Owing to the hasty growth of communication technologies in the Underwater Internet of Things (UIoT), many researchers and industries focus on enhancing the existing technologies of UIoT systems for developing numerous applications such as oceanography, diver networks monitoring, deep-sea exploration and early warning systems. In a constrained UIoT environment, communication media such as acoustic, infrared (IR), visible light, radiofrequency (RF) and magnet induction (MI) are generally used to transmit information via digitally linked underwater devices. However, each medium has its technical limitations: for example, the acoustic medium has challenges such as narrow-channel bandwidth, low data rate, high cost, etc., and optical medium has challenges such as high absorption, scattering, long-distance data transmission, etc. Moreover, the malicious node can steal the underwater data by employing blackhole attacks, routing attacks, Sybil attacks, etc. Furthermore, due to heavyweight, the existing privacy and security mechanism of the terrestrial internet of things (IoT) cannot be applied directly to UIoT environment. Hence, this paper aims to provide a systematic review of recent trends, applications, communication technologies, challenges, security threats and privacy issues of UIoT system. Additionally, this paper highlights the methods of preventing the technical challenges and security attacks of the UIoT environment. Finally, this systematic review contributes much to the profit of researchers to analyze and improve the performance of services in UIoT applications.

LGR4 and LGR5 encode two homologous receptors with critical, yet distinct, roles in organ development and adult stem cell survival. Both receptors are coexpressed in intestinal crypt stem cells, bind to R-spondins (RSPOs) with high affinity, and potentiate Wnt–β-catenin signaling, presumably by the same mechanism: forming RSPO-bridged complexes with the E3 ligases RNF43 and ZNRF3 to inhibit ubiquitylation of Wnt receptors. However, direct evidence for RSPO-bound, full-length LGR5 interacting with these E3 ligases in whole cells has not been reported, and only LGR4 is essential for the self-renewal of intestinal stem cells. Here, we examined the mechanisms of action of LGR4 and LGR5 in parallel using coimmunoprecipitation, proximity ligation, competition binding, and time-resolved FRET assays in whole cells. Full-length LGR4 formed a tight complex with ZNRF3 and RNF43 even without RSPO, whereas LGR5 did not interact with either E3 ligase with or without RSPO. Domain-swapping experiments with LGR4 and LGR5 revealed that the seven-transmembrane domain of LGR4 conferred interaction with the E3 ligases. Native LGR4 and LGR5 existed as dimers on the cell surface, and LGR5 interacted with both FZD and LRP6 of the Wnt signalosome to enhance LRP6 phosphorylation and potentiate Wnt–β-catenin signaling. These findings provide a molecular basis for the weaker activity of LGR5 in the potentiation of Wnt signaling that may underlie the distinct roles of LGR4 and LGR5 in organ development, as well as the self-renewal and fitness of adult stem cells.

The underwater internet of things (UIoT) has emerged as a booming technology in today’s digital world due to the enhancement of a wide range of underwater applications concerning ocean exploration, deep-sea monitoring, underwater surveillance, diver network monitoring, location and object tracking, etc. Generally, acoustic, infrared (IR), visible light (VL), radiofrequency (RF), and magnet induction (MI) are used as the medium of communication in order to transfer information among digitally linked underwater devices. However, each communication medium has its advantages and limitations: for example, the acoustic communication medium is suitable for long-range data transmission but has challenges such as narrow bandwidth, long delay, and high cost, etc., and the optical medium is suitable for short-range data transmission but has challenges such as high attenuation, and optical scattering due to water particles, etc. Furthermore, UIoT devices are operated using batteries with limited capacity and high energy consumption; hence, energy consumption is considered as one of the most significant challenges in UIoT networks. Therefore, to support reliable and energy-efficient communication in UIoT networks, it is necessary to adopt robust energy optimization techniques for UIoT networks. Hence, this paper focuses on identifying the various issues concerning energy optimization in the underwater internet of things and state-of-the-art contributions relevant to inducement techniques of energy optimization in the underwater internet of things; that provides a systematic literature review (SLR) on various power-saving and optimization techniques of UIoT networks since 2010, along with core applications, and research gaps. Finally, future directions are proposed based on the analysis of various energy optimization issues and techniques of UIoT networks. This research contributes much to the profit of researchers and developers to build smart, energy-efficient, auto-rechargeable, and battery-less communication systems for UIoT networks.

As oceans cover the majority of the earth’s surface, it becomes inevitable in extending the concepts of Internet of Things (IoT) to ocean bodies, thereby tiling the way for a new drift in the digital world, the Internet of Underwater Things (IoUT). The primary objective of IoUT is the creation of a network of several smart interconnected undersea things, to digitally link water bodies by using devices such as autonomous underwater vehicles. Since the traditional ideas of IoT cannot be merely expanded to underwater, due to the difference in environmental characteristics, this puts forward a variety of challenges for scientists to work with IoUT, and one such challenge is the network management with IoUT. This paper gives an overview on (1) underwater network management systems (U-NMS) using acoustic communication in IoUT; (2) the challenges and benefits and use cases of U-NMS; (3) fault, configuration, accounting, performance, security and constrained management (FCAPSC) functionalities of U-NMS and (4) a comparison between network management system in IoT and U-NMS system in IoUT. Additionally, this paper shows the prototype design and implementation setup of U-NMS in a laboratory environment, using lightweight machine to machine (LWM2M) and acoustic communication technology for IoUT. This paper will contribute much to the profit of researchers and industry players in uncovering the critical areas of the Internet of Underwater Things.

In this paper, several MAC scheduling methods applicable to an underwater environment are proposed. Besides, a new marine communication system model was proposed to improve the reliability of the proposed SBMAC method. The scheme minimizes transmission of control frames except for data transmission and various transmission methods and ACK methods can be used together. Simulation models are set indices and analysis of the underwater environment is established to conduct reliable simulations. Consequently, the performance improvement of the proposed method is verified with respect to delay time, data transmission rate, memory utilization, energy efficiency, etc.