First characterisation of the populations and immune-related activities of hemocytes from two edible gastropod species, the disk abalone, Haliotis discus discus and the spiny top shell, Turbo cornutus

Abstract

The disk abalone Haliotis discus discus and the spiny top shell Turbo cornutus are edible gastropod species of high economic value, mainly in Asia. Mortality outbreaks and variations in worldwide stock abundance have been reported and suggested to be associated, at least in part, with pathogenic infections. Ecology, biology and immunology of both species are currently not well documented. The characterisation of the immune systems of these species is necessary to further assess the responses of H. discus discus and T. cornutus to environmental, chemical and disease stresses. In the present study, we investigated the morphology and immune-related activities of hemocytes in both species using light microscopy and flow cytometry. Two types of hemocytes were identified in the disk abalone hemolymph, blast-like cells and hyalinocytes; whereas four main hemocyte types were distinguished in the spiny top shell, blast-like cells, type I and II hyalinocytes, and granulocytes. Flow cytometric analysis also revealed differences between cell types in immune-related activities. Three subsets of hemocytes, defined by differing lysosomal characteristics, were observed in the hemolymph of the spiny top shell, and only one in the disk abalone. Phagocytic activity was higher in H. discus discus hemocytes than in T. cornutus hemocytes, and the kinetics of PMA-stimulated oxidative activity was different between hemocytes of the disk abalone and the spiny top shell. Finally our results suggest for the first time a predominant mitochondrial origin of oxidative activity in gastropod hemocytes.

Introduction

Homeostasis in molluscs partially relies upon cells freely circulating in hemolymph and infiltrating in tissues, referred to as hemocytes [1], [2], [3]. Hemocytes are involved in various and numerous physiological functions including nutrient digestion, transportation, and distribution [4], [5], [6], and shell and tissue repair [7], [8], [9]. Hemocytes also mediate cellular internal defence in molluscs through accumulation and detoxification of chemical toxicants [10], [11], [12], phagocytosis [13], [14], [15], [16], [17], [18] and encapsulation [19], [20], [21], [22] of invading, foreign, biological material.

The characterisation of hemocytes under natural, ambient conditions is essential to further understand cell-mediated responses of molluscs to environmental, anthropogenic and pathological stresses. Types and functions of molluscan hemocytes, however, have not been fully described. Until now, most knowledge of molluscan hemocytes has been from bivalve species [2], [17], [23], [24], mainly because of their high economic value [25]. Hemocytes of gastropods have been investigated mainly in biomedically important snail species, such as Biomphalaria glabrata [26], [27], [28], [29], [30], [31] and Lymnaea stagnalis [32], [33], [34], [35], [36], [37]. B. glabrata and L. stagnalis are, respectively, intermediate hosts for the human parasitic blood fluke Schistosoma mansoni, responsible for schistosomiasis, a parasitic disease affecting roughly 200 million people throughout the world [38], and the avian Schistosoma parasite, which can accidentally infect humans [39]. Gastropod hemocyte characterisation was also reported in the abalone species Haliotis diversicolor [40], [41], Haliotis asinina [42], Haliotis rufescens, Haliotis cracherodii [43], [44] and Haliotis tuberculata [45], [46], [47] and in only a few other gastropods [48], [49], [50], [51].

Classification of molluscan hemocytes commonly has been based upon morphological and biochemical features analysed by light and electron microscopy [1], [40], [48], [49], [50], [51], [52]. Flow cytometry also has been applied to molluscan hemocyte studies but, until now, mostly in bivalve species [53], [54], [55], [56], [57], [58], [59], [60], [61], with only very few studies of gastropods [36], [37], [46], [47], [62], [63].

Although controversies still persist about molluscan hemocyte classification, two main classes are generally accepted: granulocytes, containing many intra-cytoplasmic granules, and agranulocytes, with no or fewer granules [1], [2], [24], [64]. While granulocyte population may appear to be homogeneous, various agranulocyte subpopulations were reported, including different subtypes of hyalinocytes [24], [27], [40], [48], [49], [56], [58], [60] and juvenile or blast-like cells [27], [40], [46], [49], [61], [65], [66], [67]. It is not clear currently whether such diversity in hemocyte subtypes represents distinct cell lineages, differences in differentiation and/or physiological state, or variations in methodology being applied.

The disk abalone Haliotis discus discus and the spiny top shell Turbo cornutus are two representatives of the vetigastropods, a very ancient evolutionary lineage of marine gastropods. Both species are edible molluscs of high economic value in Asia. Worldwide stock abundance of abalones and top shells has been impacted by mass mortalities and large fluctuations [68]. Understanding the immune systems of these species is necessary to better assess the relative contributions of environmental factors, pathogenic infections and culture-related stresses in mortality outbreaks. In the present report, we applied light microscopy and flow cytometry to characterize the populations and immune-related activities of the circulating hemocytes of the disk abalone and the spiny top shell.