular article, superior tooth small, subacute; scaphocerite relatively slender, lateral tooth reaching to midlength of third antennular article, lateral margin straight, blade remarkably reduced; carpocerite slender, extending beyond antennular peduncle.

Mouthparts not dissected. Third maxilliped (Fig. 5F) pediform; lateral plate with small hooked-shaped projection pointing laterally; antepenultimate article about 3 times as long as widest part; penultimate article short, about a fourth of antepenultimate article length; ultimate article very stout, not tapering distally, slightly shorter than antepenultimate article, tip widened, with 13 spiniform setae; exopod slender, reaching to about 0.8 of antepenultimate article length.

Major cheliped (Fig. 6A–C) with coxa unarmed; basis and ischium short; merus robust, dorsal margin convex, about twice as long as broadest part, dorsodistal margin blunt; carpus short, cup-shaped; palm swollen, ovoid, about 1.9 times as long as broad, dorsodistal margin with small projection; fingers about 0.6 times as long as palm, pollex subequal to dactylus; plunger slightly reduced. Minor cheliped (Fig. 6D–F) with coxa unarmed; basis and ischium short; merus relatively robust, about 2.5 times as long as broadest part; carpus vase-shaped; palm ovoid, nearly 0.7 times as long as merus, about 0.7 times as long as broad; fingers subequal, about 0.8 times as long as palm, broadened, with minute accessory projection at tip; dactylus spoon-shaped.

Figure 6. 

Synalpheus gustavi sp. nov., holotype (FLMNH-UF-71394). A major cheliped, B same, chela, C lateral view same, D mesial view minor cheliped, E same, fingers, dorsomesial view, F same, ventromesial view.

Second pereiopod (Fig. 7A, B) with basis short; ischium relatively slender, approximately three times as long as broad; merus slender, five times as long as wide, about 1.5 times as long as ischium; carpus slender, about 1.4 times as long as merus, with four subdivisions, proximal article longest, slightly longer than sum of other articles, second article slightly longer than third one, distal article about 2.7 times as long as third article; palm slightly longer than fingers; fingers very slender.

Figure 7. 

Synalpheus gustavi sp. nov., holotype (FLMNH-UF-71394). A second pereiopod, B same, distal segments, C third pereiopod, lateral view, D fourth pereiopod, lateral view, E fifth pereiopod, lateral view.

Three last pereiopods (Fig. 7C–E) shorter and more robust compared to general shape of walking legs in Synalpheus. Third pereiopod (Fig. 7C) robust, coxa armed with small spiniform setae on dorsodistal margin; ischium vase-shaped, slightly longer than widest part; merus robust, about 2.2 times as long as wide, about 2.6 times as long as ischium; carpus slightly widening distally, 1.5 times as long as ischium, with single spiniform seta on distoventral margin; propodus relatively robust, about 0.7 times as long as merus, with five spiniform setae on ventral margin in addition to distal pair; dactylus biunguiculate, superior unguis remarkably longer (around 3 times) than inferior one. Fourth pereiopod (Fig. 7D) generally similar to third pereiopod, slightly less robust; coxa lacking spiniform setae on dorsodistal margin. Fifth pereiopod (Fig. 7E) generally similar to third pereiopod, less robust; carpus about 0.7 times as long as merus, without spiniform setae on distoventral margin; propodus subequal to merus, bearing three spiniform setae on ventral margin and one pair distally, furnished with five transverse rows of microserrulate setae on distal half.

Uropods (Fig. 5E) with mesial tooth of protopod blunt, lateral tooth subrounded with subdistal blunt projection on mesial side; exopod broad, with small distolateral tooth next to slender spiniform seta; diaeresis lacking except for lateral margin terminating to small tooth; endopod slightly longer than exopod.

North of Madagascar: Nosy Vorona.

All the specimens were collected from cryptic sponges situated among dead coral and coral rubble.

The new species, S. gustavi sp. nov., shares a distinctive spoon-shaped feature of the minor cheliped, along with the presence of an orbitorostral process, with several species found in the Indo-Pacific region. These species include S. anceps AH Banner, 1956; S. dorae Bruce 1988; S. harpagatrus Banner and Banner, 1975; S. laticeps Coutière, 1905; S. neptunus neptunus (Dana, 1852); S. neptunus germanus Banner and Banner, 1975; S. paradoxus Banner and Banner, 1981; S. paralaticeps Banner and Banner, 1982; and S. theano De Man, 1910. Among these morphologically similar species, we included S. neptunus neptunus, S. anceps, and S. theano in the phylogenetic tree to explore their relationship with the new species. However, the results of the phylogenetic analysis reveal that the spoon-shaped minor chela has evolved independently in S. gustavi sp. nov. compared to other Indo-Pacific species exhibiting similar characteristics in the minor chela.

The new species can be distinguished from the aforementioned species based on four distinct characteristics, allowing for a quick and accurate differentiation. These traits are as follows: 1) very shallow frontal margin of the carapace, both the rostrum and orbital teeth, 2) unique shape of the ultimate segment of the third maxilliped, 3) presence of a four-articled carpus in the second pereiopod, and 4) exceptionally narrow and slender fingers in the second pereiopod.

Synalpheus anceps, which was reported from Saipan in the Northern Mariana Islands (AH Banner 1956), is the only species, along with the new species, that possesses a very shallow frontal margin. However, there are notable differences between the two species. In S. anceps, the front (including the rostrum and orbital teeth) is less pronounced, the blade of the scaphocerite is complete, the ultimate segment of the third maxilliped is normal Synalpheus like, the second pereiopod carpus has five articles with relatively normal-shaped fingers, and the third pereiopod is generally less robust compared to that of the new species. Additionally, in S. anceps, the superior unguis of the last three pereiopods is slightly longer than the inferior unguis. On the other hand, the new species can be distinguished from S. anceps by having a squame reduced to a small projection, a distally broad ultimate segment of the third maxilliped, a four-articled carpus in the second pereiopod with very slender fingers, and the three last pereiopods with superior ungui of the dactylus significantly longer than the inferior ones.

Synalpheus dorae shares one of the four distinctive characteristics of S. gustavi sp. nov., i.e. the second pereiopod with four-articled carpus. However, there are several other notable differences between the new species and S. dorae: the less developed superior tooth of the basicerite in the new species compared to that of S. dorae, the more developed scaphocerite blade in S. dorae, the major chela palm terminating to a small projection dorso-distally in S. gustavi sp. nov. while bearing a prominent tooth in S. dorae, more robust three last pereiopods with dorsal ungui about three times as long as ventral ungui in S. gustavi sp. nov. compared to slightly longer superior ungui in S. dorae, very strong dorsal spiniform setae of telson, and very narrow posterior margin of telson in S. dorae with mesial spiniform setae being juxtaposed.

Three species, namely S. harpagatrus, S. laticeps, and S. paralaticeps, are distinguished by the specific shape of the fingers on the minor cheliped, which possess 3 or 4 teeth at the tip. In contrast, the new species has a single tooth and a tiny accessory tooth on the fingers. Additionally, the following differences, in addition to the four main distinctions, can be observed between these three species and the new species: the longer stylocerite (overreaching the first antennular article) and more developed blade of the scaphocerite in the three of them compared to S. gustavi sp. nov.; typical ultimate segment of the third maxilliped in S. laticeps and S. paralaticeps compared to broad distally and armed with 13 slender spiniform setae in S. gustavi sp. nov., and narrow distally armed with 10-12 short heavy spiniform setae in S. harpagatrus; stouter general shape of the minor cheliped in S. harpagatrus; armed merus of the third pereiopod in S. harpagatrus and S. paralaticeps but unarmed in S. laticeps and S. gustavi sp. nov.; proportion of the superior unguis of the third pereiopod dactylus to the inferior one being approximately 3 in S. gustavi sp. nov., 2 in S. paralaticeps and 1 in S. harpagatrus and S. latirostris; fully developed diaeresis in S. harpagatrus and S. paralaticeps compared to the reduced one to a lateral tooth in S. gustavi sp. nov.; straight posterior margin of the telson in S. gustavi sp. nov. compared to the convex one in the other three species.

The Red Sea-inhabiting species, S. paradoxus, can be further distinguished from S. gustavi sp. nov. by the following characteristics, in addition to the four previously mentioned differences: its very strong superior tooth of the basicerite; half-developed blade of the scaphocerite; major cheliped merus armed with a strong tooth dorso-distally, palm with strong tubercle on the dorso-distal margin, and the dactylus longer than the pollex; minor cheliped dactylus with two definite rows of setae situated dorsally and mesially; third pereiopod dactylus with the superior unguis about twice as long as the inferior unguis; complete diaeresis; and triangular distolateral angles of the telson.

The three remaining species S. neptunus neptunus, S. neptunus germanus and S. theano, are separable from the new species by various characters, in addition to the four mentioned differences (De Man 1911; Banner and Banner 1972, 1975). Regarding the frontal appendages, S. theano, S. neptunus neptunus, and S. gustavi sp. nov. have a shorter stylocerite falling short of distal margin of the first antennular article while it overreaches the first antennular article in S. neptunus germanus; S. theano and S. gustavi sp. nov. possess a longer superior tooth of the basicerite reaching to about midlength of the second antennular article or end of it, while in the other two species it fails to reach the midlength of the second antennular article; the scaphocerite blade is absent in S. neptunus germanus, reduced to a small bump in S. gustavi sp. nov., partially reduced in S. neptunus neptunus, and with no reduction in S. theano. Regarding the thoracic appendages, S. neptunus neptunus and S. neptunus germanus possess a typical shape of the ultimate segment of the third maxilliped, S. theano lacks the crown of spiniform setae replaced by setae, and that of S. gustavi sp. nov. is broadened distally with 13 spiniform setae; the major cheliped palm terminates to a tiny projection in S. gustavi sp. nov. while the other three to prominent tubercles; the minor cheliped dactylus is furnished with two rows of setae, one dorso-mesially and one mesially in S. neptunus germanus, while there is only one mesially located row of setae in S. theano and S. neptunus neptunus, and S. gustavi sp. nov. has no row of setae; the superior unguis of the third pereiopod is remarkably longer in S. gustavi sp. nov. compared to the inferior unguis, whereas it is only slightly longer in the other three species. Regarding the pleonal appendages, three differences are notable. First, the diaeresis is complete in S. theano and S. neptunus neptunus, while it is limited to its lateral tooth in S. neptunus germanus and S. gustavi sp. nov. Second, the position of dorsal pairs of spiniform setae on telson are more mesially (especially the posterior pair) in S. gustavi sp. nov. (for detailed comparison see De Man 1911: fig. 61a for S. theano, Banner and Banner 1972: fig. 3M for S. neptunus neptunus, Banner and Banner 1975: fig. 12i for S. neptunus germanus, and Fig. 5E for S. gustavi sp. nov.). Third, the posterior margin of the telson is straight in S. gustavi sp. nov., almost straight in S. neptunus germanus, and concave in S. theano and S. neptunus neptunus.

Phylogenetically speaking, S. gustavi sp. nov. was initially included in the first and only worldwide phylogenetic study of the genus Synalpheus (Hultgren et al., 2014) as S. aff. brevifrons. The new species is broadly grouped with the S. gambarelloides species group, consistent with the findings of the present study.