Arthropoda, Volume 3, Issue 1 (March 2025) – 5 articles

Here, we confirm the presence of two species of caridean shrimps for the first time in Brazil: Latreutes parvulus and Ambidexter cochensis. During active samplings conducted in December 2023, April 2024 and September 2024 on the rocky shore of Prainha da USP (Ubatuba municipality, São Paulo state, southeastern Brazil), we captured five ovigerous females of L. parvulus and fifteen individuals of A. cochensis (five males, three non-ovigerous females and seven ovigerous females). All the females of L. parvulus exhibited eyestalks with small, truncated processes separating the cornea from the stalk, a unique feature of this species. On the other hand, all specimens of A. cochensis exhibited a unique combination of characteristics: a rostrum with an apex bifid, a stylocerite bearing a lateral spinule and a transverse row of setae between the anterior pair of spines, and three longitudinal rows of setae on the telson. Also, males of A. cochensis had the tip of their appendix masculina armed with four simple apical setae and another two setae positioned below them. All analyzed shrimps had fully developed gametes in their reproductive systems, indicating breeding populations in this area. This is the first record of L. parvulus on the Atlantic coast of South America and a new meridional distribution limit for A. cochensis. Full article

The palaemonid shrimp genus Pseudopalaemon Sollaud, 1911, is endemic to South America, comprising seven freshwater and low salinity species. This study aimed to compile an overview of the genus, including an illustrated identification key for species and updated distributional data. Diagnostic morphological characters of the species were analyzed using specimens from several museums and other scientific collections. Current knowledge about the genus reveals significant knowledge gaps, particularly in species ecology. The data and insights from this study support future research and highlight a need for further studies on this group. Full article

Trilobites, as typical euarthropods, possess compound eyes. In 1901, Lindström was the first to describe them in detail; on the one hand, we reconsider his descriptions of the different modes of trilobite eyes; on the other hand, we expand this by compiling the observations that have been possible in recent years. There are two, perhaps three kinds of trilobite compound eyes. The first are the primordial holochroal eyes, which are actually apposition compound eyes, similar to those of many modern diurnal crustaceans and insects. The abathochroal eyes, often referred to as the second form, are probably a subtype of the holochroal eyes. Consequently, the second is the schizochroal eye of phacopid trilobites, which are hyper-compound eyes composed of numerous small compound eyes below each of the big lenses, which appear from outside as one big lateral eye each. Thirdly, one may call the maculae light-sensitive organs, but this is still uncertain. Comparing what are probably the oldest trilobite eyes described so far with other forms, it is possible to conclude that the sensory apparatus is much older than the fossil record of trilobite eyes and probably developed in Precambrian times. The refractive apparatus, however, was developed later and separately within the systematic groups. This explains why, for example, the mandibulates have a lens and a crystalline cone. Still, the chelicerate xiphosurans, such as horseshoe crabs or eurypterids, possess a lens cylinder with an index gradient but no crystalline cone. Furthermore, this can explain why the calcite character of trilobites is unique in the arthropod kingdom. An important discovery is the probably epidermal, lens-building cells encompassing a prospective lens of Schmidtiellus reetae Bergström 1973 from the early Lower Cambrian of Estonia. We reconsider the morphology of hypostome maculae and interpret them as a potential phylogenetic relict and a potential predecessor of all arthropod ommatidial compound eyes. It will be of great relevance for future research to understand the evolution of compound eyes and vision because we witness the emergence of the first lenses in the trilobite, if not the arthropod kingdom. Full article

Mastigoproctus Pocock, 1894, is the most speciose genus in the thelyphonid subfamily Mastigoproctinae Speijer, 1933, with eighteen described species distributed from the Southern United States to Colombia and Venezuela. Ten of these species occur in Mexico. In the present contribution, Mastigoproctus spinifemoratus, sp. nov., is described based on an adult male and two juveniles from Eastern Nuevo León and Southwestern Tamaulipas, Mexico. It differs from five other species of Mastigoproctus, in which spiniform tubercles are present on the retrolateral surface of the pedipalp femur, in the ventrally directed epistome of the carapace, and the absence of an accessory spine on the prodorsal margin of the pedipalp trochanter. The new species raises the number of Mastigoproctus species to nineteen and the number in Mexico to eleven. Full article

Rock shrimps (Sicyonia dorsalis and Sicyonia typica) are commonly caught as bycatch during shrimp trawling along the Brazilian coast, but are not commercially exploited due to their small size and hard carapace. This study evaluated their spatio-temporal distribution, size classes, and sex ratio near the Almada River Estuary, Ilhéus, Bahia, Northeastern Brazil, and tested correlations between environmental factors and species abundance. Samples were collected monthly using double-rig trawl nets in the estuary and along transects at depths of 5–35 m. Bottom water and sediment samples were obtained for analyses of environmental factors. In total, 5336 individuals of S. dorsalis and 303 individuals of S. typica were collected. No individuals were recorded in the estuary. Both species were significantly more abundant between 25 and 35 m, where fine sediment with high levels of organic matter occurred. Considering the temporal variation, their abundance decreased during the rainy season, coinciding with increased river flow. Organic matter content, salinity, and water transparency were the primary environmental factors influencing abundance. Females were generally larger and predominant compared to males, likely due to life cycle dynamics. Despite being congeneric and sympatric, the species exhibited distinct population patterns, possibly to avoid niche overlap and competition. Full article