Does larval supply explain the low proliferation of the invasive gastropod Crepidula fornicata in a tidal estuary?

Abstract

Human-mediated transport and aquaculture have promoted the establishment of non-indigenous species in many estuaries around the world over the last century. This phenomenon has been demonstrated as a major cause of biodiversity alterations, which has prompted scientists to provide explanations for the success or failure of biological invasions. Crepidula fornicata is a gastropod native from the East coast of North America which has successfully invaded many European bays and estuaries since the 19th century, with some noticeable exceptions. Its spread over Europe has been explained by a combination of human-mediated transport and natural dispersal through its long-lived planktonic larva. We here investigated whether larval supply may explain the failure in the proliferation of this species within a particular bay, the Bay of Morlaix (France). Patterns of larval distribution and larval size structure were analysed over ten sites sampled three times (20 July, 4 August and 21 August 2006), regarding characteristics of the adult population and environmental features. Our results evidenced a strong spatial structure in both larval abundance and size at the bay scale, even if larval abundances were low. In this scheme, the location of spawning adults played a critical role, with high numbers of early larvae above the main spawning location. The larval size structure further showed that settlement-stage larvae were rare, which suggested that released larvae might have been exported out of the bay. The use of an analytical model aimed to study the effect of tidal currents on the potential for larval exportation confirmed that larval retention within the bay might be low. The limitation in larval supply resulting from the interactions between spawning location and local hydrodynamics may thus impede the proliferation of this species which is well established for more than 50 years. This study provided an example of factors which may explain the failure of the transition between two major steps of biological invasions, i.e. sustainable establishment and proliferation.

Appendix

The descriptions and the values of the parameters used in the analytical model are given in Table 2. Figure 6 gives the schematic representation of the study bay used in the analytical model. Two regions are considered. The region B is delimited in the north by a section of surface s ₁(t). The adult population is located within the region B at a distance x of the section s ₁(t). The region E is defined as the area outside the bay where the larvae released from the adult population can be exported by the tide: its width L is equal to x _T  − x, where x _T is the maximal tidal excursion. The surface s ₁(t) of the section separating the regions B and E varies with the tidal cycle and is given by:

$$ s_{1} (t) = W \times (h + \varepsilon (t)) $$

(5)

with W the length of the region E, h the mean depth of the bay, and ε(t) the free surface elevation.

Table 2 Values used to parameterize the analytical model

Fig. 6

Schematic view of the bay used to construct the analytical model. Meaning of the letters are presented in Table 2. The two white dots indicate the innermost and outermost spawning locations which were used in the model, with a distance within the bay of 8 and 1 km, respectively

The section of surface s ₂(t) is the other section of the region E. By this section, larvae are exported outside E by the offshore residual current U. s ₂(t) also varies with the tidal cycle following:

$$ s_{2} (t) = L \times (h + \varepsilon (t)) = (x_{T} - x) \times (h + \varepsilon (t)) $$

(6)

The volume V _B (t) of the region B is calculated from the surface of the bay S, the mean depth of the bay h, and the free surface elevation ε(t) following the equation:

$$ V_{B} (t) = S \times (h + \varepsilon (t)) $$

(7)

and the volume V _E (t) of the region E is calculated from:

$$ V_{E} (t) = W \times s_{2} (t) $$

(8)

The volume V ₁ (t) exchanged between the regions B and E according to the tidal current u(t) through the section of surface s ₁(t) and during a time step Δt is defined by:

$$ V_{1} (t) = u(t) \times s_{1} (t) \times \Updelta t $$

(9)

and the volume V ₂ (t) removed from region E by the residual current U through the section of surface s ₂(t) during a time step Δt is defined by:

$$ V_{2} (t) = U \times s_{2} (t) \times \Updelta t $$

(10)

Using those equations, the final equations of the analytical model are:

• During ebb:

  $$ N_{B} (t + \Updelta t) = N_{B} (t) \times \left( {1 - {\frac{u(t) \times W \times \Updelta t}{S}}} \right) $$

  (11)
  $$ N_{E} (t + \Updelta t) = N_{E} (t) \times \left( {1 - {\frac{U \times \Updelta t}{W}}} \right) \,+\, {\frac{u(t) \times W \times \Updelta t}{S}} \times N_{B} (t) $$

  (12)

• During flow:

  $$ N_{B} (t + \Updelta t) = N_{B} (t) + {\frac{u(t) \times \Updelta t}{{(x_{T} - x)}}} \times N_{E} (t) $$

  (13)
  $$ N_{E} (t + \Updelta t) = N_{E} (t) \times \left( {1 - {\frac{U \times \Updelta t}{W}} - {\frac{u(t) \times \Updelta t}{{(x_{T} - x)}}}} \right) $$

  (14)