Environmental temperature is one of the most significant factors influencing the developmental rate and survival of embryos and larvae of many marine animals, including polychaetes. The aim of this study was to experimentally evaluate the effect of temperature increase on the embryonic development and larval survival of Spirobranchus incrassatus and S. cf. corniculatus. Adult worms of both species were collected from the western margin of the Gulf of Tehuantepec, Mexico. Embryos and larvae were obtained from these worms and exposed to four temperature treatments (28, 30, 32 and 34°C). The optimal temperature for embryonic development of S. incrassatus and S. cf. corniculatus was 30 and 28°C, respectively. For both species, the maximum critical temperature was 32°C and the lethal temperature was 34°C. The embryonic stages of S. cf. corniculatus were most negatively affected by elevated temperatures. Larval survivorship of S. incrassatus and S. cf. corniculatus was higher at 30°C and 28°C (76.2±2.8%; 72.6±4.2%) and lower at 34°C (28.9±2.6%; 26.3±2.9%), respectively. These results suggest that S. cf. corniculatus has the lowest thermal tolerance. Both Spirobranchus species already live near their upper limit of thermal tolerance in the study region (30°C). In the near future, under a global warming scenario, the distribution of both species could be modified, causing a loss of biodiversity, changes in the trophic chain, and alterations in the water column, such as excess organic matter.

Species of the genus Spirobranchus are sessile polychaetes with an important role in marine ecosystems since they are suspension-feeding invertebrates. However, for most species almost nothing is known about their early and larval development, so we describe the early development of two species of Spirobranchus from the Southern Mexican Pacific. The early trochophore larva of S. incrassatus was formed 12 h post-fertilization at 28°C and was described up to a 24-day-old metamorph, whilst it was only possible to follow S. cf. corniculatus to the eight-day-old early trochophore larva. With this study, we describe the early embryological stages, which provide an understanding of the reproductive biology of these polychaetes. Furthermore, this type of information is the basis for future experimental work on the possible response of embryos and larvae to changes in environmental variables, such as temperature.

Understanding the physiological mechanisms that facilitate invasion success is critical for predicting the persistence and spread of non-native species under increasing thermal variability. We examined thermal tolerance, acclimation capacity, and metabolic performance in juveniles of the invasive red swamp crayfish Procambarus clarkii (Girard, 1852) under controlled laboratory conditions. Juveniles were acclimated for 30 days to four ecologically relevant temperatures (20, 24, 28, and 32°C), and upper and lower critical thermal limits (CTmax and CTmin), thermal tolerance breadth (thermal polygon area), acclimation response ratios (ARR), and thermal metabolic scope (TMS) were quantified. Both CTmax and CTmin increased significantly with acclimation temperature, indicating pronounced thermal plasticity. CTmax rose from 37.4 to 40.1°C, whereas CTmin increased from 8.5 to 16.4°C across the acclimation range. The resulting thermal polygon area (286.0°C²) revealed a broad thermal tolerance consistent with a eurythermal strategy. ARR analysis showed asymmetric acclimation capacity, with greater plasticity at lower than at upper thermal limits. Aerobic metabolic performance, expressed as TMS, exhibited a unimodal response across acclimation temperatures, peaking at 28°C and declining at the thermal extremes, yet remaining consistently high and far from physiological collapse across all treatments. Together, these results demonstrate that juvenile P. clarkii combines broad thermal tolerance, asymmetric acclimation capacity, and sustained aerobic performance across a wide thermal range. This suite of ecophysiological traits likely enhances survival, establishment, and spread in thermally heterogeneous freshwater environments, providing mechanistic insight into the invasion success of this globally invasive crayfish under current and future climate variability.

Pomatostegus Schmarda, 1861 is a serpulid genus with three recognized species, mostly from tropical waters. Pomatostegus stellatus (Abildgaard, 1789), was described from the Caribbean Sea, and has been widely recorded in the Pacific and Indian oceans; P. kroyeri Mörch, 1863 was described from Puntarenas, Costa Rica; however, most records from the Tropical Eastern Pacific were referred to P. stellatus. In this work, the two species are redescribed using morphological characters. The main differences are in the shape of the Spirobranchus-type collar chaetae, opercular plate shape and the number of “free” circlets of spines without accompanying of opercular plates. Comments about characters of P. actinoceras Mörch, 1863, described from Philippines, and an identification key for the three species, are included.