African Diplostomum (sensu Dubois 1961): Minireview on taxonomy and biology

Freshwater fi sheries has a signifi cant contribution to development as an important source of human proteins as well as in sport fi shing and aquarium. Despite their importance, both wild and aquaculture fi sh suffer from a problem of parasitism, notably Diplostomum species, causing fi sh mortalities particularly in fi ngerlings. Although a considerable effort in understanding Diplostomum species taxonomy, biology and control of fi sh parasites has been well developed in the northern hemisphere, the knowledge of these aspects in Africa is not clearly known. Therefore the present work reviews the aspects of biology and taxonomy of African Diplostomum. The traditional approach to uncover these aspects would be to complete the life cycles in the laboratory, which would lead to morphological characterisation of all life cycle stages. However, establishing and maintaining life cycles of Diplostomum species is diffi cult, laborious and expensive. Although, molecular methods have been proven to provide an alternative solution, are not common in Africa due to lack of equipment and expertise. However, improvement of some weaknesses for some studies like providing pictures or diagrams of the Diplostomum species found is recommended. In addition, for laboratories that have the capacity to do molecular analysis, the use of a familiar molecular marker like a barcode region could be a prospective development in future. Review Article African Diplostomum (sensu Dubois 1961): Minireview on taxonomy and biology Fred D. Chibwana* Department of Zoology and Wildlife Conservation, University of Dares Salaam P.O. Box 35064, Dar es Salaam, Tanzania Received: 16 August, 2018 Accepted: 24 September, 2018 Published: 25 September, 2018 *Corresponding author: Department of Zoology and Wildlife Conservation, University of Dares Salaam P.O. Box 35064, Dar es Salaam, Tanzania, E-mail:


Introduction
Freshwater fi sheries has a signifi cant contribution to development as an important source of proteins. More than 90% of all freshwater fi sheries, i.e. wild capture and aquaculture, occurs in developing countries [1]. Besides providing food and a livelihood for millions of the world's poorest people, freshwater fi sheries contributes to the overall economic income by means of export commodity trade, tourism and recreation [2]. As a consequence, freshwater fi sheries has become an important economic activity for both rural and urban populations in Africa and globally. The widening gap between supply and demand for fi sh products, has further made capture fi sheries to be the largest extractive use of wildlife worldwide. Increases in human population, rising incomes and increasing urbanisation coupled with stagnation or decline of supplementary proteins, further exacerbates the situation. As a result most communities globally have responded by venturing into aquaculture to supplement capture fi sheries. Despite its progress, African aquaculture needs to address a cascade of challenges including lack of national policies to guide aquaculture development, unfavourable investment policies, the absence of linkages between farmers, lack of research/technology development and extension, and unfavourable investment climates, inadequate quality seed and feed, and above all infectious and parasitic diseases [3,4].
Diplostomum species, especially larval stages namely cercariae and metacercariae are among the main agents of important diseases in fi sh and aquaculture systems. Diplostomum species are strigeoid digeneans of the family Diplostomidae [5,6]. The family Diplostomidae has four subfamilies: Diplostominae, Crassiphialinae, Alariinae and Codonocephalinae, which are classifi ed on the basis of host specifi city and metacercariae types [7]. However, the present review focuses on members of the compound genus Diplostomum [8], that includes three subgenera, often elevated to genera level [9]. They are Diplostomum, Tylodelphys and Dolichorchis. These parasites are ubiquitous in freshwater systems, but the most frequently encountered stages are the metacercariae that reside unencysted in the eyes, cranial and brain cavities of freshwater fi shes. Diplostomum can be highly pathogenic to fi sh and thus threaten the natural and aquaculture practices globally. Fingerlings in particular may experience high rates of mass mortalities when heavily infected with metacercariae, or as a result of massive cercarial penetration [5]. The metacercariae can also impair the fi sh escape response, diminish fi sh crypsis and thus may increase their vulnerability to predation [10]. In addition, cercarial penetration can increase the chances of bacterial infection in fi sh [11] and productivity of farms could be severely compromised. glabrata have been reported as the potential fi rst intermediate [19]. In Africa, on the other hand, the range of snail hosts are far from completely understood, despite several experimental attempts undertaken [13,20]. The only known life cycle is that of Tylodelphys xenopi, in which the freshwater snail Bulinus tropicus is the fi rst intermediate host [21]. Beverley-Burton (1963) also tried to infect Radix natalensis (Lymnaeidae) with Diplostomum (Tylodelphys) mashonense, but could not obtain cercariae. Even so, strigeoid cercariae purported to belong to the genus Diplostomum have been reported from Biomphalaria species from three fi sh farms in Kibos area within Kisumu, Kenya [22]. Similarly, at Mindu dam in Tanzania, Diplostomum-like furcocercariae had been reported once from the snail Biomphalaria pfeiff eri [23,24], but since then B. pfeiff eri have not been spotted in the dame. Thus the consistently high prevalence of the metacercariae of T. mashonense in C. gariepinus at Mindu Dam [23,25], brought a suspicion that nonlymnaeid snail species could serve as snail hosts, particularly because lymnaeid snails have not been recorded within or around the dam. As such it was hypothesised that other snails besides lymnaeids and Biomphalaria, could be responsible for the transmission and Bulinus spp were shown to support that hypothesis [25], as shown in fi gure 1.

Many studies on
In Diplostomum species, host specifi city is mostly restricted in the fi rst intermediate host while less specifi c in the second intermediate host and fi nal hosts. For instance in Europe and North America, metacercariae of Diplostomum species have been recorded from over 150 species of fi sh from families Percidae, Salmonidae, Coregonidae, Clupeidae, Gobiidae, to name a few [26,27] and a broad range of piscivorous bird species serving as defi nitive hosts [10]. Similarly in Africa, Diplostomum species have been found in almost every fi sh family i.e. Characidae, Centrarchidae, Cichlidae, Claridae, Cyprinidae, Hepsetidae, Salmonidae, Schilbeidae to mention but a few [28,29]. However, the catfi sh Clarias gariepinus (family Claridae) is the most examined fi sh [29] and references therein. Nonetheless the range of fi sh hosts in Africa is not well known as the level of Diplostomum studies in Africa is still at an infant stage.
As far as defi nitive hosts are concerned, in Africa adult Diplostomum have been reported from the Egyptian kite Milvus migrans aegypticus, the Egyptian moorhen Gallinula chloropus chloropus and the giant heron Ardea goliath in Egypt [20,30,31], Pel's fi shing owl Scotopelia peli in Ivory Coast [32], the grey heron Ardea cinerea in Zimbabwe and Tanzania [13,23,33] the African darter, Anhinga rufa rufa in Ghana [34] and the great white egret Ardea alba in Tanzania [23,33]. Generally, studies of Diplostomum species in bird defi nitive hosts in the African continent are scarce and limited. This is attributed to (i) low sampling efforts in the tropical countries due to inadequate expertise and resources (ii) diffi culty in getting study permits to sacrifi ce some birds as they are either found in the protected areas (national parks and game reserves) or lack of interests in fi sh parasitology.

Taxonomy of diplostomum (Sensu Dubois, 1970) species in africa
Precise identifi cation of members of the genus Diplostomum is usually diffi cult because of remarkable morphological similarity within and among species at almost every developmental stage [6]. Also, lack of a well-defi ned criterion further exacerbates the delineation diffi culty within the Diplostomum group [6,35]. Furthermore, the taxonomic problem is aggravated by deformation of the body in the course of fi xation, staining and mounting of permanent preparations [6]. In addition, many species have been described on the basis of one or two life cycle stages as a result different stages of the same species have been given different names or different species are known by the same name [6].
The taxonomy of Diplostomum species in Africa in general remains not well understood because the full range of species that may occur in Africa is far from completely known. As a consequence the biology of all reported species such as the range of hosts and life cycle stages (eggs, miracidia, intramolluscan stages, cercariae) is poorly understood. So far in the whole of Africa the adults of only fi ve Diplostomum species have been described; they are D. tregenna [12], D. marahoueense [32], D.
xenopi [21]. Nevertheless, more than 15 Diplostomum species have been reported as metacercaria in different fi sh hosts species [29,37,38]. Although some authors tried to name the metacercarial stages to species level, for example D. garrae, D.
Moreover the taxonomic status of some Diplostomum species that have been recorded from some fi sh hosts is confusing.
For instance, when the genus Clarias was reviewed by [39], and Zhokhov [38] who considered Diplostomum tregenna as Dolichorchis tregenna. At the metacercarial stage the fundamental features that enable distinguishing genera or species are the shape of fore-and hindbodies, presence or lack of additional organs of attachment like pseudosuckers, the structure of the holdfast organ, structure of the reserve bladder, the shape and spread of the calcareous bodies [35].  of Congo (DRC) [36] and T. xenopi from an experimental host, the African darter Anhinga melanogaster in South Africa [21] and T. mashonense from Ardea cinerea and A. alba [13,33]. Other   [13,20].
In addition, maintaining life cycles using in vivo systems is diffi cult, laborious and expensive and in vitro cultivation is almost impossible [6]. All these considerations indicate that both the identifi cation of the larval stages and uncovering the diversity of Diplostomum species in Africa is challenging.  has been widely adopted by other biological fi elds, the parasitological community has barely used it. In parasitology and Diplostomum in particular, DNA barcoding has been able to reveal diversity and specifi city of metacercariae in hosts [52], disentangle cryptic species [53] and linking life cycle developmental stages [25]. As already shown by Besansky et al. (2003), DNA barcoding is potentially a great tool to improve the rate of discovery of parasites' species diversity and life cycles. The author of this review, therefore recommends the increased use of this genetic region in African diplostomids in order to fi ll the taxonomic gaps prevailing in Diplostomum species emanating from various morphological challenges.
It will also enable quick identifi cation of diplostomid species irrespective of their developmental stage once their sequences are deposited in the public nucleotide databases.

Conclusion
Since the fi rst Diplostomum species in Africa was described However, improvement of some weaknesses for some studies like providing pictures or diagrams of the Diplostomum species found. For laboratories or researchers that have the capacity to do molecular analysis, the use of a molecular marker that is commonly used like barcode region CO1 and ITS could be a prospective development in future.