Previous research has been focused on terrestrial decomposition and entomology as a post-mortem interval (PMI) estimation method for remains in forensic casework [1]. More recent case studies [2,3,4,5] have determined a high frequency of remains discovered in aquatic environments, but with little controlled research on aquatic decomposition there is a knowledge gap around aquatic decomposition and estimating post-mortem submersion intervals (PMSI) in aquatic cases. Most recent studies on aquatic decomposition have largely focused on large water bodies such as rivers [2, 3] and the ocean [4, 5], investigating factors in full submersion scenarios such as algal colonisation, aquatic decomposition patterns and scavengers. However, there is limited documentation of decomposition in environments such as tidal areas, shallow waters where remains are subject to both terrestrial and aquatic factors.

PMI is the term frequently used in terrestrial research and casework to refer to the time since death. In contrast, PMSI refers to the submersion interval of remains and is the term used in aquatic casework and research. A PMSI is established to determine the time the remains were submerged for but doesn’t consider the time the remains may have been outside of the water before submersion. This is an important distinction to consider when discussing casework. Further, in terrestrial cases there is the need to consider the pre-appearance interval (PAI) of insect colonisation as an unknown period, thus a minimum PMI is established (PMImin); this is considered the minimum amount of time the remains have to have been there for based on the time required for the insects to have reached that life stage/level of growth [8]. In an aquatic setting there may be the PAI to consider after remains have floated or become partially submerged, but there is also an additional unknown submersion period to account for.

Terrestrial decomposition follows a well-established pattern of stages: fresh, bloat, active decay, advanced decay and dry/skeletal [1]. These stages are accompanied by predictable characteristics and documented insect succession where Calliphoridae are primary colonisers in the fresh to active decay stages, followed by secondary dipteran colonisers and Coleoptera in active decay stages and towards the advanced decay and dry stages [6, 7]. In contrast, there are no true obligate necrophagous insects in aquatic environments [7, 8], with most insect activity relevant to aquatic decomposition being flying insects with access to the remains only during periods of bloat and float [9, 10]. Aquatic decomposition has its own defined decomposition stages, being submerged fresh, early float, floating decay, floating deterioration, floating remains, and sunken remains [10]. However, these stages are less predictable in onset and duration than terrestrial stages and are impacted by variables such as float and bloat pattern and orifice access for insects [11].

Recent aquatic studies have focused on complete submersion and use of the total aquatic decomposition score (TADS) [12, 13] as a method to determine PMSI. These studies were able to establish a PMSI using a combined approach of accumulated degree days (ADD) and physical morphology of remains, however presented large confidence intervals and in Dalal’s model a consistent overprediction of PMSI [13]. Further, this method has only been researched in complete submersion cases and cannot be directly applied with confidence to hybrid/partial submersion cases without further research.

Other studies have evaluated the use of terrestrial based PMI estimation methods such as insect activity in aquatic environments, stemming from Payne and King’s initial work on aquatic decomposition in 1972 [10]. Heo et al. [9] researched insect succession in aquatic decomposition using a man-made freshwater pond system. Though insect colonisation in this study was high, there was an unknown submersion interval of time and significant insect death due to the carcass sinking that determined this to be an inaccurate method to estimate PMSI. Bray et al. [11] also presented the lack of ability to use insects in aquatic cases as a result of an unpredictable submersion interval, emphasising a lack of predictable insect succession. Similarly, the difficulty in determining the unknown time period between initial submersion and floating remains adds further potential discrepancies for PMSI estimation. This unknown submersion period confounded with the pre-appearance interval (PAI) of insects to the now accessible corpse creates considerable potential error for PMI/PMSI estimations utilising insects.

This has highlighted the need for further studies into increasing the confidence of PMSI estimations, achievable through further research of aquatic variables, the adaptation of approaches, and combining approaches for increased accuracy. Several recent studies [4, 5, 9, 11, 14,15,16] have investigated aquatic decomposition of completely submerged remains, yet no studies have considered how partial submersion might result in dual zone scenarios with both aquatic and terrestrial components, outside of the float and bloat stages of aquatic decomposition. Methods such as TADS [13] are only applicable when the remains are entirely subject to an aquatic environment, otherwise discrepancies in physical characteristics will be unable to provide a reliable PMSI. Further, although entomology is well established in terrestrial casework for PMI estimations, without understanding the effect the proximity of water has on insect attraction and colonisation of partially submerged remains insects cannot be utilised in aquatic case work for PMI/PMSI estimations. When considering tidal, draught and flood environments where remains are likely to be only partially submerged, there needs to be an understanding established of the potential for remains to be segmented into distinct aquatic and terrestrial regions, if there is cross over between the two environments, and what this means for insect succession and the use of entomology for PMI estimation.

This study considered the potential of hybrid situations between terrestrial and aquatic decomposition, with the aim of establishing how the presence of two distinct decomposition environments affects the rate of decomposition of a single carcass comparatively. Further, this study aimed to establish a recommended approach for estimating PMSI in such scenarios.