A new species of Isotogastrura from marine littoral sand of Fernando de Noronha Islands, Pernambuco State, Brazil is described and illustrated. It is characterized by the absence of prelabral setae, presence of a spatulate mucro, fusion of the dorsal part of Abd. IV and V and one pair of long circumgenital setae on males. It is similar to I. veracruzana from México. A key for the 9 species known in the genus is provided.
The genus Isotogastrura is distributed in tropical coasts and comprises 8 known species, all described from sand in tropical litoral areas (Pota-pov et al. 2011).
We describe here a new species of Isotogastrura from Fernando de Noronha, a group of oceanic islands in equatorial Atlantic, Northeastern Brazil. This is the first record for the genus in South America, extending its known distribution to the southern tropics, and reinforcing its pan tropical distribution pattern.
The phylogenetic position of the monotypic family Isotogastruridae—between the families Isotomidae and Hypogastruridae, which are representatives of Orders Entomobryomorpha and Poduromorpha respectively—is controversial and poses taxonomic uncertainties (Fjellberg 1995). The new species brings interesting new features that contribute to this discussion, i.e., dorsal fusion of Abd. IV and V; and this condition is seen also in the Entomobryomorpha, but not in the Poduromorpha in which the apical sensory organ of Ant. III has been displaced to the base of Ant. IV (Bonet 1946).
Material and Methods
The material studied comes from marine littoral sand of Fernando de Noronha Islands, Pernambuco State, Brazil. Collecting was done by a washing technique and the extraction of floating specimens manually with the aid of brushes. Specimens were fixed in 80% ethanol, cleared on KOH and lactophenol, and mounted under a cover in Hoyer's solution for subsequent analysis by phase contrast microscopy. Ten specimens, not included in type material, were dehydrated and metalized for observation by scanning electron microscopy.
The new species was compared with all available bibliographic information and the types of I. ahuizotli and I. veracruzana were checked. Abbreviations used in the text are: Ant. = antennal segment; Abd. = abdominal segment; Th. = thoracic segment.
Isotogastrura Thibaud & Najt 1992
Members of this genus are very small, less than 0.5 mm long, with appearance intermediate between the families Isotomidae and Hypogastruridae. Body with short setae and sensorial setae. Antennae shorter than head. Ant. organ III partly displaced to Ant. IV, with 2 fanlike sensilla and 2 cuticular conic papillae. With square and prognathous head. Four eyes per side. No postantennal organ. Mandibles with apical teeth and molar surface, maxillae with fringed lamellae. Pronotum well developed with 4 cuticular vesicles but without setae. Dorsal chaetotaxy of second and third thoracic tergites and all abdominal tergites with 3 rows of setae. Most tergites subequal in size, some with a pair of posterior cuticular vesicles. No microsensilla on Th. II and III. Unguis with a big dorsal teeth and unguiculus filiform, longer than ungues. Abdomen segment V with a dorsal glandular opening and often with one median posterior cuticular tubercle. Tenaculum and furcula well developed. No anal spines.
Type species: Isotogastrura arenicola Thibaud & Najt 1992
Holotype: male, BRAZIL: Pernambuco, Fernando de Noronha Islands, Boldró, from marine littoral sand, 20-VII-2012, coll. Lima and Zeppeli-ni, deposited at Coleção de Referência em Fauna de Solo da Paraíba (LSCC-CRFS/UEPB) number 3680,
Paratypes: 6 males and 2 females, same data as holotype, deposited at LSCC-CRFS/UEPB number 3681-3688. Two male adults, same data as holotype, deposited at Museu Nacional da Universidade Federal do Rio de Janeiro number 2427 and 2428. One female and 1 male adult from the same locality will be deposited at senior author's institution (LESM/UNAM) number 2991 and 2992.
Body length (n = 10): 0.35 mm (range 0.3–0.4 mm). White in alcohol, with grey pigmentation uniformly distributed over dorsal areas except for the darker eye patches. Body shape typical of genus, only with primary hexagonal granulation, ventral side of abdomen wrinkled with some smooth areas (Figs. 1, 2 and 5). Setae smooth and short about 5-8 μm. Sensorial body setae about 10 μm, thicker than setae with blunt tips. Head large, with exerted mouth parts as common for the genus. Th. I with 4 dorsal tubercles. Posterior edge of Abd. IV and anterior edge of Abd. V dorsally fused (Fig. 13), separation only noticeable by the lateral constrictions between them, ventrally well separated; with a dorsal glandular opening on Abd. V partly covered by cuticular fold. One pair of small tubercles present at middle posterior edge from Th. II to Abd. II (Figs. 11 and 13).
Ant. I and II with 7 and 11 setae, respectively. Sensorial organ of Ant. III with 2 conical cuticular papillae granulated, 2 fan-like inner sensilla, and 2 outer tubular bifid sensilla, 1 of which is grouped together with inner ones, another 1 more proximal and associated with lateral microsensillum, which is small and pointed (Figs. 1, 2 and 10). Ant. IV with 8 sensilla, 2 of which are thicker and longer (Figs. 1 and 10). Subapical organ small and strongly depressed. Labrum with 0/3,3,4 setae and 4 lobules. No prelabral microsetae. Maxillary outer lobe with apical palp and 2 sublobal hairs. Labium with 5 basolateral and 4 basomedian setae postlabial setae (Fig. 4). Some elements of labial palp difficult to interpret. Mandibles slender as typical for the genus, each with 4 apical teeth and molar surface well developed. Maxillary head with most lamellae strong and serrated. Head with 4+4 ocelli, 2 inner smaller (Figs. 3 and 11). Postantennal organ absent. Dorsal chaetotaxy shown in Fig. 11 and 13. Th. II-Abd. III with 3+3 axial setae each. Number of sensilla 2,2/1,1,1,1,1, microsensilla absent. Sensilla longer than setae and distinguished from them by hyaline appearance and more blunt tips. Legs chaetotaxy from subcoxa 1, subcoxa 2, coxa, trochanter, femur and tibiotarsus is: 1,1,4, 6,11,12; 1,3,7, 6,11,12 and 2,3,9,5,10,11 from I to III, respectively. Ungues without inner teeth but a big dorsal tooth ¾ the length of ungues (Fig. 6). Unguiculus filiform, sinuous and longer than ungues (Fig. 6), easily bent (Fig. 7). Thorax without ventral setae. Ventral tube with 6+6 lateral paired setae (4+4 in distal and 2+2 in basal position) and 1 unpaired posterior seta (Fig. 14). Tenaculum with 3+3 teeth, setae absent (Fig. 5). Manubrium without anterior setae, posteriorly with 8+8 setae on main part and 5+5 on basolateral fields (Figs. 8 and 14). Anterior subcoxae furcalis with 5 setae, posterior with 2 setae. Dens with 3 posterior and 1 anterio-median setae (Figs. 8 and 9), 1 inner setae thicker than others, mucro spatulate (Fig. 9). Abdominal segment IV and V fused in the dorsal side (Fig. 13). Female genital plate with 4 pairs of pregenital setae, 2 circumgenital setae and 2 eugenital setae (Fig. 12). Male genital plate with 4 pairs of pregenital setae, 5 circumgenital setae, 2 of them longer than others, and 4 pairs of short eugenital setae (Fig. 15). Each lateral anal valve with 14 setae + 2 microsetae, posterior valve with only 2 setae (Fig. 14). No anal spines.
The new species is named I. mucrospatulata sp. nov. after the shape of the mucro.
Habitat and Distribution
The new species is known only in the sand beaches of Fernando de Noronha Islands. The type locality is located at Boldró, S 03 50′ 44.5″ W 32 25′ 47.1″. Small body size of I. mucrospatulata sp. nov. indicates that it inhabits narrow passages among the grains of sand (Potapov et al. 2011). Interstitial Collembola look like typical euedaphic species, but are flexible and slender enough to be able to move between sand grains of small size without changing the pore architecture (Thibaud 2007). All specimens in this study (total of 22) were found in sand at the intertidal zone. The species I. mucrospalulata sp. nov. was the only collembolan species found in sand samples from 6 different beaches in Fernando de Noronha and there are no records of if from elsewhere.
Isotogastrura mucrospatulata sp. nov. is characterized by the absence of prelabral setae, presence of one spatulate mucro, fusion of the dorsal part of Abd. IV and V and one pair of long circumgenital setae on males. It is similar to I. veracruzana from México in the absence of prelabral microsetae, but differs in the shape of mucro, dental setae shape, fusion of the dorsal part of Abd. IV and V (noticeable by the absence of thinner granulation in intersegmental areas) and setae of genital plate of male. When dorsal body chaetotaxy is compared, the stability of the chaetotaxic pattern within the genus is remarkable. All the segments have the 3 rows of setae, anterior, median and posterior. We noticed that only I. mucrospatulata sp. nov. lacks 2 setae on the m row of Th. II and III (only 5 setae on m row versus 7 in I. veracruzana; see Table 1). The stronger setal reduction on row m of Abd. I occurs on I. trichaetosa. All the species have only one seta on row m of Abd. V (Table 1).
According to Potapov et al. (2011) Isotogastrura trichaetosa is the most primitive species of the genus, having more homonomic axial chaetotaxy of abdomen (3,3,3,3) than is common in the genus (3,3,3,2), normal shape of body, (elongated in other species) and thin sensilla on Ant. IV (very variable in different species). Another primitive character, the simple (vs. bifurcate) dorsal and ventral guard sensilla of antennal organ III, is shared with I. coronata (Canary Islands) and I. madagascariensis (Madagascar). It is interesting the fact that species from Canary Islands, Madagascar and China have the dorsal and ventral guard sensilla of the sensorial organ of Ant. III simple while all the species from America have bifid guard sensilla (Table 2). Other characters have no correlation with the continental distribution of the genus.
Most species of the genus occur between the Tropic of Capricorn and the Tropic of Cancer, except I. coronata, which extends into the Mediterranean. The new Brazilian species indicates that Isotogastrura is also distributed in the tropical part of South America, and thus the genus is completely pan tropical as proposed by Potapov et al. (2011).
Usually, littoral species are distributed widely along the coasts because of transportation possibilities by water and by similar habitat conditions. With respect to Collembola, Thibaud (2007) remarked that many species from interstitial littoral sands have trans-oceanic distributions; even though this is true for several species, some genera seem to have many species with restricted distributions, such as Isotogastrura.
Systematic Position of Isotogastruridae
The family Isotogastruridae Thibaud & Najt 1992 is monotypic, and only the genus Isotogastrura is known. It has an intermediate position between the orders Poduromorpha and Entomobryomorpha (Thibaud & Najt 1992). It is generally accepted that the family belongs to Poduromorpha although its phylogenetic position is still not fully understood (Fjellberg 1995). The character which is taken into account to compare, is the individualized prothoracic tergite, together with many special characters shared either with the Isotomidae or the Hypogastruridae.
When the morphology of the antennae of Isotogastrura is compared with other genera, it is found that the sensorial organ of Ant. III has been displaced to Ant. IV. Also in the genus Tafallia (Hypogasturidae), Bonet (1946) clearly described this fact: “en el Ant. III sólo queda la sensila lateroventral que en tamaño y forma es muy semejante a la sensila externa del Ant. IV”. Therefore this character seems to have a common origin in both genera, Tafallia and Isotogastrura. The organ in both genera is partly displaced to Ant. IV in that 2 sensilla and 1 guard sensillum are placed at the base of Ant. IV, while only the ventral guard sensillum and the microsensillum remain on the apex of Ant. III. Similar condition occurs in other basal Poduromorpha (e.g., Onychiuridae and Tulbergiidae). This feature supports Fjellberg (1995) hypothesis, that Isotogastruridae is a Poduromorpha more related with Hypogastruridae.
Comparison of chaetotaxy among the species of Isotogastrura (by demi-tergite).
Anal valves and Abd. VI chaetotaxies resemble those of some species of Friesea (Neanuridae) and to Willemia (Hypogastruridae), both belonging to Poduromorpha. The presence of cuticular tubercles has been also found in Willemia meyboholae Palacios-Vargas 1987 (Hypogastruridae ) from marine littoral sand.
The fusion of abdominal segments is seen in different families of Entomobryomorpha, but not in Poduromorpha, this is an interesting character that is in discordance with Fjellberg (1995), and must to be taken into account when comparing the families. The reduction of Abd. VI to anal valves also occurs in Actaletidae and some members of Isotomidae, indicating that this condition must be developed several times during the evolution of different groups of Collembola.
A phylogenetic analysis should establish the actual relationship of the genus with the family Hypogastruridae and Order Entomobryomorpha.
Key for the species of Isotogastrura
1.Abd. IV with 3+3 axial setae, Abd. VI with 2 posterior cuticular tubercles I. trichaetosa
—. Abd. IV with 2+2 axial setae, Abd. VI without 2 posterior cuticular tubercles 2
2.Abd. V dorsally with a posterior median tegumentary tubercle 3
—.Abd. V dorsally without a posterior median tegumentary tubercle 5
3.Abd. I ventrally with 2 median setae; Abd. III with a pair of posterior median tegumentary tubercles I. arenicola
—.Abd. I ventrally with only 1 median setae; Abd. III without a pair of posterior median tegumentary tubercles 4
4.Prelabral microsetae absent 8
—.Two prelabral setae present I. ahuizotli
5.Abd. III dorsally with a pair of posterior median tegumentary tubercles I. litoralis
—.Abd. III dorsally without a pair of posterior median tegumentary tubercles 6
6.From Th. II to Abd. II dorsally with a pair of posterior median tegumentary tubercles I. coronata
—.From Th. II to Abd. II dorsally without such tubercles 7
7No prelabral microsetae; sensillum of sense organ of Ant. III bifid; dens with 4 setae; Abd. I sternite with 1 seta I. atuberculata
—.Two prelabral microsetae; sensillum of sense organ of Ant. III simple; dens with 3 setae (sometimes 4); Abd. I sternite with no seta I. madagascarensis
8.Th. II with 21+21 setae, no genital setae of male modified I. veracruzana
—.Th. II with 19+19 setae, 2 genital setae of male modified I. mucrospatulata sp. nov.
Comparison of traits of Isotogastrura species.
The senior author is grant-supported by PASPA program (DGAPA, UNAM). Zeppelini is grant-supported by CNPq # 301803/2012-9. The Postgraduate Program on Ecology and Conservation (State University of Paraiba) partially supported the research. Biol. Elihú Catalán, Fac. Ciencias, UNAM assisted in the elaboration of line figures and María de Jesús Martínez assisted in the preparation of figure plates.