Our observations are the first qualitative description of the reproductive behavior in the rhomboid mojarra (Diapterus rhombeus). This species is one of the most common and well-studied estuarine species found along the Brazilian coast. In Brazilian waters, rhomboid mojarras are known to typically spawn during the summer months (i.e., December to April; [20]), but may also spawn in other periods, for instance, from August to November [17]. The distance between these studies is nearly 2500 km, which may explain why such differences in the reproductive period exist (e.g., regional environmental characteristics such as water temperature and salinity). In the Brazilian central coast (present study), the environmental variables are similar to those reported by [17]. Previous observations made on D. rhombeus revealed that adults typically spawn in areas deeper than 10 m, whereas juveniles use the shallow waters of estuaries, bays and mangroves. Moreover, [20]) suggested that smaller individuals (i.e., < 150 mm TL) present evidence of previous spawning events. However, here, we show that shallow habitats, such as mangroves, are highly suitable spawning habitats for this species.
The sequence of events observed herein, along with the video footage we produced, allows the subdivision of the behavior into distinct modalities. First, the apparently disoriented high-speed swimming of individuals in group is indicative of a breeding aggregation. Incidentally, this behavior appears to detract other typical mangrove fishes (e.g., snappers, sea bass and pilchards) from the area, as these species were never observed during these breeding aggregations. Second, we observed a sequence of repetitive movements beneath the roots (i.e., nest clearing), leading many fish into a ‘numbness’ or lethargic state.
Distinct hypotheses can be put forward to explain the observed aggregations of mojarras and their unexpected lethargic behavior. For example, these fish may spawn in shallow areas that are difficult for most predators to access [25]. Furthermore, the presence of streams and mangrove creeks near a spawning event (JAR-F, unpublished data) may result in increased food availability (i.e., from the inputs of organic matter; [26, 27]), which may thereafter be essential to support a large aggregation of offspring. By itself, the lethargic state in which fish were found hiding themselves among prop roots may be a consequence of their state of fatigue after spawning, in which fish would be particularly vulnerable to predation. Thus, it appears that mating interactions are plausible explanations for the observed aggregations. This theory is further corroborated by the fact that most of the individuals captured were sexually mature. Moreover, the seemingly erratic and high-velocity swimming we observed is typical of groups undergoing nest preparation [28], which also involves modifying the benthic habitat to accommodate the eggs to be released [29]. Nest builders of marine substrata typically clean nesting areas by removing and/or farming preferred substratum before egg release [30, 31]. However, given that nest preparation behavior seldom occurred during our observations, group breeding aggregations appear to be the main reproductive strategy that rhomboid mojarras undergo in shallow water mangroves. Overall, breeding in shallow-water mangrove habitats provides the main advantage of reduced predation risk for offspring and spawning adults as well as ample resources for newborns.
While it is difficult to conclusively establish the precise functions of the seemingly complex behavioral display presented by D. rhombeus, the success (i.e., abundance and wide distribution) of this species in numerous embayments of the Brazilian coast is tied to its life history strategies (see [32,33,34,35]). Thus, it is important to consider that the observed reproductive strategies may not necessarily be common in D. rhombeus, as this species is able to adjust to a wide range of conditions and thereby show plasticity in its behavior [34], a key element to exploit a wide range of environmental conditions. Therefore, whether this behavior occurs in turbid estuarine conditions remains to be determined, and to this effect, multibeam sonar may be able to provide a definite answer.
To protect this species (and its habitat), it is paramount to understand its reproductive behavior and how it is tied to habitat requirements. Notably, the rhomboid mojarra is an important component of the artisanal fishing sector in the South American continent [33, 36,37,38] and may suffer unforeseen effects from small-scale fisheries, which may be difficult to properly assess. An understanding of this organism’s behavior and habitat requirements is an important step in this direction.